PART IV DATA ANALYSIS & RISK ASSESSMENT (Chapters 9-11)

Transcription

1 PART IV DATA ANALYSIS & RISK ASSESSMENT (Chapters 9-11) Australian Centre for Geomechanics 50

2 9. SEISMIC MONITORING SYSTEM DATA ANALYSIS This chapter describes the analysis, interpretation and presentation of quantitative SMS data. The objective is to specify the seismic fingerprint or personality of each mine sector. This can be done using MS-RAP features. A considerable effort should go into analysing the data after the SMS database is validated. Obviously analysis will be limited by inadequate attention to foundational factors such as SMS coverage or data processing. It is necessary to ensure a robust seismicity database. AS/NZS 4360:2004 Risk Management Framework 3.3 Identify risks What can happen, where and when? Tools and techniques 3.4 Analyse risks 9.1 Analysis Principles Routine analysis is mandatory, to allow personnel to become adept at analysing and interpreting, to become proactive about hazards, and to relate anomalous seismic response to any other indicators. There are numerous techniques for the analysis and interpretation of event data from a seismic monitoring system. Some are direct and practical and can be done in limited time on mine sites. Some are more the domain of consultants or researchers. Some suit certain types of seismic behaviour. Regular analysis will establish which techniques are best for the Mine. The engineer performing this task must have a powerful computer with dual screens, and familiarity with the mine s visualisation software package. It is important to note that MS-RAP analysis depends on the source of the energy causing failure being due to the mining itself (e.g. pillar failure, abutment failure, which are directly induced by mining) rather than being regional (e.g. slip on major faults, which may be triggered by mining in a rockmass that is already loaded by stress and close to a state of instability). 9.2 Frequency of Analysis and Reporting Hudyma and Brummer (2007) note that future seismicity in a mine is strongly related to past seismicity in a mine. Our greatest opportunity to become proactive about mine seismicity hazards and risk is through routine back-analysis and interpretation of seismic data. Despite this, the impetus to back-analyse seismic data is often a very large event or a major incident. There is no question that the more seismic analysis we do, the better we get at doing the analysis and interpreting the results. This will undoubtedly lead to a better understanding of the local seismic response to mining, and to the capabilities and limitations of the seismic monitoring system. Alternatively, if seismic data is not routinely analysed, it is possible that a data collection problem will arise in the seismic monitoring and go undetected for an extended period of time. Only when it is suddenly important to analyse the seismic data, is it realised that there is a problem, or that the data is inadequate. The outcome of analysis and interpretation is to be documented. Ensure good communications of monitored data - updates in pre-shift meetings, regular geotechnical presentations on seismicity, display seismic monitoring results. Analysis is necessary on several time levels: a) daily and weekly verify system functionality; identify larger events and recent trends affecting production; look for anomalous rockmass response; activate any required re-entry restrictions. b) after-blast review look for anomalous rockmass response; activate any required re-entry restrictions. c) short-period review, e.g. monthly update and communicate seismic hazard status. d) long-period review, e.g. yearly, or an external review or audit knowledge synthesis. Australian Centre for Geomechanics 51

3 Periodic formal reports, say monthly, are suggested to summarise seismicity, and disseminate feedback information sufficiently frequently and regularly. Mercier-Langevin & Hudyma (2007) and Hudyma & Brummer (2007) suggest that such reports may: Show plan and section of the seismically active areas List and discuss all large events and their suspected causes List all damaging events and the support system in use that was damaged Present facts per mine sector (emergence of new clusters of events, spikes in activity, inferred causes) Identify short term (1 month) abnormal trends in seismicity often associated with blasts Identify long term (1 year) abnormal trends in seismicity (often associated with large scale structures or geometric configurations) Present a detailed seismic risk analysis from MS-RAP including ramifications for production Identify seismicity management failures and successes To make these periodic reports possible, it is clearly necessary for the Mine to keep very good historical records, and have a robust system for records and information management. MSRRM Major Sponsor mines have the benefit of an analysis of their seismicity data by MSRRM staff. State if this has been done, and if so when, where the records are, and the key findings. 9.3 Clustering and Cluster Tracking Create and update seismic clusters and groups in MS-RAP. Analysis is much more successful if logical groups of seismic events are analysed together. This is discussed further in Appendix A. Create clusters and allocate to groups. Set up groups of events defined by location, faults, structures, dykes, other mining or geological features. The premise is that SMS clusters give advance notice of where significant activity may be during stoping, and give early identification of seismically active structures. Groups need 100 or more events over some months to provide a degree of confidence in the analysis. Avoid analysis that is highly dependent on one or a few large events. Update MS-RAP frequently (e.g. monthly) with surveys, faults/structures, geological refinements, mining development and stopes The site Operation Manual needs to specify the location of these files and how to perform the updates. The opening screen of MS-RAP is the Cluster Tracking Monitor (or CTM), an example of which is shown in Figure 22. The CTM contains the listing of seismic cluster groups at the mine, and their activity over time periods selected by the pull down menus at the top of each column. Also shown for each time period selected are the total number of events in each group (#), the number of significant events (#S) and the number of large events (#L). The Seismic Hazard Scale rating, the confidence in the rating and mechanism for each group is shown on the far right. It is important to remember that the data which is ticked (note the check boxes next to the cluster group s names) and the time setting for the far right column (set to All in Figure 22) determines which seismic events will be included in all subsequent analysis. The tabs above the CTM also allow seismic data filtering by time, space or quality. New clusters will appear at the bottom of the CTM as unnamed. It is important to group these clusters and give them names according to nearby locations, mining geometries or geological features. CTM cells shown as orange indicate that the frequency of events for that cluster group and for that time interval is 2 to 5 times the long term average. Red cells indicate the frequency is above 5 times the long term average. This is simply to highlight which clusters or areas of the mine are currently seismically active and whether or not this activity may be anomalous. You can also sort the columns in the CTM by clicking the #, #S or #L headings. This is a good way to get a quick indication of which areas of the mine are seismically active. Australian Centre for Geomechanics 52

4 Figure 22 - Cluster tracking monitor from a MSRRM sponsor s mine. 9.4 How to Perform Daily Seismic Data Analysis At most mines, there is a desire or need to get a quick overview of the recent seismicity in the mine, particularly the events in the last shift or last day. This information is often required at an operations meeting at the start of the morning shift. Typical information required may be: Where have the events been over the last day? Has there been an unusual pickup of activity in the mine in the last 24 hours? Have there been any large magnitude events in the mine in the last 24 hours? What was the seismic response to recent mine blasting? These questions generally relate to short-term seismic hazard in the mine. Specifically: Where is it active in the mine? Is there an increased likelihood of a large magnitude event? While there is no good seismological technique for measuring short-term seismic hazard, with time and experience people develop a feeling for short-term seismic hazard by looking at recent event rates and the location of recent significant events. This section presents a standard procedure for analysing and interpreting seismic data on a daily basis with MS-RAP. Ideally, by following standard procedure, rock mechanics engineers and technicians use the available tools to develop a better feeling for what is normal and unusual at the mine. An important feature of MS-RAP is to generate fast reports to allow quick interpretation of seismic data. In a few minutes, a user should be able to get a very good idea of the location, size and number of events that have occurred in the last 1 to 3 days, and generate a graphical report containing this information. The flowchart in Figure 23 is a model for integrating MS-RAP into a daily seismic data interpretation routine. There are five main steps: Australian Centre for Geomechanics 53

5 1. Import unprocessed seismic data into MS-RAP (typical duration: 1 minute), 2. Generate a daily report in.pdf format (typical duration: less than 2 minutes), 3. Short-term seismic hazard assessment (typical duration: 5 to 15 minutes), 4. Manually process seismic data on seismic system (typical duration: 0.5 to 4 hours), and 5. Update MS-RAP with manually processed events and make seismic hazard interpretation (typical duration: minutes). After the first 3 steps, the user should be able to make an assessment of the recent events in the last day or few days. Note that most Australian mines with ISSI seismic monitoring systems now make use of the ISSI remote processing service. When the geotechnical engineer arrives on site each morning, much of the previous 24 hours worth of seismic data has been processed. At these sites, step 4 is likely to be far less time consuming than what is shown in Figure 23. Each of the steps is described in detail below in the following sections. Figure 23 - Flowchart for performing daily seismic data analysis. Typical Duration 1 minute Import Data into MS-RAP Use MS-RAP Scheduler to load all new events into MS-RAP Trigger short-term update 2 minutes Generate Daily Report Make list of events Plot mine plans with significant and large events with report generator minutes Seismic Hazard Assessment Look for areas with: High frequency of events Abnormal number of significant events Any occurrence of large events Watch for mine blasts that have slipped past the blast filters Print key charts Prepare to distribute or show the key pages of the report or discuss recent seismicity in a morning/operations 0.5 to 4 hours Process data manually Focus on: Location of significant and large events Elimination of significant and large outliers Process data manually on seismic system minutes Update MS-RAP: Update events Update mine plans, if necessary Input blast details for Omori analysis Final Data Interpretation Load manually processed events into MS-RAP Events changed or deleted in manual processing will be automatically changed or deleted in when MS-RAP is updated Identify significant/large outlier events blasts tag as outliers and blasts in MS-RAP Look for new clusters Look for unusual activity rates, particularly significant size events Australian Centre for Geomechanics 54

6 Import Data into MS-RAP Key Steps: 1. Start the MS-RAP Scheduler. 2. Click on Trigger button for Import Recent Data. This will update MS-RAP with new events or changes in events in the last 14 days. 3. You can shut down the MS-RAP Scheduler, if you chose. It is no longer required until more data is to be loaded into MS-RAP. Note that MS-RAP is designed to replace or update previously imported events. In addition, if an automatically processed event is later deleted during manual processing with WaveVis TM or JMTS TM, MS-RAP will purge the automatically processed event from the MS-RAP database in the next data import. Basically, you should have no concerns about importing automatically processed events into MS-RAP. Generate a Daily Report Key Steps: 1. Start MS-RAP. 2. Open the Report Manager in MS-RAP (Figure 24). 3. Ensure that the desired report settings are selected. Recommended report settings should include: o List of events, o Summary of the Cluster Tracking Monitor o Long section of the mine, showing all event locations, o Plan view of all large events or if there are 3 or more significant events on a plan, and o A Magnitude vs Time chart for all of the events. Ensure that the Backdate Report button IS NOT ticked (Figure 24). Figure 24 - Report Manager in MS-RAP. A reasonably good seismic sensor array should give sufficiently accurate automatic source locations to identify unusual seismic event behaviour. If your automatic source locations are not reliable enough for a first pass data analysis, re-calibration or improvements to the seismic system should be considered. Australian Centre for Geomechanics 55

7 Daily Seismic Hazard Assessment The Cluster Tracking Monitor and charts generated in the daily report will show whether there have been unusual levels of activity recently. If there is little activity, or the level of activity is normal a short-term seismic hazard assessment may not be required. If there is unusual activity, steps should be undertaken to investigate the unusual seismic activity. What is unusual recent seismic activity? A substantial increase in event frequency may be expected following a mine blast, but blastinginduced stress-change events are mostly small events. Unusual recent seismic activity may be: o An substantial increase in event frequency not related to mine blasting, particularly in one area of the mine, or o A substantial increase in the number of significant events, particularly in one area (the CTM should identify this), or o The occurrence of any large events. Key Steps: If unusual recent seismic activity is found or suspected, interactive use of MS-RAP should be used to investigate the unusual behaviour. 1. Look for areas in the mine with a high frequency of events. You can do this by looking at the mine plan plot, or with the summary of the Cluster Tracking Monitor. o Are there unusual concentrations of seismic events? Has there been a mine blast to cause these events? o Is there a recent concentration of significant events? o Are there any large events? 2. Magnitude Vs Time Chart. This is the most important tool. o What is a normal event frequency? The cumulative number of events line in the Magnitude vs Time chart will show whether the number of events is above normal. A change in slope of the cumulative line represents an increase in event frequency. o Has there been a mine blast that caused unusual event increases? Mine blasts typical cause a Step in the cumulative number of events line in the Magnitude vs Time chart. The number of events after blasts may increase dramatically for a few hours. o In general, what is the shape of the cumulative number of events line in the Magnitude vs Time chart? Step-like behaviour is seismicity induced by mine blasting. A gradual slope is seismicity independent of blasting. o For this part of the mine or cluster group, using a Magnitude vs Time chart over the last 12 months, when are the large events occurring? Are the large events occurring primarily with blasting (in conjunction with steps in the chart) or are the events occurring unrelated to blasting. The Diurnal chart will also show the time of day of the significant seismic events. 3. Are there large events? o Have large events occurred in this area in the recent past? Recent large events in this area should be considered a very strong warning sign of high short-term seismic hazard. o Have large events ever been recorded in this area. Large events that have ever occurred in an area imply a relatively high seismic hazard. It is rare for the ground to fail and become destressed" causing a decrease in seismic hazard. It is necessary for a user to become very comfortable with using MS-RAP to investigate individual cluster groups using the Magnitude Vs Time, b-value, diurnal and mine plan plotting analyses in MS-RAP. This will enable you to conduct these analyses quickly during a daily seismic hazard assessment. Watch for mine blasts that may have been missed by the MS-RAP blast filters. These will be significant or large events that occur at blast time. Australian Centre for Geomechanics 56

8 Make note of significant and large events that occur with no small events preceding or following the significant and large events, especially if these events do not follow directly after mine blasts. This is unexpected behaviour, and a strong sign of geologically controlled (particularly fault-related) seismicity. Manual Data Processing Manual data processing is the most time consuming task in operating a seismic system, typically taking 30 minutes to several hours to complete. It is important that the manual processing is done routinely so that the best possible data is included in MS-RAP for future analyses. From a seismic data processing perspective, it is far more important to eliminate blasts from the seismic system, than it is to get good source location on small seismic events (local magnitude -2). Below is a recommended seismic data processing list of priorities: 1. Remove or tag all blasts on the seismic system. 2. Locate large events (Richter magnitude +1) as accurately as possible. 3. Locate significant events (Richter magnitude 0) as accurately as possible. 4. Locate smaller sized events (-1 Richter magnitude < 0). 5. Locate small events (-2 Richter magnitude < -1). 6. Locate very small events (-2 Richter magnitude). Ideally, on a daily basis, steps should be done as soon as possible at the start of each day. Steps 4 and 5 should be kept up to date as much as possible, preferably keeping up within a day. Ideally, step 6 is kept up to date when possible, but if something has to be left undone, it is the processing of the small events (-2 local magnitude). Typically the very small events comprise more than half of the events on the seismic system and may not be easy to manually process or locate accurately. It is imperative that all mine blasts are tagged as blasts with the seismic system software (WaveVis TM, JMTS TM ). Blasts pose a serious problem for seismic data interpretation, as they occur frequently (a few to several per day) and in MS-RAP have the appearance of large seismic events. Including only a few blasts per month as real events will severely compromise our ability to understand seismic hazard. Final Data Interpretation Key Steps: 1. Start the MS-RAP Scheduler. 2. Click on Trigger Button for Import Recent Data. This will update MS-RAP with the changes in events made during the manual processing. 3. You can shutdown the MS-RAP Scheduler, if you chose. It is no longer required until more data is to be loaded into MS-RAP. 4. Allocate ungrouped clusters to cluster groups. As new seismicity is recorded and new clusters are generated by MS-RAP, these clusters may potentially be part of existing cluster groups. 5. Allocate significant and large un-clustered events to clusters. Very large seismic events may not be well located and are frequently not clustered. Often the events that occur after large events identify which clusters these large events should be associated with. 6. Add mine blasts to the blast database. The blast database is very useful for building cause-effect relationships between mining and seismicity. Information in the blast database can be displayed in most of the MS-RAP data analysis techniques. If significant or large events are not clustered, check to see if these events can be manually allocated to a cluster (see the user s manual). Ideally, the vast majority of significant and almost all large events are associated with clusters and cluster groups. Chapter 11 describes some approaches for conducting short term, day-to-day seismic hazard assessment. Australian Centre for Geomechanics 57

9 9.5 Other Points Also note the following miscellaneous comments and cautions: Ensure that any seismically quiet periods are correctly handled and understood: If due to SMS downtime or non-operation of certain sensors, the effect of those periods must be removed from or accounted for in charts, graphs and calculations. If due to a temporary lack of mining, be aware that this quiescence does not reflect the seismic potential of an area when mining is active. If due to a theorised physical change in the rock mass (such as an area becoming destressed, or certain pillars fractured, or punching into the footwall) then the truth and extent of the change needs to be demonstrated. Beck (2003) cautioned that some source parameters (source radius, apparent stress, corner frequency, stress drop) produced by seismic monitoring systems are very heavily model dependent, also that the stress drop inferred from SMS will not match the real stress change in the rock. Errors arise due to assumptions, sensor distribution, and asperities. Notes on the Gutenberg-Richter plot (b-value plot): The larger events in a Gutenberg-Richter (GR) plot are few in number by definition, so statistically they appear more variable. It is not appropriate to use these few larger events alone to attempt to forecast a maximum size event. The GR plot should always be accompanied by a statement of the time period (weeks, months etc) of the data comprising the plot. Comparisons of such plots for different clusters/groups should be on the basis of similar time periods. The GR plot should show a fairly linear slope. If not linear, and the number of events is statistically sufficient, review the cluster for: o Appropriate cluster definition does it comprise two (or more) mechanisms superimposed; o Effects resulting from seismic system changes e.g. if a nearby sensor becomes nonfunctional, lower-end events may stop being recorded, warping the GR plot. Australian Centre for Geomechanics 58

10 10. THE SEISMICITY KNOWLEDGE BASE (SKB) This chapter deals with explaining the seismicity in light of the mine environment and mining practices. The repetitive nature of mining provides the opportunity to apply, to future design, experience gained in every prior mining step (Kaiser, 1996). This implies procedures and methods be developed to integrate seismic data with important nonseismic information and considerations (such as mine sequencing, presence of major geological features, stress, numerical modelling). Damage is highly variable according to local site factors, so the latter need to be collected, filed and interrogated to determine their relevance and influence. It is not necessarily easy to develop an engineering appreciation of all the factors that have a part in directing seismicity. The Mine may have some of this material in current and archived documents and papers, which should be consulted accordingly. The outcome of the SKB process will ideally: Define how seismic character is related to the various physical aspects of the Mine. Provide sufficient objective information to allow the rigorous evaluation of the merits and usefulness of the SKB topics (based on Durrheim et al, 2007). Provide material for research, continuous improvement and back analysis. AS/NZS 4360:2004 Risk Management Framework 3.3 Identify risks Why and how can it happen? 3.4 Analyse risks Evaluate existing controls 10.1 Definition of the Seismicity Knowledge Base (SKB) The Seismicity Knowledge Base (SKB) is a database that integrates everything that is known about the seismicity at the Mine. Rockburst risk questions are very complex and often subjective to answer. A large amount of information, both seismic and non-seismic in nature, needs to be synthesised or integrated to arrive at knowledge conclusions. It is necessary to formalise data in an objective and robust manner. It is proposed that for a robust SRMP, the synthesis method is to concisely summarise what is known in a database called the SKB as follows: The SKB contains facts. It summarises knowledge of the specific aspects of the Mine that are considered to be related to seismic damage. The SKB is a layered database, and may have a dozen or more layers. It is a major document requiring time and effort to compile. The SKB has similarities to the Ground Conditions Model (GCM) 2 launched by Western Mining Corporation (WMC) a decade ago. The term Knowledge Base is adopted in the SRMP instead of alternatives such as Model, as the latter can be confused with numerical modelling, with results or interpretations instead of facts, and with geological models. 2 The GCM (Singh et al, 2002) presented data as a series of overlays of parameters such as mine history, structural geology, Q-rating, blast quality, installed ground support, bursts, falls, adverse ground conditions, and stability problems. The GCM was regularly updated with information from the various data gathering methods. The primary function of the GCM was to synthesise the relevant information from the geological /geotechnical databases into an appropriate format for application to mine design, planning and ground support requirements. See also Newmont Australia Ltd (2003). Australian Centre for Geomechanics 59

11 The SKB also represents a feedback loop. It is the knowledge repository and a place where improvements are decided. Entries made to the SKB may trigger further entries to the SRMP, GCMP and related procedures. The SKB is a live database new data is added as it becomes available. The SKB is part of the continuous improvement process. It facilitates the trending of and repair of systematic failures and deviations. As noted by Beck (2003) every failure... should be treated as a failure of the rock mechanics hazard management system... must investigate, and implement into the system the fundamental requirements to avoid or manage the event. It is usually difficult to visually discover relationships among a large quantity of disparate data. Many aspects and uncertainties can muddy the waters. There are tradeoffs between many parameters. It takes time and a comprehensive study of these features to identify the fundamental controls on seismicity. A powerful technique known as CSIRO Self Organising Maps or CSOM (Fraser et al, 2006) is available and is recommended to better understand relationships between the SKB parameters The SKB Structure It is likely that the SKB will comprise layers in more than one form. Possibilities are: A written summary a statement of the factual links between seismicity and a range of parameters. Links can sometimes be expressed as a series of rules. The written summary clarifies thinking. A pictorial SKB (e.g. sets of Level plans) can be viewed and interrogated when assessing seismic clusters and trends. Map on Level plans the extent of and degree of damage due to events, including notes on the installed support, whether that support was adequate, and proximity to stoping, and estimated distance from the source. Via the MS-RAP Minodes function. A future capability will be to attach files to Minodes e.g. incident reports, photos, monitoring data. A database this could contain data relevant to specific occurrences, such as significant seismic events. A set of charts when data lends itself to plotting, do so. Construct charts combining several of the measured or observed parameters. These charts will become tools to develop knowledge and understanding of seismicity at the Mine. Repeat for each mining area if more than one. Most importantly, SKB knowledge is a synthesis of data per location, not per topic (although topical knowledge can and should be extracted from it). The structure of the SKB must be based on location. Whatever the particular structure used physical or electronic it must be designed to collate and view the full range of data available per location. In practical terms, the unit of location will most likely be the stope or the drive The SKB Contents Practitioners are urged to consult Singh et al (2002) for illustrations of the types of information that belong in the SKB. The SKB needs to address the following topics, and perhaps others as relevant, and the interactions between these topics: Location Numerical modelling results Geology Rock Types Australian Centre for Geomechanics 60

12 Rock Properties Significant Geologic Structure Structure Mechanical Properties Rockmass Characterisation Groundwater Stress Regime Ground Conditions Ground Control Measures Excavation Span, Geometry o Development headings o Creation of stress attractors Mining Sequence o Stope sequencing o Stope retreat direction Stope blasts Production rate - rate of extraction of rock Workplace activities Backfill Previous Mining Refer to Appendix E for more information on each of these topics. Australian Centre for Geomechanics 61

13 11. SEISMIC HAZARD AND RISK ASSESSMENT This chapter is about how to define the level of seismic hazard and risk in The Mine. It describes techniques for assessing seismic hazard in the short term, as well as a procedure for the demarcation of high seismic hazard areas and assigning a risk level, based on the seismic system data analysis plus key mining / environment parameters. A quantitative framework for risk assessment is used in MS-RAP. It stands in contrast to the qualitative approaches. Qualitative methods use words to describe consequences and likelihoods. They are characterised by being based on experience, taking time and skill to develop at a new Mine, relating hazards to similar conditions (which can over-estimate some hazards and miss others) and relying on correct interpretation of seismic mechanisms. The MS-RAP quantitative approach uses numerical values for likelihood, consequences and risk. This permits the objective measurement of the success or failure of the particular risk assessment. This approach has the benefit of being based on real time data, but it is important to note that: Estimates are completely dependent on data quality and an adequate seismic system Robust data management systems are necessary A wildcard with both qualitative and quantitative approaches is that future seismic hazard is not guaranteed to be related to past seismic hazard. If for example tectonic stresses are locked into the rockmass, future seismicity can be much larger than could be inferred from past seismicity. Both these methods rely on input from past seismicity, and cannot make provision for such as these. Therefore it is critical to refer to the SKB for guidance in the risk assessment process. AS/NZS 4360:2004 Risk Management Framework 3.4 Analyse risk Consequences and likelihood Types of analysis Sensitivity analysis 3.5 Evaluate risks 11.1 Short-Term Seismic Hazard Assessment There are no consistently reliable short-term seismic hazard assessment techniques. However, at some mines, rock mechanics personnel have developed a local intuition or understanding of seismic hazard by reviewing and analysing seismicity on a daily and weekly basis. On a regular basis, rock mechanics personnel are required to use their best judgement or intuition with regard to the likelihood of hazard seismicity. MS-RAP can aid that judgement, by: Providing real-time data analysis tools, The means of back-analysis of past seismicity, Development of intuition is still in the hands of the operator. MS-RAP is simply an application to aid that development. One of the first applications of seismic monitoring systems was to analyse recent activity rates, particularly following mine blasts and large events. If the seismic event occurrence rate was unusually high, a management decision was made to exclude or keep out of the active areas. The activity rate to cause exclusion is based on local site experience, allowing significant potential for bias and subjectivity. The nature of failure in a rock mass is that it is rarely instantaneous. It is a gradual process that is caused by a set of conditions related to stress, geological features, and the influence of mining. Australian Centre for Geomechanics 62

14 The process evolves over time gradually growing and accelerating. This process generates significant seismicity that often gives strong indication of the seismic hazard (maximum potential event size) and the seismic source mechanism. The objective of proactive use of MS-RAP is to identify high seismic hazard situations and to identify patterns in seismicity. Objectives There are three primary considerations in short-term seismic hazard assessment: Where is seismic hazard elevated, When is a high hazard event most likely to occur, and How big could the event be. There are different MS-RAP applications for each of these considerations. Tools and Techniques In MS-RAP, the main tools for short-term seismic hazard assessment are: Spatial plotting of event magnitude, Frequency-magnitude analysis, Magnitude-Time History analysis, Apparent Stress Time History analysis, Event frequency and the Cluster Tracking Monitor, Instability analysis (EICAV), Daily histograms, and Diurnal analysis. For clustered seismic events in MS-RAP, each of these techniques has been subjectively rated for their value in determining where, when and how big an event may be (Table 8). The success rate of each technique has also been subjectively rated. Table 8 - Success in short term seismic hazard assessment. Short Term Seismic Hazard Assessment Techniques Where? When? How Big? Success Rate Spatial Plotting of Magnitude Frequency-Magnitude Analysis Magnitude Time History Analysis Apparent Stress Time History Cluster Tracking Monitor Seismic Hazard Map Instability Analysis Daily Histogram of Event Frequency Diurnal Analysis Notes: =Not useful =Rarely useful =More successful =Most successful No single technique is universally applicable or infallible. Seismic hazard is best evaluated using a combination of techniques. The following pages describe some of the techniques available in MS-RAP for short-term seismic hazard assessment. A more detailed discussion of the techniques can be found in Appendix A and in the MS-RAP electronic help file or manual. Australian Centre for Geomechanics 63

15 Spatial Plotting of Event Magnitude Where? When? How Big? Success Rate Spatial Plotting of Magnitude Description: Plotting of seismic events in 2D and 3D can be very revealing about future seismicity. When possible, plotting should be done on up-to-date mine plans and include all past stoping, and geological features. Seismic Hazard Considerations: The largest local event is an indicator of the local worst case event. Where are the significant and large events? Future significant and large events are likely to be close to past events. How many significant and large events have occurred? If there are numerous significant and large events, there is a significantly greater likelihood of future events. If there are few or no significant or large events, the likelihood of such an event in the near future is reduced. Frequency-Magnitude Analysis Where? When? How Big? Success Rate Frequency-Magnitude Analysis Description: Frequency-magnitude analysis is the fundamental technique for seismic hazard assessment in mines. Seismic Hazard Considerations: A lower b-value generally implies that there is a greater proportion of large events in a particular population of events. This fact alone suggests that there is an increased likelihood of larger events. The x-axis intercept of the theoretical frequency-magnitude relation is often a good indicator of the largest potential event in the group. This is particularly true when the frequencymagnitude relation is well-behaved and linear. Groups of events that have few or no significant and large events have a lower seismic hazard. Groups of events with numerous significant and large events have a higher relative seismic hazard. Magnitude-Time History Where? When? How Big? Success Rate Magnitude Time History Analysis Description: Magnitude-time history analysis is perhaps the most valuable tool for investigating seismic hazard. Australian Centre for Geomechanics 64

16 Seismic Hazard Considerations: Look for an increase in the number of events (change in slope in the blue Cumulative Number of Events line). Larger events often occur during periods of increasing event frequency. Kijko and Funk (1994) give a very crude rule of thumb of the largest expected event: where: m max = X max + (X max X n-1 ) m max is the largest expected event, X max is the largest event that has occurred, and X n-1 is the second largest event that has occurred. So if the largest event is local magnitude +1.5 and the second largest event is +1.1, the largest expected event would be Are step increases in event frequency due to mine blasting? Do the significant and large events tend to occur during periods of high event frequency? This suggests that large events are primarily caused by stress change due to mine blasting. Do significant and large events occur at times of low event frequency? This suggests that the significant and large events occur largely unrelated to mine blasting. This is typical of fault-slip seismicity. There is a setting to allow SHS to be shown for a cluster. This is helpful for identifying situations in which unexpected large events could occur. In the Blast Panel of MS-RAP, there is a setting to allow mine blasts to be shown on the magnitude-time history chart. This is helpful for building a relationship between mine blasts and mine seismicity. When events are occurring, is the maximum event magnitude increasing and are there changes in event frequency? This is analogous to the local deformation rate. Apparent Stress Time History Where? When? How Big? Success Rate Apparent Stress Time History Description: Apparent Stress is a seismic source parameter measuring the relative amount of energy in a particular seismic event. It has been shown that when stress is increasing, Apparent Stress is higher than expected. Apparent Stress Time History is a means of quantifying changes in Apparent Stress over time. Seismic Hazard Considerations: In many cases, large seismic events occur shortly after stope blasting. Typically the seismic events following mine blasting have unusually high Apparent Stress. Apparent Stress Time History can be used to show when stress change is occurring and the likelihood of large seismic events is higher (elevated seismic hazard). To successful apply Apparent Stress Time History, it needs to be used in back-analysis to best determine analysis dependent parameters. Apparent Stress Time History is less likely to be an indicator of high seismic hazard for seismicity that is not related to stress change, in particular to seismicity on mine faults. Large events on mine faults often do not have any significant precursory seismicity, so the Apparent Stress Frequency is likely to be low. In the Blast Panel of MS-RAP, there is a setting to allow the date of mine blasts to be shown on the Apparent Stress Time History chart. Australian Centre for Geomechanics 65

17 Event Frequency and the Cluster Tracking Monitor Where? When? How Big? Success Rate Cluster Tracking Monitor Description: The Cluster Tracking Monitor (CTM) is used to evaluate seismic event frequency for clustered seismic data. The CTM allows the user to view the number of seismic events in mine clusters for four different time periods. Typically one time period is selected for: Short term (1 to 3 days), Medium term (1 to 3 months) Longer-term (in the range of 3 months to 1 year), and Very long term (a few years or more). For each cluster and group in MS-RAP, the long term frequency is calculated for the number of events, the number of significant events and the number of large events. For each time frame, the number of events recorded is compared to the long term frequency. If the frequency of events in a particular time frame is more than two times the long-term frequency, the time frame is coloured orange. If the frequency of events in a particular time frame is more than five times the long-term frequency, the time frame is coloured red. Seismic Hazard Considerations: The CTM alone is the single most important feature in MS-RAP. It shows where and when events are occurring, in particular the significant and large events. If a group of events has had few or no significant and large events, the seismic hazard is low. For example, in Figure 25, group G20 has had more than 4727 seismic events, but only 8 significant and no large seismic events. Despite the high number of events, seismic hazard is relatively low If a group of events has had many significant and large events, the seismic hazard is high. For example, in Figure 25, group G17 has had 1525 events, which is relatively low, but it has 62 significant and 9 large events. This group has the second highest number of significant and large events (after group G13). The CTM also shows the number of un-clustered seismic events. If there are a considerable number of un-clustered seismic events (particularly significant and large seismic events), the level of confidence in the MS-RAP clustering is reduced. In Figure 25, only 29 of the significant and only 1 of the large events have not been clustered. This is very good clustering, with almost all of the significant and large events in clusters. Figure 25 - Example of a Cluster Tracking Monitor. Australian Centre for Geomechanics 66

18 Each Time-Magnitude cell in the CTM shows if the event frequency is 2 times or 5 times greater than the long-term frequency. When a particular group of events shows high event frequency for multiple cells, the seismicity can be considered anomalous. The Backdate feature can be used to investigate historical event frequencies. This is a useful exercise prior to large events or during the blasting of particular stopes to investigate past seismicity rates, and to develop local rules of thumb on what are normal and abnormal levels of seismicity. Seismic Hazard Mapping Where? When? How Big? Success Rate Seismic Hazard Map Description: Seismic hazard mapping is a combination of three MS-RAP seismic hazard techniques: spatial event plotting, the Cluster Tracking Monitor, and frequency-magnitude analysis. MS-RAP generates Minodes, which are automatically generated nodes spaced on about 5 metre spacing for all mine development. There are typically several thousand Minodes calculated for an average size mine. Seismic hazard is calculated for each Minode and is updated with the occurrence of each new seismic event loaded into MS-RAP. Seismic Hazard Considerations: The greatest benefit of a seismic hazard map is that it is overlain onto mine development allowing the worst-case seismic hazard to be viewed for the mine development. This information can be compared to the location of past events and used in determining ground support requirements and exclusion zones following mine blasts. Instability Analysis (EICAV) Where? When? How Big? Success Rate Instability Analysis Description: Use of Instability Analysis (EICAV) is relatively common in South Africa. These analyses are rarely successfully used at Australian or Canadian mine sites. A local level of confidence in the application of Instability Analysis should be achieved before it is used as a short-term seismic hazard tool. This is done through back-analysis of past seismicity, focussing on tuning the Energy Index averaging parameters. Seismic Hazard Considerations: An increasing or variable Energy Index (EI) is purported to be an indicator of increasing stress. A decrease in EI is potentially related to rockmass destressing. This is often seen as the key characteristic of this analysis. A sharply decreasing EI suggests that significant rockmass failure has occurred and may be a precursory indicator of a large event. The Cumulative Apparent Volume (CAV) is related to the rockmass cumulative deformation. A flat CAV suggests that no significant rockmass deformation has occurred. A steadily increasing CAV suggests accumulating rockmass deformation. Large steps in CAV suggests that seismic energy is being released in large seismic events. There is a setting to allow the date of mine blasts to be shown on the EICAV chart. Australian Centre for Geomechanics 67

19 Limitations: Successful use of Instability Analysis requires a high degree of local experience in applying the technique. Confidence in this technique can only be gained through successful back-analysis of local seismic data. The technique also relies on a high level of system sensitivity in the zone it is applied (possibly as far down as Richter Magnitude -2.0). Daily Histogram of Event Frequency Where? When? How Big? Success Rate Daily Histogram of Event Frequency Description Amongst the first applications of seismic monitoring was identifying anomalous event frequency levels. If there are an unusually high number of events, the ground is perceived to be working and is more likely to have a significant ground failure. This is still the most widely used approach, particularly after mine blasts. Seismic Hazard Considerations: A daily histogram allows the user to compare recent daily event rates to past daily event rates. There is a setting to allow the user to calculate a past moving average. The default is 30 days. This allows the user to look for long term trends, such as a steadily increasing event rate. In the Blast Panel of MS-RAP, there is a setting to allow the date of mine blasts to be shown on the daily histogram chart. Diurnal Analysis Where? When? How Big? Success Rate Diurnal Analysis Description: Diurnal (or time of day analyses) can be used to determine whether the majority of events are occurring directly as a result of blasting, or whether they are independent of blasting. Diurnal charts are very simple. They show the hour of the day in which the seismic events occur. Seismic Hazard Considerations: A diurnal chart allows the user to determine whether or not seismic events (particularly significant events) occur directly following blasting or with no obvious relationship to blasting. Commonly seismicity associated with faults or other major structures exhibit little or no relationship to blasting. Whilst the diurnal chart shows little information about the level of seismic hazard, it provides some indication as to the time when this hazard may occur. Other Considerations Most Probable versus Worse Case Seismic Event The user has to choose whether seismic hazard should consider the most probable large seismic event, or the worst-case seismic event. Selecting the worst-case seismic event will provide the most conservative seismic hazard evaluation, but this may be unduly pessimistic. The most probable seismic event may be more appropriate: Australian Centre for Geomechanics 68

20 In cases of relatively low mining extraction, During development mining activities, When there has been no recent stope mining in proximity of a workplace (in the last few months). The worst-case seismic event may be more appropriate: In areas of high extraction, i.e. near large underground voids, or in mine pillars, When production blasting is in progress in the proximity of a workplace, Near geological features, with a past adverse seismic history, which show signs of movement. In MS-RAP, the entire seismic record should be used when assessing the worst case seismic event. A most probable event may be more realistic for areas that have not experienced a high event rate for a long period of time. In this case, the user could consider using only a more recent time period for estimating the largest potential event. For instance, if an area of the mine has not had a significant event rate for five years, a most probable event could be calculated using frequencymagnitude relation for the last 3 or 4 years of seismic data. The Role of Clustering in Understanding Seismicity The seismic hazard methodology proposed for MS-RAP is founded upon the concept that the seismic related failure in mines occurs as a result of some combination stress, geological structure, and the influence of mining. Clusters of seismic events represents rockmass failure processes, or sources of seismicity. If an adequate seismic record has been collected, the seismic hazard (or worst case seismic event) associated with each seismic source can often be estimated. There are numerous assumptions and limitations with this regard, which are detailed in the MS-RAP manual or electronic help file. Exceptions There will always be a percentage of seismic sources for which the seismic hazard potential is much greater than could be inferred by past seismicity. In some of these cases, tectonic stresses are "locked" into the rockmass, and the influence of nearby mining triggers seismic events much larger than would be expected. MS-RAP cannot make provision for such cases. However, based on several years of experience of detailed analysis of seismicity in mines, the relative frequency of occurrence of unexpected, large, "triggered" seismic events is very low. There are some similarities in some of the cases of these events: They occur associated with faults that are significant in spatial extent, and They tend to occur during development mining or at relatively low levels of extraction. It is assumed that a seismic hazard methodology can be developed without the consideration of the occurrence of unexpected, large, "triggered" seismic events. Australian Centre for Geomechanics 69

21 11.2 Mine Blasting and Mine Seismicity There are often cause-effect relationships between mine blasting and hazardous seismicity. When a blast is fired (development or stoping), the local seismic response is likely to be similar to the seismic response of past blasts. If there is a history of significant and large events after blasts, one should expect similar seismicity for future blasts. The short-term seismic hazard may be high. If there is no history of significant and large events after blasts, there is a much lower chance of a significant or large event. The short-term seismic hazard is much lower. This underlines the importance of maintaining the blast database in MS-RAP, in particular for all stope blasts and slot raise blasts. One of the earliest applications of seismic monitoring in mines was to monitor whether the level of microseismic activity being recorded exceeded the normal background level of microseismic activity. The basic premise was that if the level of seismic activity was elevated, then the ground was still adjusting to recent mine blasting. The potential for a large seismic event was considered to be greater than normal, and workers were often kept out of the workplaces being affected. The hourly or daily event count is used as a simplistic measure of short-term seismic hazard. An effective application of seismic event frequency analysis is to determine an appropriate time for the workforce to re-enter a workplace following a mine blast. Following a mine blast, there may be an elevated level of seismic activity while the rock mass adjusts to the new local stress conditions. Local re-entry procedures are often developed based on the number of hours before seismic activity returns to a normal level. Re-entry analysis can be split into two categories: Retrospective / back analysis (e.g. Omori analysis, identification of trends in re-entry times across the mine), and Real-time (e.g. keeping track of CTM changes, event rates and trends in the magnitudetime chart, apparent stress time history). Either type of analysis requires an adequate system resolution (as low as -2.0 Richter Magnitude) to adequately capture the rockmass response and seismic decay following blasting. Approaches to assess the seismic system sensitivity across the array should be used to determine where these analyses can be applied with confidence, such as those described in Chapter 4. It is important to keep in mind that activity on some seismic sources, particularly those related to shearing along faults or other major structures, often show no relationship to mine blasting. In these cases, re-entry analyses may give misleading results and give the illusion of safety when in-fact a significant or large seismic event could occur at any point in time, irrespective of blasting. Other forms of analyses are likely to be more appropriate in these circumstances. Retrospective / Back Analysis This form of re-entry analysis typically requires the full seismic decay following blasting to be recorded before an assessment of re-entry time can be made. Omori analysis is the most common method. The analysis considers all the seismic events that occur within a defined radius around the blast co-ordinates within a defined time period after the blast. By reading the cumulative percentage of number of events or cumulative percentage of seismic energy released, it is possible to define a point in time at which it may be considered appropriate to re-enter a blasted area. Once the blast details have been entered in MS-RAP, the program can automatically produce an Omori chart. An example from a sponsor s mine is shown in Figure 26. In the example shown, by the 6th hour after the blast, 90% of the cumulative seismic energy has been released. Alternatively, 90% of the total number of events has been recorded by the 7th hour after the blast. If the Omori power law estimate is used (which is based on the number of events per hour), the 90% value occurs at the 10th hour. The 90% cumulative seismic energy release is a typical criterion employed by other MS-RAP users. Using that criterion in this example, at 90% of the total seismic energy released, the rockmass would be considered to have adjusted to the new Australian Centre for Geomechanics 70

22 stress state, and the seismicity which continues to occur is similar to pre-blast levels. The chart shows that seismic events which occurred after the 6th hour were small energy (and small magnitude) events. The choice of 90% of the total seismic energy as a re-entry criterion in this case is completely arbitrary. A management decision is required as to what criteria should be used. This form of analysis is typically retrospective because we require seismic data for the entire postblast decay. In the example shown in Figure 26, we could not necessarily make a decision on reentry 3 hours after the blast, because we would not have adequately captured the full decay until around the 7 th or 8 th hour. Figure 26 - An example of an Omori analysis chart generated in MS-RAP. This form of analysis is well suited to examining re-entry time trends across a mine and building up a re-entry time profile by stoping block or mining area. These trends can then be compared to other parameters such as tonnes blasted, blast depth or stress conditions at the blast location to establish a better understanding of the rockmass response to blasting. Identifying which clusters or groups are active following a blast is also a useful tool for determining the area over which re-entry periods apply. For example, some blasts may trigger a seismic response on nearby faults which intersect other working areas. An example of a study examining trends in re-entry times by mining area is shown in Figure 27 and Table 9. If possible, the SRMP should provide details of the results of studies into re-entry times and trends across the mine. Australian Centre for Geomechanics 71

23 Figure 27 An example of a re-entry study showing re-entry times determined by Omori analysis at a sponsor s mine, organised by mining area. CL-97 CL-99 AL-89 AL-89 E SBL-088 Slash SBL-88 UH SBL-090 AL-87 SBL-88 DH 0, 0, 0, 0 SEAL-103 6, 0, 0, 0 SAL-98 W 8, 1, 8, 0 SAL-99 DH SAL-99C DH 4, 6, 9, 0, 6, 1, 4, 6, 7 SAL-97E 9, 0 0, 0 0, 0, 0 0, 5 0, 5, 9, 0 8, 10, 0, 5, 4, 0 0, 6, 0, 7, 8 BL-61 CL-61 CL-66 CL-67N 0, 0 BL-81 3, 6, 4 0, 0 0, 0, 0 BL-53 0 CL-43 0, 0, 0, 0, 0 CL-53 CL-48 UH 0 SAL-102 UH 4, 8, 0, 0 WAL-83 4, 8, 4, 8 WAL-85 UH 8 WAL-87 7 AL-82 0, 0, 0 AL-63 AL-61 0, 0 SEAL-111 3, 0, 7, 0 WAL-62 UH 2L-24 PRODUCTION BLASTS IN MS-RAP DATABASE RE-ENTRY TIMES FROM OMORI ANALYSIS (SELECTED BLASTS FROM SEPTEMBER 2006 MAY 2007) Australian Centre for Geomechanics 72

24 Table 9 - A table summarising the rockmass response to blasting at a sponsor s mine, based on Omori analysis of blasts shown in the previous figure. Mining Area SEAL-103, SAL-98 W, SAL-99 DH, SAL-99C DH, SAL-97E WAL-83, WAL-85, WAL-87 AL-89, AL-89 E, SBL-088 Slash, SBL-88 UH, SBL-090, AL-87, SBL-88 DH AL-82 Re-entry time (Hours) Re-entry time (Hours) Re-entry time (Hours) Re-entry time (Hours) Distribution of Re-entry Times Number of Instances Number of Instances Number of Instances Number of Instances Description of Seismic Response to Blasting Prone to tight, stiff response locally around stopes and in X-cut shears, as well as shear seismicity deeper in the Saddle. Blasts here generate an Omori decay in most cases. Seismic system coverage could be improved in this area. Maximum recorded re-entry time = 9 hours. Areas affected Locally around stope, Saddle and X-cut shears. Very tight, stiff response following blasting, often including both shear and non-shear significant events. SBL-Cindy and Cindy Pillar groups the most active. Response to blasting here suggests that this area is carrying load (both abutment stresses and shear stress on seismically active geological structures). Maximum recorded re-entry time = 8 hours. Areas affected Locally around stope, SBL- Cindy and Cindy Pillar groups. Limited coverage in stopes with higher elevation. A combination of responses here. Small tonnage blasts in AL-89 and AL-89E have shown low total energies or no local response, while other stopes (SBL-090, AL-87 and SBL-88 DH) generate limited response locally but may generate a response elsewhere in the mine, such as shear seismicity in the vicinity of the WAL-83/85/87 stopes or possibly in the X-cut Shears. Maximum recorded re-entry time = 10 hours. Areas affected Some locally around stope, WAL-83/85/87 area and X-cut Shears, No Omori response evident, low total energies. This area appears to be carrying less stress than those located at similar elevations more inside the Saddle, which may explain the lack of seismic response. Maximum recorded re-entry time = 0 hours. Australian Centre for Geomechanics 73

25 Real-Time There are several methods for assessing re-entry times immediately following a blast, or in realtime, during the period when the seismic decay is underway. These methods typically rely on some assessment of the rate of occurrence of events. They do not provide the same level of confidence that a retrospective analysis would allow, however they do aid in decision making and can be used to verify re-entry criterion applied to a mining area based on studies such as those shown previously. The simplest method is to make use of the MS-RAP Cluster Tracking Monitor (CTM) to keep track of which clusters are showing above average activity rates, as was demonstrated in Section 11.1 and Figure 25. If a cluster group is experiencing event frequencies of at least 2 times more than long term average frequency for an event category, that CTM cell will be highlighted. Another simple method is to examine short-term Magnitude versus Time charts, which can indicate both the rate and magnitude of events occurring. Two blasts are shown in the example in Figure 28 (indicated by the red stars). The rate of events is indicated by the blue line (the cumulative number of events). After the first blast, the slope of the blue line returns to pre-blast levels only 1 hour after the blast. After the second blast, the event rate takes longer to return to pre-blast levels, probably around 7 hours. Figure 28 - An example of a short-term Magnitude vs Time chart showing seismic events occurring after two blasts. Australian Centre for Geomechanics 74

26 A second method makes use of the Apparent Stress Time History analysis tool in MS-RAP (more information is provided in Appendix A). According to seismology theory, seismic events have a higher apparent stress when stress in the rockmass is increasing. This is typical after a blast, which alters the mine geometry and results in stresses readjusting to the new geometry. The ASTH tool can be used to give an idea if this stress readjustment is underway. An example is shown in Figure 29. Figure 29 - An example of using the Apparent Stress Time History Tool to assist in determining re-entry times following two blasts. In the example above the same two blasts are used as were shown in Figure 28. A 3 hour leading average is used to give the blue line the frequency of high apparent stress events per hour (in this case seismic events with apparent stress above 10kPa are considered high ). After the first blast, most of the seismic events occur in the first hour. This is reflected in the elevated apparent stress frequency following the blast, although because a 3 hour average is used, the frequency (the blue line) does not decrease until after the third hour. After the 2 nd blast, the apparent stress frequency is shown as being elevated for 4 hours. It is recommended that a combination of methods are used to assess re-entry in real-time, and one should not rely on any single method. In the example of the second blast, using ASTH alone may have given misleading results, as the event rate was still elevated after the fourth hour. If the short term magnitude-time chart had been use, it would have been evident that the overall event rate was still elevated after the fourth hour. Australian Centre for Geomechanics 75

27 11.3 The MS-RAP Quantitative Seismic Hazard and Risk Assessment Framework MS-RAP uses a probabilistic approach to the evaluation of seismic risk. Figure 30 shows a flowchart representing the risk assessment process applied within MS-RAP. Figure 30 - Flowchart representing the risk assessment process applied within MS-RAP. Increasing sophistication Seismic Hazard x Excavation Vulnerability x Exposure SHS Rating of Clusters Seismic Hazard Map Rockburst Damage Potential Map Seismic Risk Map Seismic Data + Mine geometry / mine plans + σ 1, UCS, ground support, span, geological structure + Personnel exposure = SEISMIC RISK In MS-RAP, seismic risk is defined as a function of several elements: Seismic hazard: here defined as the likelihood or probability of occurrence of events of a certain magnitude, and is quantified in MS-RAP by the Seismic Hazard Scale (SHS) rating. The derivation of the SHS is provided in the MS-RAP manual or electronic help file. Rockburst Damage Potential: (RDP): This is the consequence and is itself a function of two elements: Excavation Vulnerability Potential (EVP): This quantifies the proneness of an excavation to damage from a postulated seismic hazard Peak Particle Velocity (PPV): The actual seismic hazard, expressed as the likely maximum PPV at the locality (which is related to the SHS rating of that locality). Exposure: this term quantifies the exposure of personnel to possible damage sites. MS-RAP does not include features for quantifying exposure. It is up to the mine to decide on measures to manage personnel exposure (e.g. improved protection, access restrictions, re-entry protocols, etc). Seismic risk: here defined as: Australian Centre for Geomechanics 76

28 Seismic Risk = Seismic Hazard x Excavation Vulnerability x Exposure Where are the events? How big? (Magnitude) What frequency of occurrence? A function of: Span Ground support Ratio of Stress / Rock Strength at the location being assessed Presence of seismically active major structures What area or task? Production drilling Mucking Charge-up Workshop Decline etc Many of the inputs for risk estimates are imprecise, e.g. the actual stress or the continuity of a particular structure may not be well known. A sensitivity analysis may be carried out to test the effect of selected uncertainties. This can be easily done using the MS-RAP EVP function. The MS-RAP assumptions related to seismic hazard are listed in the MS-RAP manual or electronic help file and were also discussed in Section They include: Seismicity is due to local failure conditions, not regional mechanisms. There is an adequate seismic record that is representative of the seismic response to mining. The SHS has been developed using seismic data up to Richter magnitude +3. It has not been tested extensively for seismic sources capable of generating events larger that this. Future seismic hazard is related to the mechanisms of past seismic events. Mining influences will be similar in the future compared to the past. Practitioners must assess how closely their mine meets these assumptions, and therefore how strong their assessments and forecasts are. The material collated in the SKB will assist in making this assessment. NOTES: It is important to remember that system sensitivity will affect the accuracy of the RDP maps. If the seismic hazard is underestimated for an area due to the fact that small seismic events are not being recorded, the rockburst damage potential will also be underestimated. Practitioners should be aware that MS-RAP is being continually improved and refined, and can expect changes to the risk estimation procedures from time to time. MS-RAP requires Minodes to be set up for the Mine in order to proceed MS-RAP Hazard and Risk Assessment Procedure Seismic Hazard This is computed as an SHS rating within MS-RAP for each seismic cluster. Seismic hazard is mapped to mine development (Minodes) to produce a seismic hazard map. Variables (which must be documented for any analysis) are: Years of data used for analysis Clustering and grouping It is recommended that the SRMP include an up to date set of seismic hazard maps of the mine, as well as a tabulation of areas identified as having elevated seismic hazard. It is important to remember that seismic hazard ratings are constantly changing as new data is imported into MS- RAP. If the MS-RAP database is kept up to date with good quality seismic data, the seismic hazard map is a near real-time representation. An example of a seismic hazard map is shown in Figure 31. Table 10 shows the colours corresponding to the SHS ratings (maximum expected event size) in MS-RAP. Australian Centre for Geomechanics 77

29 Figure 31 - An example of a seismic hazard map generated in MS-RAP. Table 10 - The Seismic Hazard Scale (SHS) used in MS-RAP, as well as the corresponding maximum expected seismic event magnitude. SHS Rating Qualitative description Approximate Magnitude of Largest Expected Event < -2 Nil M R Max < SHS < -1 Very Low -2 M R Max < -1-1 SHS < 0 Low -1 M R Max < 0 0 SHS < 0.5 Low to Moderate 0 M R Max < SHS < 1 Moderate 0.5 M R Max < 1 1 SHS < 1.5 Moderate to High 1 M R Max < SHS < 2 High 1.5 M R Max < 2 2 SHS < 2.5 High to Very High 2 M R Max < SHS < 3 Very High 2.5 M R Max < 3 3 SHS < 3.5 Very High to Extreme 3 M R Max < SHS < 4 Extreme 3.5 M R Max < 4 Excavation Vulnerability Potential (EVP) Rockburst damage is known to be highly variable. For any given seismic event at a given distance from an excavation, there can be considerable variation in the amount of rockburst damage done. Excavation vulnerability potential (EVP) describes a particular excavation s proneness to rockburst damage. That is, given a seismic event occurs near an excavation, what is the likelihood of rockburst damage occurring and how severe is that damage likely to be? EVP was developed using an extensive database of rockburst case histories, consisting of over 250 individual instances of rockburst damage from Australian and Canadian mines. EVP is described briefly here, although a more thorough explanation is provided in Heal et al. (2006). The tools for building an EVP model for a mine have been included in MS-RAP. Australian Centre for Geomechanics 78

30 EVP can be used to empirically quantify the effect of local site conditions on rockburst damage. It is a simple index, calculated for any excavation in a mine using four parameters: o o o o Stress conditions (E1), Ground support system capacity (E2), Excavation span (E3), and Influence of geological structure (E4). The empirical EVP index has two components: a Damage Initiation Factor and a Depth of Failure Factor, defined as: Damage Initiation Factor = E1 / E2 Depth of Failure Factor = E3 / E4 The Damage Initiation factor accounts for the local site parameters conducive to the generation of dynamic rock mass failure. Increasing static stress conditions contribute to rock mass failure. Ground support increases the dynamic strength of a rock mass, reducing the likelihood and depth of rock mass failure. The depth of failure factor accounts for the local site conditions that will enhance the depth of a dynamically induced rock mass failure. The influence of geological structure factor accounts for favourable and unfavourable rock mass conditions which may hinder or enhance dynamically driven rock mass failure. The Excavation Vulnerability Potential (EVP) index is defined as: EVP = (Damage Initiation Factor) x (Depth of Failure Factor) = (E1/E2) x (E3/E4) A description of each of the four EVP factors should be provided in the SRMP. These descriptions should include how each of the factors was obtained and where to find the information required to update them. E1 Stress Conditions For the Excavation Vulnerability Potential index the local static stress conditions are quantified as the percentage of static loading to the intact strength of the rock, or: Stress Conditions σ 1T (E1) = 100 UCS where: σ 1T = the total maximum principal stress (pre-mining plus mining induced, MPa) at the location of interest (not necessarily the seismic event location), and UCS = the intact unconfined compressive strength (MPa) of the rock at the location of interest. Total maximum principal stress is best evaluated using numerical modelling, considering the effect of nearby stopes and large excavations. It is an estimate of the total maximum principal stress due to nearby stope geometry, rather than a precise calculation of stress at a particular point of a drift wall or back. In most cases there is no need to model the influence of drift scale excavations. Figure 32 contains a few examples of total maximum principal stress estimates. Australian Centre for Geomechanics 79

31 Figure 32 - Modelled total maximum principal stress using Map3D. In the geometry shown, the total maximum principal stress for estimating E1 would be 55 MPa at Point A, 60 MPa at Point B, and 30 MPa at Point C (Heal et al. 2006). The recommended procedure for obtaining σ 1T values using Map3D is: Prepare a Map3D numerical model of the Mine with the required mining steps. Export Minodes from MS-RAP into MAP3D using the Export for Map3D button at the base of the Minodes panel. If the Map3D model has drives included as voids, you will need to either add or subtract 5m from the Minode elevation to ensure the point does not plot inside void in Map3D. This can be done manually (e.g. in Excel) or by using the options in MS-RAP Preferences. The format of the exported Minodes will be a.txt file. Use the Visualisation option in Map3D to plot the Minodes in the Map3D model. Ensure that the locations are correct (e.g. Northing and Easting are defined in the same order for the Minodes and the model) and that the Minodes are not plotting inside void. Use Map3D s MSCALC tool to obtain values of σ 1T at each of the Minode locations. Map3D will write the σ 1T values for each Minode to the.txt file. You only need to set MSCALC to export S1, the other principal stress magnitudes are not required for the EVP calculation. In MS-RAP, under Preferences, import the stress values using the Import Map3D MSCALC File tool. Plot the Minodes as sigma1 to ensure the import work correctly. You can step through the mining steps in the Map3D model by using the sigma1 Step option in the Minodes panel in MS-RAP. For UCS, either manually apply UCS values using in a 2D mine plan view and the Edit tab in the Minodes panel. Alternatively, if the mine has a UCS block model, this can be imported using the Import UCS.csv folder option in MS-RAP preferences. E2 Ground Support System Capacity The type, length and characteristics of surface support and rock reinforcement (which combined are referred to as the ground support system ) have a large effect on the level of damage that may be done by a significant dynamic load. Kaiser et al. (1996) evaluate the energy absorption of different ground support systems (in terms of kj/m 2 ). This evaluation is used as the basis of the EVP Ground Support Factor, E2 (Table 11). These numbers are in basic agreement with results from dynamic testing (Stacey, 2004) and in-situ testing using simulated rockbursts (Heal & Potvin, 2007). Australian Centre for Geomechanics 80

32 Table 11 - Evaluating the Ground Support Factor (E2) based on the energy absorption of the ground support system (Heal et al. 2006). E2: Ground Support Classification Surface Support Reinforcement E2 Rating Low None Spot Bolting (spacing > 1.5m) 2 Moderate Mesh or Pattern Bolting Fibrecrete (spacing 1-1.5m) 5 Pattern Bolting with a second Mesh or Extra bolting pass of Pattern Bolting 8 Fibrecrete (overall spacing < 1m) High static strength Very high dynamic capacity Mesh or Fibrecrete Dynamic Surface Support Pattern Bolting and Pattern Cablebolts Pattern Dynamic Support Example Spot bolting with split sets or solid bar bolts, minimal surface support Pattern bolting with split sets or solid bar reinforcement, with mesh or 50 mm fibrecrete Pattern bolting with split sets with mesh or 50 mm fibrecrete. Plus an additional pass of pattern reinforcement, such as solid bar bolts. Pattern bolting with split sets or solid bar reinforcement, with mesh or 50 mm fibrecrete. Plus pattern cablebolting. Pattern bolting with dynamic ground reinforcement such as conebolts, with a dynamic resistant surface support system. Include a tabulation of the standard ground support systems in use at the mine, and their corresponding E2 value based on the broad guidelines shown in Table 11. An example is shown below in Table 12. Table 12 - An example of E2 ratings for a mine s standard ground support types. Standard Locations Brief Description E2 Rating 01 Orebody 1 cross-cuts near shear Split Sets and Mesh + Gewis + Fibrecrete 8 02 Remainder of Orebody 1 cross-cuts Split Sets and Mesh + Gewis + Fibrecrete 8 03 Orebody 1 footwall drives Splits Sets and Mesh + Gewis + Fibrecrete 8 04 Orebody 1 and 2 hangingwall drives Split Sets and Mesh + Fibrecrete 5 05 Truck loading bays, stockpiles, Progress Split Sets and Mesh + Securabolts + Fibrecrete 8 06 Orebody 3 ore drives Split Sets and Mesh + Securabolts + Fibrecrete 8 07 Main Decline Split Sets and Mesh + Securabolts 8 Intersections All intersections As above + Cable bolts 10 Reinforced Arch Rehabilitation As above + heavy gauge mesh reinforced shotcrete arches 25 By ensuring that the E2 values are up to date in MS-RAP, the Minodes tool also provides an electronic record of all installed ground support at the mine. E3 Excavation Span The span of an excavation often has a direct influence on the depth of failure for both dynamic and gravity related rock mass failures. The excavation span factor (E3) is simply the diameter of the largest circle that can be drawn internal to the excavation. The units of measure for the excavation span factor are metres. MS-RAP calculates spans automatically based on survey files. Australian Centre for Geomechanics 81

33 Figure 33 - Span of excavations estimated using the diameter of the largest internal circle (Heal et al. 2006). MS-RAP requires good quality survey files to accurately calculate spans. Where there are breaks or false lines in survey strings, the spans calculated may not be correct. 3D.dxf files or.dtm s with corresponding.str files can be used to generate Minodes and automatically calculate spans. Often these files contained false walls which needed to be manually removed. An example is shown in the following Figures. Figure 34-3D dxf file used for generating Minodes with spans. Note the closed end of the ore drive. Australian Centre for Geomechanics 82

34 Figure 35 - Spans calculated automatically by MS-RAP using the dxf file. Note the ore drive / access intersection has smaller than actual spans shown, due to the false walls present in the dxf file. Figure 36 - Spans calculated automatically by MS-RAP using the edited dxf file. Note the intersection span is now correct. Australian Centre for Geomechanics 83

35 Figure 34 shows a 3D dxf provided for an individual level. Individual layers of the dxf file have closed ends, as shown. When spans are calculated in MS-RAP using dxf files with these closed ends, smaller spans are calculated for intersections (as shown in Figure 35). Figure 36 shows the correct spans for that level, calculated by MS-RAP after the survey files were manually repaired. If future development is added to the Excavation Vulnerability Potential model, ensure that these closed ends are removed. Having accurate spans in is extremely important. If the span is underestimated (as it was in the case in Figure 35), the seismic risk at that point will also be underestimated. E4 Influence of Geological Structure The E4 factor accounts for the presence of specific rock mass features such as faults, lithological contacts, or if the rock mass structure in general promotes rock mass failure. These features may enhance the potential for rock mass failure, or significantly increase the depth of failure. In particular, the presence of seismically active major structures is a major contributor to severe rockburst damage. Three ratings are used for E4, as shown in Table 13. Table 13 - Influence of Geological Structure rating (E4). E Description Seismically Active Major Structure. Major structural features such as faults, shears or discrete contacts intersect the location and act as a potential failure surface promoting rock mass failure. Example: The rock mass fails back or along a major fault, increasing the depth of failure considerably more than would otherwise occur in the rock mass. Unfavourable rock mass / No major structure. The orientation of the rock mass discontinuity fabric may promote or enhance rock mass failure. Generally, this factor is applied when there are local cases in which the rock mass discontinuities promoted falls of ground much larger than would be expected. Example: A heavily jointed, blocky rock mass with kinematically unstable rock mass blocks. The rock mass is prone to deeper than normal gravity driven failure mechanisms. 1.5 Massive rock mass / No major structure. The rock mass is essentially massive or non-persistent rock mass discontinuities may exist, including possible minor blast related fracturing. There are no major structures such as faults or shears, which may promote or enhance rock mass failure. The recommended procedure is: Define wireframes for faults, structures, contacts, dykes, and other features considered to be seismically active. Perform underground checks on the accuracy of the wireframes used to define the E4 rating (ground truth and wireframes are unlikely to fully agree) Introduce the features into MS-RAP via the Surveys function Manually identify all Minodes intersected by or near these features, and assign an appropriate E4 value. The SRMP should include a description of the seismically active major structures present at the mine. At many mines, excavations which intersect seismically active major structures have been known to be the sites of severe rockburst damage. Such structures are often easy to identify due to the pronounced clustering of seismic events upon them. Australian Centre for Geomechanics 84

36 Rockburst Damage Potential (RDP) Whilst the EVP index alone is useful for giving some idea as to the rockburst proneness of an excavation, it does not consider the magnitude of the seismic event causing the damage and indeed whether or not a damaging seismic event is likely to occur at all. Nor does it consider the distance from the event source to the damage location. It shows that given there is a significant dynamic load on an excavation (that is, a rockburst occurs), there is a relation between the EVP index of the excavation and the amount of rockburst damage done (see Heal et al. 2006). To consider the likely magnitude and distance of the seismic event responsible for rockburst damage, the EVP index can be combined with the peak particle velocity (PPV) generated by a seismic event. For assessing rockburst risk, the maximum expected PPV can be used, which is directly obtained from the SHS rating (maximum expected seismic event magnitude) for an excavation. MS-RAP maps maximum expected peak particle velocities onto excavations, the same way it does for seismic hazard. The combination of the two parameters is termed Rockburst Damage Potential (RDP): RDP = EVP x PPV Based on probabilistic analysis of over 250 instances of rockburst damage, Heal et al. (2006) defined RDP values for which certain levels of rockburst damage are expected to occur. The rockburst damage was categorised using Table 14. The data used to build the RDP system are shown in Figure 37, which also shows iso-contours of the probability of serious rockburst damage (R4 or R5). The expected damage levels are shown in Figure 38. Table 14 - The rockburst damage scale used to assess the rockburst data. Rockburst Damage Scale Rock mass Damage Support Damage R1 No damage / minor loose No damage R2 Minor damage, less than 1 tons displaced Support system is loaded, loose in mesh, plates deformed R tons displaced Some broken bolts R tons displaced Major damage to support system R tons displaced Complete failure of support system Australian Centre for Geomechanics 85

37 Figure 37 - The EVP and PPV data for over 250 instances of rockburst damage used to develop the rockburst damage potential system. The data are coloured according to the level of rockburst damage that occurred = Probability of R4 or R5 Damage (Greater than 10t ejected) From Logistic Regression 5.0 EVP.PPV LINES ISO- PROBABILITY LINES R1 4.0 PPV (m/s) R2 R3 R4 ROCKBURST DAMGE SCALE 1.0 R EVP Figure 38 - EVP vs PPV chart showing expected damage R2 R3 R4 R5 EVP.PPV RATING R1 R2 R3 R4 R5 EXPECTED OUTCOME 4.0 PPV (m/s) R EVP Australian Centre for Geomechanics 86

38 MS-RAP assesses the RDP at a Minode by calculating the maximum expected PPV at that location based on the SHS rating of the Minode. MS-RAP produces RDP maps, which are updated in near real-time as new seismic data is imported. An example of an RDP map generated in MS-RAP is shown in Figure 39. Table 15 shows the colours corresponding to the RDP ratings, which represent the expected damage level for the maximum expected seismic event magnitude. Figure 39 - An example of a rockburst damage potential map generated in MS-RAP. Table 15 - Colours used to define rockburst damage potential ratings in MS-RAP. RDP (= EVP x PPV) Rating Colour Expected Damage Due to Maximum Expected Seismic Event Magnitude Unrated Unrated (missing data, e.g. no UCS or sigma1 value) 0 to 25 R1 (No damage / minor loose) 25 to 50 R1 (No damage / minor loose) 50 to 130 R2 (Minor damage, less than 1 tons displaced) 130 to 170 R3 (1-10 tons displaced) 170 to 230 R4 ( tons displaced) > 230 R5 (100+ tons displaced) Exposure Certain types of excavations or tasks represent higher workforce exposure to rockburst risk, based on the time spent in an excavation and the level of worker protection. For example a workshop would have a higher exposure rating than a decline because personnel spend far more time exposed to the hazard (if one exists) in the workshop. Broadly speaking, the following provides a guide to the level of exposure for different excavation types or tasks: Australian Centre for Geomechanics 87

39 o o o o o o o o Restricted access (no entry) Decline or travelway with no active mining Travelway - mining on the level Production mucking area Busy level/travelway drive/access Development mining Production drilling or production charge-up Infrastructure areas / workshops Increasing workforce exposure Exposure is the final component in the seismic risk equation. Detailed studies to quantify personnel exposure have been conducted at some mines (e.g. Owen, 2004). If this data is available it can be combined with rockburst damage potential to produce a risk matrix. Table 16 gives an example. Each task (exposure rating) is shown vertically, while the rockburst damage potential is shown horizontally to give an indication of the relative seismic risk of various tasks for different RDP levels. The exposure ratings are a function of the amount of time spent carrying out the task, the level of workforce protection and the spatial footprint or number of personnel involved in carrying out the task (from Owen, 2004). Generic ratings are shown, although specific ratings could be developed for a particular mine. Table 16 - An example of a seismic risk assessment matrix incorporating quantitative exposure (vertically) with rockburst damage potential (EVP.PPV horizontally) (Heal et al. 2006). The exposure ratings are from Owen (2004). The seismic risk ratings (VL to E) are generic qualitative descriptions. EVP.PPV < > 280 Criteria: P(R4,R5) < > 0.9 Excavation type / activity Exposure rating [17] Restricted access (no entry) 100 VL VL L M M H H Decline 1000 VL L M M H VH VH Travelway - no active mining 1000 VL L M M H VH VH Travelway - mining on the level 2000 VL L M M H VH VH Production mucking area 3000 VL L M M H VH VH Busy level/travelway drive/access 4000 VL L M M H VH VH Development mining 7000 L M M H VH VH E Production drilling M H H H VH E E Production charge-up M H H H VH E E Infrastructure areas / workshops M H VH VH E E E (SEISMIC RISK RATINGS: VL = Very Low, L = Low, M = Moderate, H = High, VH = Very High, E = Extreme) 11.5 Evaluation of Seismic Risk With the risk assessment process finalised, it is lastly necessary to evaluate the computed risk, that is, to respond to, or make decisions about: Which risks are intolerable, that is, of sufficient gravity that they need treatment. Decide treatment priorities. Australian Centre for Geomechanics 88

40 Clearly, this first requires definition of the Mine s tolerable level of seismic risk. For illustration, a rating of M or a product of 300,000 in the risk assessment matrix may be selected as the maximum tolerable risk. The tolerable level of risk needs to be specified by mine management for the Mine, as a target or expectation, in order to measure the effectiveness of the SRMP. While zero risk may be an attractive statement, it is an unrealistic and most likely unachievable target (refer Section to 3.2). Corporate management need to define what level of risk is tolerable. The evaluation should be done by mine personnel on a regular basis, at least monthly, by considering/reviewing the following questions: What is the seismic risk criteria for the Mine i.e. what risk is tolerable? What is the wider context of the risk? For example, assess to what degree higher potential losses may be associated with higher potential gains. If production value, volume requirements and/or environmental considerations are important to the Mine, more extensive or expensive management options could be considered. What localities in the Mine exceed the seismic risk criteria, and by how much? Document the areas of concern. Does the risk level require that the Mine undertake further analysis? Have there been significant changes over time (as mining proceeds, ratings for SHS, EVP and RDP will change). Are there proactive indications that certain areas have become more or less prone to rockburst damage? Instigate remedial action for any locality that exceeds the seismic risk criteria. Is the SRMP effective? (For example, what is the frequency of intolerable seismic risk?) This process may result in identification of risks that require notification ( notification is preferred terminology over warning ) to mine management. Seismic risk assessment is not usually sufficiently accurate to justify issuing of warnings. Issuing of warnings can be problematic. If this is to be done, mine management require a policy directing their response to a warning such a policy may have elements in common with the tiered response described in Chapter Quantifying Changes to Seismic Risk Revisiting the seismic risk equation shown in Section 11.3, we can use the tools to assess seismic hazard and risk to quantify the effect of risk mitigation strategies: Limit wide spans in new development Where where are the possible events? Ensure ground support suitable for How strong big? dynamic loading is (Magnitude) installed in areas of elevated seismic hazard What frequency of occurrence? Seismic Risk = Seismic Hazard x Excavation Vulnerability x Exposure Avoid for new development where possible A function of: Span Ground support Ratio of Stress / Rock Strength at the location being assessed Presence of seismically active major structures Reduce exposure (eg re-entry protocols, access restrictions, improved protection) where / when necessary What area or task? Production drilling Mucking Charge-up Workshop Decline etc Australian Centre for Geomechanics 89

41 Ground support is the most obvious change that can be made to reduce rockburst damage potential. It is also often the most effective. Simple arithmetic dictates that upgrading ground support from a system having an E2 rating of 2 to one with an E2 rating of 10, reduces the rockburst damage potential by a factor of 5. Likewise, in an intersection with an E2 rating of 10, the rockburst damage potential can be reduced by a factor of 2.5 with upgraded ground support. An ideal integrated ground support system for rockbursting conditions has surface support which is strong enough to transfer load to individual reinforcing elements and engage the full capacity of yielding rockbolts or cables. Experience has found that a ground support system that does not have well-matched surface support and rock reinforcement may have considerably reduced overall dynamic characteristics. An example would be using a dynamic rock reinforcement system such as dynamic cable bolts or cone bolts, while having a weak surface support system, such as un-reinforced shotcrete, or a relatively weak mesh. Upgrading existing support with dynamic or yielding cables linked by heavy gauge mesh or mesh straps could reduce seismic risk in many areas which have high RDP ratings. Some MSRRM sponsor sites are investigating the use of their rockburst risk model as a rationale for ground support selection in rockburst prone areas. As an example, the rockburst damage potential map (EVP.PPV) for a particular level is shown in Figure 40. The section of the ore drive shown is currently supported by a pattern of Posimix bolts and Split Sets with mesh (E2 = 8). Figure 41 and Figure 42 demonstrate the effect of upgrading ground support in the drive by adding cablebolts or by upgrading to a heavy duty dynamic support. The SHS rating of the drive is 2.0, that is, the scaled largest expected event magnitude for the drive is around M R = Figure 40 - Actual RDP map for the level. The section of the ore drive of interest has ground support with an E2 rating of 8. Expected damage for the largest expected seismic event here ranges from R3 to R5. RDP Values: (Expected Damage Outcome): 208 (R4) 176 (R4) 157 (R3) RDP Values: (Expected Damage Outcome): 228 (R4) 221 (R4) 277 (R5) Australian Centre for Geomechanics 90

42 Figure 41 - The effect of adding cablebolts in the ore drive. RDP values are reduced and the expected damage for the largest expected seismic event is now R2 to R4. RDP Values: (Expected Damage Outcome): 166 (R3) 140 (R3) 126 (R2) RDP Values: (Expected Damage Outcome): 183 (R4) 177 (R4) 221 (R4) Figure 42 - The effect of upgrading ground support in the ore drive to heavy duty dynamic support, such as yielding cables with heavy duty mesh straps. The RDP values are reduced even further and the expected damage from the largest expected seismic event is now R2 (damage contained by support). RDP Values: (Expected Damage Outcome): 67 (R2) 56 (R2) 50 (R2) RDP Values: (Expected Damage Outcome): 73 (R2) 71 (R2) 89 (R2) Australian Centre for Geomechanics 91

43 The reduction in seismic risk can also be represented as a percentage using the iso-probability contours in Figure 37. The point furthest to the right in the previous three Figures is used as an example. The change in ground support results in a reduction in the RDP value from 277 (an expected outcome of R5 damage for the maximum expected seismic event magnitude) to 221 (an expected outcome of R4) to 89 (an expected outcome of R2). This equates to a reduction in the probability of serious rockburst damage (greater than 10t ejected) from 90% to 73% to 16% occurring for the maximum expected seismic event magnitude (M R ~ +2.0 in this case). This is shown graphically in Figure 43 and Figure 44. Figure 43 - The effect of upgrading ground support on RDP ratings for the furthest right point, as shown in the previous three Figures. 6.0 R3 R4 R5 5.0 R2 4.0 PPV (m/s) R1 Add cables Existing Ground Support 1.0 Upgrade to heavy duty dynamic support EVP Australian Centre for Geomechanics 92

44 Figure 44 - The effect of upgrading ground support on the probability of serious rockburst damage for the furthest right point = Probability of R4 or R5 Damage (Greater than 10t ejected) PPV (m/s) Add cables 1.0 Upgrade to heavy duty dynamic support Existing Ground Support EVP MS-RAP also allows to the user to manually change the SHS rating (the maximum expected seismic event size) for an area to determine what the future rockburst damage potential may be. This is done using the Override SHS feature in the Edit tab of the Minodes panel. In the example shown in Figure 45, a rockburst damage potential forecast is made for an ore drive where mining has not yet occurred. Let s say that at this particular mine, seismic events in the vicinity of M R ~ +2.0 are known to occur in the ore drives once mining commences. Once a User SHS is applied the effect of upgrading ground support for certain areas can be quantified, as was demonstrated in the previous few Figures. A future area of mine seismicity research is attempting to forecast seismic hazard using numerical modelling. These forecast seismic hazard maps could then be used to forecast seismic risk. Australian Centre for Geomechanics 93

45 Figure 45 - Example showing the procedure of manually changing the SHS rating of an area to determine what future rockburst damage potential may be. Current Seismic Hazard Map SHS rating for ore drive changed to +2.0 Current Rockburst Damage Potential Map Rockburst Damage Potential Map with changed SHS rating More information on seismic risk mitigation is presented in the next few Chapters. Australian Centre for Geomechanics 94

46 PART V CONTROL OF SEISMIC RISK (Chapters 12-17) Australian Centre for Geomechanics 95

47 12. RISK CONTROL MEASURES GENERAL COMMENTS The risk needs to be considered together with costs and mine-specific constraints to identify appropriate risk management approaches. This becomes the basis for action to adopt measures to reduce the seismic risk to a tolerable level. Decisions on risk treatment need to be based on relevant criteria. Criteria can depend on: Regulatory requirements Stakeholder perceptions (e.g. surface vibration limits) Operational limitations the preference to keep production going, versus the perceived level of seismic risk Technical capabilities the confidence level in the analyses AS/NZS 4360:2004 Risk Management Framework 3.6 Treat risks. Selecting the most appropriate options involves balancing the costs of implementing each option against the benefits derived from it Preparing and implementing treatment plans General notes Six classes of risk control measures may be defined according to the conventional hierarchy of controls. However for seismic risk, a division into tactical and strategic controls has been found to be appropriate. No control method should be used in isolation. Seismicity is complex and no single control can do everything. Also not much can be expected from one control step if other steps are inadequate, or if the directives are not carried out. Seismicity represents a hazard not just to personnel, but to production and machinery. All need to be properly considered. The selected treatment/s need to be formulated and resourced in accordance with the Mine s methods for such works. The Mine must have a robust system for ensuring that all action steps are undertaken adequately and the results are monitored and verified. Refer to Chapter 15 for assistance in making this process run efficiently and effectively on Mine sites. Durrheim et al. (2007) present a survey of South African seismic risk management practices. Australian Centre for Geomechanics 96

48 13. TACTICAL RISK CONTROL MEASURES Select the tactical measures to be adopted by the Mine to reduce the seismic risk to a tolerable level. Tactical measures are variously called low level, reactive, immediate, or operational. They are generally physical measures adopted to address the situation. The process requires that the effect of the tactical action be continually monitored (WMC Resources Ltd, 1997). AS/NZS 4360:2004 Risk Management Framework 3.6 Treat risks 13.1 Upgrade Ground Support and Reinforcement The successful design, installation and quality control of rock support and reinforcement systems that are appropriate for seismically active ground conditions is a very important issue facing the WA mining industry (Department of Industry and Resources, 1997). Note that ground support can also be a strategic measure. It is emphasised that dynamic support does not replace static support design as per the Mine s GCMP. Installing dynamic-capable ground support, as factor E2 in the EVP computation, is often the best available option to reduce seismic risk. Evaluate the effect or sensitivity of modifying ground support by trialling different E2 values within the MS-RAP risk assessment procedure (Section 11.2). The following are presented as approaches that have been applied in various situations, and could be considered by the Mine: Select dynamic-capable ground support adequate to provide safe working conditions. Select support to limit or confine the damage that occurs, not prevent it. As well as dynamic considerations, ensure adequate static support of dead weight of worstcase wedges that might develop following seismic activity. Control the support pressure requirement by arching development drive backs, and bolting walls to control span. Expect ground support designs to feature mixtures of bolts rather than just one bolt type. A mixed pattern has advantages in seismic ground. Ensure that surface support is able to transfer the dynamic load to reinforcement units. Use shotcrete to cement in place a zone of blast-affected damaged rock around the excavation. Do not first scale back to competent rock. This reduces exposure to strainbursts, attenuates the impact of seismic surface waves, and keeps the induced stress zone further from the excavation. Study design plans to identify key locations requiring extra support / dynamic support. Consider the concept of a Rockburst Support Multiplier when designing support for personnel access openings in high seismic hazard areas. That is, back-analyse situations at the mine where the installed support has not been "adequate" for seismic situations and estimate what the shortfall was. Express the shortfall as a multiplier on what computations suggest. The decisions regarding suitable dynamic-capable support need to consider the following practical questions: To what distance from the face should dynamic-capable support be installed (e.g. within 20m) Is yielding support required What fracture thickness is allowed / assumed / known Are there any different requirements for backs, shoulders, walls What support element length is needed to contain potential wedges Is a one-pass system required or not Australian Centre for Geomechanics 97

49 Can dynamic-capable support be retro-fitted (i.e. risk arises later, not now) What is the lifespan of the drive What is the importance of the drive Does mesh need to be the final layer Is fibrecrete required right up to the face Is fibrecrete required on the face itself Are there any stope-specific areas where inspection is required to finalise exact support Refer Appendix E for more information on dynamic ground support selection Reduce Personnel Exposure Exposure is a key factor in the Seismic Risk computation. Reduction of exposure is often a practical short-term risk control measure. It attempts to avoid places and times when the seismic hazard is highest. The following approaches are presented for consideration: Schedule high exposure activities to occur while the Rockburst Damage Potential is lower. o For example, development mining is a high exposure activity, so it may be preferable to develop accesses early, before the advancing front of seismicity arrives at the level. o Balance this option against the necessity for rehabilitation due to deterioration over time in the access drives nearer to the date of mining on the level. Use exclusion zones and periods (which may vary for different drives and stopes) as standard requirements after blasts. Mercier-Langevin & Hudyma (2007) describe the specification of exclusion procedures in a Canadian mine. It may be appropriate to assess seismicity diedown at the end of a nominated exclusion period, and increment the period if activity is insufficiently reduced. Use blanket entry restrictions. Use revised work procedures. If specific situations (such as open stope brows) are known to be seismic-prone, refine work practices (such as drilling) to minimise exposure at these locations. Consider the possible effect of every stope blast on the stability of all nearby mapped structures. Use modern blasting practices (electronic detonators). Fire critical blasts from surface. Use remote controlled or automated equipment Make Ad-hoc Refinements to Mine Designs The following practices are documented by Singh et al. (2002): Locally vary development location, development size and pillar size. This is one reason why development should always be extended past a proposed turnout, so that a geological and geotechnical assessment can be made to check for wedges and large structures and the turnout relocated if adverse conditions are encountered. Vary excavation profile. Where high levels of strainbursting are encountered, it may be necessary to arch the face into a concave dish shape, to reduce strain bursting at the face. In cases where new development is designed to intersect an existing excavation, the last cut or two may act as a thin pillar and can be seismic-prone. If so, implement a procedure to control this situation, whereby personnel are aware of the situation, and the last cut is a fulllength cut, not a short cut. Similarly, avoid leaving a thin wedge of rock between a structure and an oncoming stope brow. The wedge may be ejected into the drive. Australian Centre for Geomechanics 98

50 13.4 Destressing Destress blasting techniques may be viewed as tactical or strategic. Tactical destressing is viewed as that done on a cut-by-cut basis as development is advanced. Recently, Tarasov & Randolph (2007) and Tarasov & Ortlepp (2007) have identified a mechanism whereby an extending fracture can become briefly frictionless even under considerable confinement (high normal stress), then be severely shock-loaded when friction is restored. High confinement is a critical requirement for this process to occur. This suggests that high confinement close to excavations should be avoided. Using destressing to create a zone of fractured rock surrounding an opening is therefore helpful as it reduces the ability of the rockmass to provide high confinements. Itasca Consulting Group Inc and Richard Brummer Associates (1998) undertook a major review of destress blasting practices, and wrote that there can be little doubt that it is possible to control violent rock behaviour by means of destressing under appropriate circumstances. The report also commented that: Destressing is an area that is practiced routinely, but is not well understood, and in which experience rather than engineering tends to guide practice. The main goals of destressing are (a) to promote fracturing, and/or (b) to promote shear on existing fractures. the rock volume needs to have some freedom of movement to generate shear on pre-existing fractures implying that destressing may not be effective if done away from immediate excavations with widely-spaced blastholes. Create a crush zone around the opening, in order to attenuate the seismic energy before it reaches the opening wall. Caution drilling of destress holes can itself trigger seismicity. Use a Creighton-style destressing strategy to push seismicity away from development faces. The German coal industry has largely solved their rockburst problem by means of the [destressing] techniques outlined. See also O Donnell (2001) and Andrieux et al. (2006) for important aspects of destressing experience Groundwater If structures in excavations are wet, wait while drainage proceeds, or install drainholes to intersect and drain the water sources. Australian Centre for Geomechanics 99

51 14. STRATEGIC RISK CONTROL MEASURES Describe the strategic measures adopted by the Mine to reduce the seismicity hazard to a tolerable level. Strategic measures are variously called high level or proactive. They generally relate to mine design and planning activities and are incorporated at that stage. Strategic methods require a degree of investigation or research to decide on the most appropriate physical change to implement and are normally considered at the mine planning stage (WMC Resources Ltd, 1997). Strategic methods alone cannot completely eliminate the seismic risk. Many strategic methods involve some form of stress management. Some of the strategic approaches may be perceived as less economical or productive. However the opposite is usually true, because these strategies allow mining to proceed with far less problems and interruptions, less risk of losing ore, and less exposure to seismic risk. AS/NZS 4360:2004 Risk Management Framework 3.6 Treat risks 14.1 Sequence Adopt a sequence that will favourably manage the way the seismic energy in the mine will be released (Morrison, 1995). Examples of favourable sequences include: Retreat from the centre of the mining block towards solid unmined abutments. Retreat away from potentially unstable geologic structures or rock types (e.g. Active porphyries). If approaching a potentially unstable structure, approach perpendicular to the structure, or as near to that as practicable certainly at no less than 30 degrees to the structure. Avoid leaving pillars, which interrupt the progressive convergence process and increase the seismic risk later on. Utilise stress shadows where feasible. Leave a generous spacing between blocks that are to be extracted simultaneously. One mine requires simultaneous crown pillar extractions to be at least 150 m apart and not advancing towards each other. Other principles include: The preferred sequence is top down centre out panels. The principle is to achieve the most uniform convergence of the surrounding rock as the ore is extracted. Use numerical modelling to study sequence options to assist in selection. In some situations (such as remnant mining) there is little flexibility available in sequencing and a compromise is needed. The direction of mining may affect which side of a stope is likely to be damaged. If this is observed, put access and infrastructure on the other side. See also Potvin & Nedin (2003) for a summary of pillarless mining advantages and issues. Australian Centre for Geomechanics 100

52 14.2 Control Span of Development and Stope Accesses Adopt a reduced excavation size. The single most important thing at depth is to go smaller (Morrison, 1995). Span is a direct driver of rockburst hazard. In the MS-RAP formulation database of over 250 rockburst damage instances, the vast majority of very serious rockbursts (R5 damage, with greater than 100 tonnes ejected) occurred in wide span intersections (Heal et al. 2006). Therefore: Choose an excavation shape (profile) that is not aggressive in the stress field. Choose mining methods that do not require wide spans. Control spans by controlling widths of drives (design, blasting, wall bolting). Limit the formation of large spans at intersections. Eliminate 4-way intersections totally. Avoid tight angles (thin pillar noses) at 3-way intersections. Any large spans will need specific management. A 4x4 m drive has only 56% the profile area of a 5x5 m drive. Canadian mines function well with smaller size development and low profile machinery. Coal mines use continuous haulage by conveyor, eliminating both the trucks and the need for large profile drives. Development size is driven mainly by the choice of loading and trucking for ore handling (Morrison, 1995). Reducing stope sizes works because it reduces the rate of change it controls the size of each incremental mining step, and thus of the energy that can be released (Morrison, 1995) Smaller versus Larger Blasts Consider the tradeoffs between smaller and larger blasts (including whether the mine viability depends on a certain blast size). This is an area of debate (see below), and the Mine needs to obtain site data to be able to define the appropriate strategy. This is done by charting blast size (tonnes broken) against the magnitude of following events. The arguments for smaller blasts are: As stresses increase with depth, more energy is stored in the rockmass and released with each increment of mining. Mining in smaller steps is necessary to maintain smaller (or at least the same) amounts of energy release (Morrison, 1995; Beck, 2003). Smaller blasts make smaller changes to the system, or more gradual stress changes, so that the response is less dynamic or less seismic (Morrison, 1995). Several smaller seismic events are preferable over one very large event. The arguments for larger blasts are: Large blasts trigger seismic activity at a time and place of our choosing. The time of energy release after blasting is critical, so a mass blast is used to trigger seismicity and get rid of the energy Small blasts can also trigger significant activity. As they nibble away, small blasts may not release energy that is locked in due to asperities or offsets, until a critical point is reached and a significant event occurs anyway (Morrison, 1995) Bulk non-entry mines can afford large blasts because personnel exposure can be well managed Pillars The ideal situation is to completely avoid pillars (crown or rib) in seismic ground select continuous face advance schedules rather than pillars. Creating pillars in bursting ground is often a problem. Traditional cut and fill mining with a diminishing (progressively reducing) pillar as the next level is approached is the classic example (Beck, 2002). Australian Centre for Geomechanics 101

53 A pillarless sequence relies on (a) the rockmass of the next stope becoming fractured and destressed by the mining of the current stope, and (b) achieving a more uniform rate of convergence of the host rocks as mining proceeds. This may be sufficient to reduce the magnitude of local seismic events. If pillars cannot be avoided, due to scheduling and production problems, then consider the following: Pillar Geometry Design pillars with W:H dimensions that are unlikely to fail violently. Pillars should be either of substantial size to be permanently stable, or small enough to yield and fail immediately they are formed and stressed, or preconditioned (see below) to create correctly oriented failure planes through the pillar. In hard rock, the critical W:H ratio may range from 0.5:1 to 1.5:1. Avoid substantial stiff pillars of this geometry range. Pillar Structures Avoid highly stressed meta-stable pillars with the chance of failure by shearing on structures intersecting the pillar. Pillar Stress Reducing high stress in the pillar by destressing can be considered, but carries the risk of initiating seismicity. Pillars are isolated volumes of ground, so their stiffness (their geometry) controls their behaviour (Itasca Consulting Group Inc and Richard Brummer Associates, 1998). Destressing could be used to alter the stiffness of the pillar. Reduce the ability of the pillar to carry high stress, by preconditioning (see below) Reduce maximum stress levels in pillars by any practicable means, e.g. wider pillars, tandem pillars. If a small pillar is deemed essential, consider constructing a replacement pillar using CRF. For pillar crush events, consider the maximum stress in the pillar in relation to rock mass strength. Pillar Mining Sequence Analyse the stresses associated with different primary-secondary-tertiary stope and pillar sequences, to find a preferred sequence. Include using numerical modelling, and include the worst-case scenario of excavating secondaries before backfill is emplaced in primaries. Try to avoid diminishing pillars (shrinking back onto a sill or to a central access, another void, or a fault), including the undercutting of a pillar. Rather, form a pillar at its final size from the start including installation of undercuts or slot rises so that the new stope starts at the pillar and moves away from it. Diminishing Pillars If no way is found in which to avoid a diminishing pillar situation, then a difficult situation arises. The following options have been used in this situation, but none are guaranteed to avoid seismicity: Mass blast the pillar while it is still large, and accept the resulting seismicity. In essence this strategy moves directly from the fully stable to the fully failed condition. Recover the full remaining pillar thickness, while it is still thick, using non-entry methods. One mine ceases flatback cut-and-fill and uses longhole stoping when the crown reaches 20m thick. An option could be to mine crown pillars from the top down essentially using underhand cut and fill, and with the stope below the crown backfilled right up to the crown. While productivity may be low, the seismic risk is reduced because (a) a layer of broken rubble confines the working floor; (b) seismic failure of the base of the crown is contained by the fill; (c) the final lift down is more likely to be fractured rock unable to sustain seismicity. Bracket Pillars A special function of pillars in some mines is as bracket pillars, i.e. pillars located adjacent to or enveloping structures which could slip or burst, such as faults or porphyry dykes. If a bracket pillar is proposed, a numerical stress modelling study would be required to size and position the pillar. Australian Centre for Geomechanics 102

54 Remnant Pillars In a mine that is recovering remnant pillars, it is likely that locally higher stress conditions will be experienced. This will require special support, rules and sequencing Preconditioning Preconditioning may be defined as the intentional weakening of a volume of ground that is currently under confinement, but is destined to become a pillar or abutment later in the mining sequence. The objective is to reduce the ability of the pillar to carry high stress. The following points are relevant: There is a suggestion (Itasca Consulting Group Inc and Richard Brummer Associates, 1998) that preconditioning is not effective if no shearing is possible i.e. if the preconditioning blastholes are widely spaced and there is no freedom for movement. Therefore (in the author s personal opinion) preconditioning must be designed to suit the future orientation of stresses when the volume of rock becomes a pillar. Run a numerical model to estimate the loci of failure, assuming the pillar or abutment was not to be preconditioned, and then precondition those loci so that the pillar movement / failure occurs as soon as the pillar is formed. Preconditioned pillars need not be rectangular. Again in my opinion, design skew-shaped pillars that will (a) encourage shearing failure as soon as the pillar is formed, and (b) at the same time, provide a limited freedom of movement. Extend preconditioning beyond the planned pillar dimensions, so that it is effective even should a key blast unexpectedly pull up short. Design the preconditioning blast pattern to either (a) greatly fracture the rock, or (b) create presplit fractures that are parallel to what will be the σ 1 stress direction once the pillar is formed, or (c) both. Preconditioning is still not well understood, so practitioners contemplating this practice for the first time are urged to seek advice. See also Mikula et al. (1995) for an example of a preconditioning attempt at Mt Charlotte mine Control of Faults and Structures Distinguish pre-existing faults that slip, from fault-slip extension of existing fractures. This strategy is about the former only. Control of faults is still probably the most difficult and least well understood strategic option. The observations offered below are highly tentative. Any practitioner contemplating fault destressing is urged to consult the literature and seek advice. The first step is to identify all critical structures. The second step is to locate discretionary infrastructure away from known structures. The third step is to look for ways in which a highly loaded structure might become suddenly or unexpectedly released, e.g. via a stope blast, failure of a junction with another structure, failure of some remnant pillar, stope overbreak. These structures may need control. Fourthly, undertake a numerical modelling study of the situation and the proposed control, to investigate: Are high shear stresses generated on any structures? How does the maximum shear stress on the fault compare to shear strength. Are some structures highly shear loaded, but confined, with nowhere to go. These are likely to generate a large event if they are loaded to the point of eventual failure. Determine whether fault-slip seismicity could be driven by increasing shear stress on a fault surface, or decreasing confining stress, or both. Australian Centre for Geomechanics 103

55 The conventional thinking is that slip can result from combinations of increase in shear stress, decrease in normal stress, loss of cohesion, and failure of asperities: The result may or may not be dynamic, depending on the fault properties and the stiffness of the rock mass environment. If the mining process causes increase in shear stress on the fault, events potentially can be very aggressive. Control methods that have been attempted appear to be of two types: Encourage early movement on the fault: Excavation of a void intersecting the fault plane. In practical terms, this could mean commencing a stope at the fault (or at the junction of key structures) and mining away from it. The fault is then able to move at will, releasing energy often but in smaller amounts. However in some multi-structure scenarios, the process of mining away from one structure will mine towards another. In this case, a pro-active destressing procedure could assist. Use a stress shadow to reduce clamping stress components on certain structures, to attempt to make frequent but lower-intensity fault slip more likely. If mining is to be done under cover of a stress shadow, avoid encroaching on the edge of the stress shadow. Be aware that stress-shadowed ground can become active as it relaxes. Also, while seismic release on a structure may be more likely if a longer length of the structure is daylighted or unconfined, the event magnitude may also increase. An uncertainty with these approaches is that there is no guarantee of restrained rock movement if a fault-slip event is initiated. Once mobilised, the fault-slip momentum may continue unabated. Reduce the intensity of any movement on the fault: If a fault or structure could be compromised, e.g. become stressed in a location close to development areas, use destress blasting to tag to the structure. The idea is to reduce stress concentrations and confinement between the drive and the structure, and to push shear stresses further into the rock where the structure is less free to move. Use a destressing procedure on the fault to weaken it to reduce its capacity to carry shear stress. Blasting and fluid injection destressing have been attempted the latter may suffer from fluid leakage from the fault. A tight slot blast destressing strategy was trialled recently (Kempin et al. 2007) to manage stope-scale fault-slip issues. The premise of the strategy was to manage the fault-slip movement by ensuring critical voids were backfilled with tightly-packed broken rock, rather than being left empty. This strategy was used to advance stopes across critical structures Mining Method Adopt mining methods and targets that do not aggravate or suffer from seismic response. For example: Non-entry mining methods that reduce or eliminate personnel exposure. Entry cut-and-fill for example is more difficult to manage, as personnel exposure is greater. Selection of production rates that are optimum for the ground conditions. Many mines are pressured to raise production rates beyond optimum, and experience adverse seismicity among other consequences. Use methods such as longhole retreat mining over flatback cut-and-fill reduce the number of accesses, and avoid high stresses affecting the backs of every lift. Selection of smaller equipment. The common utilisation of large mobile equipment requires large drives, which bring increased rockfall risk, increased rockburst risk, and inability to perform effective scaling from the floor. Select mining methods that avoid having personnel working beneath stressed brows. Australian Centre for Geomechanics 104

56 14.8 Mine Planning Adopt mine plan strategies that do not aggravate or suffer from seismic response. Use numerical modelling to study stress concentrations and stress changes per mining step, and forecast localised high stress areas. Be aware that both σ 1 and σ 2 can concentrate sufficiently to cause damage do not just consider σ 1 alone. Define the preferred excavation orientation, which should be parallel to the direction of the locally applicable major principal stress. Use automation and remote technologies. Locate stope access development in waste, to allow sufficient flexibility with extraction sequencing. Minimise development in critical rock types. Limit development through areas known to be seismically active Maintain access drive stand-off distance from stopes and development; e.g. Junction mine kept excavations at least three diameters apart (Singh et al. 2002). Obey any preferred excavation orientation, such as not parallel to major geological structures, or perpendicular to intrusives / dykes / faults, or parallel to major stress. Run ore passes full, not empty. Ensure two accesses are available to any location with elevated seismic hazard, as insurance in case of seismic damage to one access. Avoid surprises, such as the orebody becoming bigger. Exploration drilling must be adequate, and include sterilisation proof around the defined ore reserves. In seismic territory surprises are usually bad news Backfill Backfilling may dampen seismicity and there is some suggestion that fill may slightly reduce fault-slip event magnitudes. Paste fill acts as a damper on stope boundaries such that surface waves generated on the free surfaces are attenuated. It also exhibits energy absorption characteristics that reduce transmissions of seismic shockwaves, particularly high frequency components (Singh et al. 2002). Adopt stringent backfilling practices. Do not fire stope blasts until the required backfilling is emplaced (Barrick Gold Corporation, 2005). Mines have paid dearly in seismicity for the decision to blast secondary stopes too soon, due to delays in fill placement in primaries. Fill is a good example of a geotechnical stop point (refer Section 15.6), whereby mining must be suspended till the fill is completed Groundwater If groundwater is a major issue, design adequate pre-drainage to dewater stopes before any excavation. Australian Centre for Geomechanics 105

57 15. OPERATIONAL SYSTEMS AND COMMUNICATIONS Establish a robust system to embed the SRMP requirements within the Mine culture. This chapter includes a description of the technical and operations planning and communications processes in place at the Mine related specifically to seismicity. AS/NZS 4360:2004 Risk Management Framework 3.1 Communicate and consult. Effective communication is important to ensure that those responsible for implementing risk management, and those with a vested interest, understand the basis on which decisions are made and why particular actions are required. 4. Establish effective risk management Develop risk management plans. Risk management is to be embedded in all the Mine s important practices so that it is relevant, effective, efficient and sustained Data and Information Sources This section describes the data sources that must be accessible to relevant mine personnel, in support of the broad range of skills required by on-site geomechanics specialists. These data sources must survive for the lifetime of the mine. Information and knowledge can easily be lost due to many factors turnover of personnel, quality of supervision, level of training, inexperience of mine management, disregard of importance and role of geotechnical advice, attrition rate at all levels of the workforce, and last but not least, changed ownership of the mine (MOSHAB 3, 1997). AS/NZS 4360:2004 Risk Management Framework Tools and techniques Technical guideline Data relevant to seismicity in the Mine is available from sources and locations such as those listed in Table 17. Include a statement describing if and how any datasets have been verified. For example, major geological structures can greatly impact the mining solutions; therefore it is necessary to verify wireframed structures against ground truth, and correct any discrepancies found. 3 Mines Occupational Safety and Health Advisory Board Australian Centre for Geomechanics 106

58 Table 17 - Example record of data sources relevant to seismicity. Source Physical Location examples Electronic Location Names and Positions of relevant accountable and responsible personnel Seismic monitoring system Seismic office / ESG / ISSI IP address Geotechnical engineer SMS manufacturer, service, Seismic office / ESG / ISSI Supplier personnel maintenance, IT support data SMS operation manuals, Seismic office / ESG / ISSI Geotechnical engineer software, instruction manuals SMS wiring diagrams, Seismic office / ESG / ISSI Electricians drawings, underground plans and layouts Geological and Geology office / 3D models Geologist geotechnical core logging Underground visual Geotechnical office / stope Geologist observations and mapping files Structural mapping Geotechnical office / stope Geotechnical engineer files Historical information Geotechnical office / stope Geotechnical engineer files Stress measurement and Geotechnical office / Geotechnical engineer monitoring database / reports Displacement and closure Geotechnical office / Surveyor monitoring database Accident and hazard Planning office / database / Geotechnical engineer reports files Investigations of and Geotechnical office / Geotechnical engineer reports on rockbursts documents Photographs of rockburst Geotechnical office / Geotechnical engineer damage Numerical modelling using / MAP3D / PHASES / ABAQUS / etc documents Geotechnical office / 3D models Geotechnical engineer Consultants reports / Electronic / document Consultants Reference documents Mine blast data Shift Supervisor office Blast engineer MS-RAP program Seismic office / ESG / ISSI Geotechnical engineer database, backup files, mine plan files, sensor files 15.2 Engineering Work Systems and Checklists An adequate system of work needs to be established on the mine site to effectively and efficiently allow SRMP activities to happen. It is essential to use a system of work that is focused on the practicalities of geotechnical data collection, seismicity assessment, and operations management. The SRMP requirements are complex, and need to be done and seen to be done. This is satisfied by establishing key written (or electronic) documentation points. As a minimum, that documentation should be: A monthly seismicity report A report on all seismic events that cause significant damage A standard seismicity risk assessment process for each new design and excavation proposal Some practitioners use checklists to guide and audit this process. Checklists are useful for proofing the process against: Turnover of personnel and loss of experience Limited experience of site personnel The temptation to focus on keeping production going The temptation to focus narrowly on a few items only, missing other significant items Lack of audit of basic items Lack of consultation between engineering and operational personnel Australian Centre for Geomechanics 107

59 A detailed seismic hazard check list for design is useful: To ensure that seismic risk aspects are adequately considered in all mining work plans and layouts. So that engineers can prepare layouts / work plans for geotechnical scrutiny based on seismic risk principles. Seismic strategy checklists should ensure consideration of the following types of questions: What is the seismic history of the area? Does a geotechnical report exist that covers any previous problems? What are the expected seismic conditions? How will the presence of high seismic levels be catered for? What response will be made if high stresses are present? Has the presence of stress induced damage been addressed in the layout? Are critical rock types present? Are critical faults and structures present? Is the excavation a standard shape and size? Has the presence of large excavation spans been addressed? Does the global block extraction sequence obey the guidelines? What support capacities are required (i.e. support resistance & energy absorption)? Is the risk profile acceptable? An example seismic design checklist from Lightning Nickel is shown in Table 18. Australian Centre for Geomechanics 108

60 Table 18 - Seismic design checklist from Lightning Nickel. In this form the column Ref indicates the appropriate section in the mine s Ground Control Management Plan. SEISMICITY DESIGN CHECKLIST REVIEW BY: DATE: FOR: 1 DESIGN GEOMETRY Ref EVALUATION (attach pages as necessary) a) Shape, size, orientation, & depth of this excavation GEOLOGY / STRUCTURE a) Identify rock types, H/wall & F/wall contacts 6.6 b) Identify porphyries in detail - plan and section views 6.7 c) Identify or interpret Faults & larger Structures. Identify active structures (open, offset, sheared) 6.1 d) Has previous mining nearby avoided particular porphyries / structures? Why? 6.5 f) Q' values STRESS FIELD a) Estimate the stress field 4.1 b) Any stress indicators? Quartz spalling, rocknoise, support loading, borehole condition 4.3 c) Can Structures / Contacts shear in this stress field & geometry? Redesign / support to suit. 4.4 d) If low stress: Assess any relaxation potential 4.8 e) If high stress: Is the design favourably oriented w.r.t. stress? Is a Map3D model required? Why? SEISMICITY a) Review MS-RAP hazard. History of seismic / rockburst activity? What PPV is expected? 8.5 b) What kinds of seismicity may occur - strainburst / pillar burst / fault slip. How will it be controlled? 8.3 c) Re-entry window constraints? GROUND CONDITIONS a) History - any old design reports / memos? What is recent performance of the area? SUMMARY POTENTIAL FAILURE MECHANISMS a) Will rocktypes structures /stresses cause adverse ground conditions? 9.2 b) What failure mechanisms? wedge - sliding - toppling - creep - unravelling - stress - seismicity GROUND SUPPORT a) STANDARD: Are the ground support designs appropriate? Meet GCMP standards? 9.5 b) SPANS: Specify wide span / intersections & extra support. Wedges to be mapped. 9.6 c) SEISMIC / PORPHYRY: what support capacity and energy capacity needed? RISKS a) What can go wrong? Fill out "risk assessment" sheet 9.9 Australian Centre for Geomechanics 109

61 15.3 Mining Instruction Signoff Forms Excavation Design Signoff Details of seismicity management are to be included on the Work Plans / Mining Instructions / for all excavations. The plan must be assessed by all relevant persons and signed off, including geotechnically by the site geotechnical engineer. Stope Blasts Signoff It is recommended that stope blasts also require signoff by the geotechnical engineer. Blasts are important occurrences that can define seismic behavioural domains. Blasts affect geometry, stress, void space, exposure of structures, etc. Some blasts can bring about large changes in seismic behaviour. A stope blast signoff functions to ensure a check is made of: Installed support, Support performance, Rock condition, Previous seismic response, Overbreak, The blast intent as compared with the original excavation design i.e. Deviations in quality control, blast sequence, blast timing, presence of backfill, occurrence of rock falls that change the geometry. In effect this represents a change analysis to identify deviations, and to instigate action if the deviations exceed tolerable limits. Any significant changes must prompt a geotechnical review / seek more advice / renew management approval. Refer Table 19 for an indicative example of a change analysis form for checking designs and blasts. After the blast, a reconciliation of those factors may be undertaken and the signoff may be updated accordingly. Australian Centre for Geomechanics 110

62 Table 19 - Example of a change analysis form for checking designs and blasts. Mine area: Original design workplan: Original design memo: DESIGN VERSUS IMPLEMENTATION CHECK Change Analysis by: Date: Item Any inadequacies in the seismic system Attach additional assessment pages where required Deviations between design Evaluation of the expectations and the intended or deviation - actual implementation tolerable or not? Action required Any inadequacies in seismic data processing Any inadequacies in geotechnical mapping Correct bolts installed - type, pattern, coverage Sequence of mining Geometry, orientation, span, backs profile, of excavations Unexpected variations in geology, rock type, structures Unexpected variations in rock mass quality, groundwater Thickness of critical pillars Sequence of relevant blasts Spikes in seismic response, unexpected responses Australian Centre for Geomechanics 111

63 15.4 Real-time Seismic System Interface It is helpful, especially for larger mines, if the mine operations centre or control room is fitted with a computer interfacing with the seismic system, to show in real time the size and location of the latest few events. These details are provided to operators on request for events they may have heard or felt, reducing the level of uncertainty and anxiety (Mercier-Langevin & Hudyma, 2007) Daily Shift Communication Meeting Commitment to seismicity safety must be clear at the daily shift communication meeting. Seismicity issues are to be noted. Decisions are needed on what items to discuss, and the following are suggested: Any recent large events in case of a larger event, location and size information is required, as are the confines of the affected area. Seismically busy / active areas Any current no entry areas Changes to support requirements Relevant charts, statistics, level plans, memos, etc, are to be posted to complement the verbal communication. More detailed discussions with Shift Supervisors may be required. Good shift communication requires effort and dedication, and suggests that the Engineers need to be present at the shift meeting. Management need to recognise that operating on multiple or long shifts can be detrimental to adequate communication. A major review of the WA industry found that safety and health information was not effectively communicated to the workforce (MOSHAB, 1997). Table 20 shows an example of a weekly sheet used to communicate ground hazards to workforce (Singh et al. 2002). Australian Centre for Geomechanics 112

64 Table 20 - Example of a weekly sheet used to monitor and communicate ground hazards to the workforce (after Singh et al. 2002). Australian Centre for Geomechanics 113

65 15.6 Management/Technical Planning Meeting Good communications, demonstrating leadership and commitment, is important to the management of the seismicity hazard. AS/NZS 4360:2004 Risk Management Framework 3.1 Communicate and consult: A consultative team approach is useful to help define the context appropriately, to help ensure risks are identified effectively, for bringing different areas of expertise together in analysing risks, for ensuring different views are appropriately considered in evaluating risks and for appropriate change management during risk treatment. Involvement also allows the ownership of risk by managers and the engagement of stakeholders. It allows them to appreciate the benefits of particular controls and the need to endorse and support a treatment plan. Seismicity issues are to be discussed at the Mine s regular multi-disciplinary engineering / technical planning meeting. The meeting should consider any: Recent significant seismic occurrences and trends Recent investigation reports and memos Mining constraints that make it impossible to achieve geotechnical best practices Risks and costs of different options Production versus safety conflicts (MOSHAB, 1997) Geotechnical Stop or hold points, i.e. Situations where work stops until further instructions are received (Barrick Gold Corporation 2005). Note that the multi-tiered response plan in Chapter 16 is in effect a series of geotechnical stop points. Any recommendations in regard to seismicity management are to be assessed by the technical team, documented, and actioned in accord with the Mine s methods for such work Site Inductions All persons working at the Mine shall complete an induction, and seismicity awareness shall be included in the induction programme. Detail which inductions related to seismicity at the mine should be taken by new personnel, contractors and other visitors to site Workforce Mine management shall employ an adequately skilled workforce. Management should consider the following in relation to ability to manage a seismic hazard: Choice of an owner-operator work force versus a contracted workforce. Bonuses and incentives these should relate to SRMP desirables, not only to productivity. (MOSHAB, 1997). Training of and active participation of the workforce in managing seismicity. How to avoid deterioration of established safe working practices. The survey (MOSHAB, 1997) found this to be a significant issue due to factors such as ineffective OHS management, intervention by a principal in contractor s matters, and confusion over reporting authorities and responsibilities. The survey reported that evidence of deterioration in this fundamental commitment was too consistent to be ignored. The effect of employee turnover. The effect of rapid elevation of young professionals to management roles without adequate training and experience. The effect of reduced commitment to provide training. The effect of non-compliance with Regulations. Inability of operators to read the ground. Australian Centre for Geomechanics 114

66 15.9 Seismicity Awareness Training Module A Seismicity Awareness Training Module, based on the SRMP, shall be regularly presented to site personnel. The purpose of the training module is to ensure all persons are aware of the basic seismicity management considerations that are relevant to the Mine. This Module can comprise a range of approaches, such as: Lectures PowerPoint presentations DVD videos Several levels of presentation can be prepared, e.g. a shorter version for all personnel, a more detailed version for supervisors and operators, and a refresher training version. Topics included in the training module may include: The Mine geotechnical environment Geology of ore bodies Stress and its effects Seismic hazard identification and control systems Ground support requirements and how ground support is used The rock noise and hazard reporting system at the Mine The frequency of the presentations needs to be specified and adhered to. Frequency depends on employee skill and turnover. The presentations need to be updated to take into account feedback and any new hazards or hazardous practices that may be identified. The objective of the presentation is to develop competency, which may be demonstrated by linking the training to the Reporting System (below). Provide details of training materials available at the mine. These may include The ACG Training DVD Rockburst Unleashing Earth s Energy The ACG Training DVD Reading the Ground The Working Safely in Active Ground Presentation (available at The MS-RAP pdf Training Modules (available at Seismic Hazards Reporting System This section describes the system at the Mine for personnel to report seismic hazards. AS/NZS 4360:2004: Risk Management Framework 3.1 Communicate and consult: Since the views of stakeholders can have a significant impact on the decisions made, it is important that their perceptions of risk be identified and recorded and integrated into the decision making process. The Mine should have a functional accident / incident reporting system, which may or may not be integrated with a rocknoise reporting system. Whatever the particular protocol, a reporting system is essential so that operators have a framework to document significant seismic-related occurrences. Operators are generally closely aware of changes in their own working environments, and need this avenue of communication. The effort to document and report rockfalls in itself...implies an effort to understand and prevent these occurrences. In some cases the analysis will be incomplete or it may not be possible to come to a definitive conclusion. However, over time, the collection of such records will result in a better Australian Centre for Geomechanics 115

67 understanding of the rock mass and will make it possible to design more effective support systems. (Barrick Gold Corporation, 2005). It is the responsibility of every person to use the system. Management should ensure that the system is effective, and not ignored, bypassed or discouraged, and that the workforce is trained and competent in the use of the system. The system could be one or more of: A Hazard Reporting Book A Ground Control Log Book Entries in the Mine s formal accident / incident reporting system A specific rock noise / ground hazard card A Workplace Inspection Form (i.e. a prestart check every shift for every workplace) Any unusual or undesirable occurrence relevant to seismicity should be communicated. For example the following types of data could be recorded as appropriate for the Mine: Seismic damage Rock noise All uncontrolled falls of ground and potential rock fall hazards Any fall of ground not previously recorded Visibly deteriorating ground conditions in any active area Unusual creaking and popping noises in any active area Pillar or side wall slabbing Movement along faults, shears or joints Opening up of fracture planes Unusual hangingwall closure or footwall heave Load being taken on rock bolt plates Drill holes that can not be completed or close rapidly upon completion Any failure of ground support or reinforcement Unusual water inflows Surface vibrations A procedure for recording, communicating, and assessing these reports is required. Essentially: Recording: the information is added to the SKB using a standard record sheet which includes plan, section, and key relevant data. Communicating: the standard record sheet or report is displayed to provide tangible data and improve understanding of mine seismicity. Assessing: significant occurrences are inspected, investigated and documented in detail by the Geotechnical Engineer, using the standard Fall of Ground (FOG) report. Refer to Section 6.1 for examples of rock noise reporting forms. Refer to Section 7.3 for a template of the ACG rock mass failure recording sheet. Australian Centre for Geomechanics 116

68 16. MULTI-TIERED SEISMICITY RESPONSE PROCEDURE This multi-tiered response procedure (Table 21) defines the management process for determining immediate actions when significant seismicity occurs. It includes emergency response to rockbursts and very large seismic events in the mine. In effect, this procedure is a series of prescribed geotechnical stop points. The multi-tiered procedure shown below is based on a similar procedure used by Lightning Nickel at Longshaft Mine (Lightning Nickel, 2003). AS/NZS 4360:2004 Risk Management Framework Customised process 16.1 Procedure Format The procedure consists of three strands: A list of indicative conditions that will trigger escalation to another tier of response. Alternative response options. The authority required for each level of action to be taken. Intermediate tiers can be bypassed if conditions so require. Figure 46 shows an example of a multi-tiered framework from Mt Charlotte mine. Figure 46 - An example of a multi-tiered framework to decide on re-entry following production blasts in a seismically active area. Australian Centre for Geomechanics 117

69 16.2 Multi-tiered Framework Table 21 - A multi-tiered overall seismicity response framework. TIER TRIGGERS ACTIONS AUTHORITY 1 Seismicity within expectations 2 Seismicity die down following blasts 3 Seismicity event rate increasing 4 Seismicity hazard level increasing 5 Isolated significant or large events distant from workings 6 Isolated significant or large events close to workings, causing damage 7 One or multiple significant or large events close to workings, or isolated events, causing injury and damage Normal operating condition Monitor Geotechnical Engineer Blast fired Monitor Omori-style decrease in events Geotechnical Engineer Operator feedback Shift supervisor feedback Seismic monitoring system indicators MS-RAP indicators Seismic monitoring system indicators Operator feedback Shift supervisor feedback Seismic monitoring system indicators Operator feedback Shift supervisor feedback Seismic monitoring system indicators Underground inspection MS-RAP review Determine reason for increase and document via Memo If hazard is also increasing move to Tier 4. Communicate to management and workforce Determine reason for increase and document via Memo Table and discuss at Mine Technical Meeting Verify adequacy of relevant seismic hazard strategies Recommend strategy changes Communicate to management and workforce Extend re-entry time MS-RAP review establish if this is truly isolated or part of a trend Evaluate and document any trend Communicate to management and workforce MS-RAP review Withdraw personnel from area Review re-entry time Underground inspection Investigate reason for event Discuss at Mine Technical Meeting Verify adequacy of relevant seismic hazard strategies Recommend strategy changes Communicate to management and workforce Rehabilitate Upgrade ground support in other areas at similar risk MS-RAP review Withdraw personnel from area or mine Activate Mine Emergency Response (triggers monitoring, reporting, evacuating, re-entry, media protocols) Review re-entry time Underground inspection if able Investigate reason for event/s Discuss at Mine Technical Meeting Verify adequacy of relevant seismic hazard strategies Recommend strategy changes Communicate to management and workforce Rehabilitate Upgrade ground support in other areas at similar risk Geotechnical Engineer Senior Geotechnical Engineer Senior Geotechnical Engineer Geotechnical Superintendent Underground Manager Note: Stages 6 and 7 require personnel to be withdrawn from a mine sector or from the entire mine. Mercier-Langevin & Hudyma (2007) provide an example of a mine that closes a mine sector for 12 hours if an event of local magnitude -0.8 or greater occurs within 20 metres of mine development. Australian Centre for Geomechanics 118

70 16.3 Response to a Large Event If a particularly large event occurs, the initial response is twofold: a) Interrogate the SMS to ascertain event location and size, with follow-up data analysis and interpretation b) Activate the Mine s Emergency Response Plan (ERP). The Mine will need to specify the triggers that will activate the ERP. Include relevant information in the ERP. The managerial team will need to consider reports to workforce, exclusion zones, investigations, and media relations. Durrheim et al. (2007) document the style of emergency response to rockbursts that is used in South African mines. Australian Centre for Geomechanics 119

71 17. MONITORING THE SEISMIC RISK MANAGEMENT PLAN The SRMP must be robust, to survive turnover of staff and all the various issues described in this document. The process for maintaining and improving the SRMP must be specified, and properly communicated to those personnel responsible, including via a formal job induction and handover instructions for new personnel. The functioning of the SRMP must be verified by audits, internal and external. AS/NZS 4360:2004 Risk Management Framework 3.7 Monitor and review learn lessons from the risk management process, by reviewing events, the treatment plans, and their outcomes. 3.8 Record the risk management process Technical guideline Sound professional engineering practice requires that the SRMP shall be reviewed and audited: At any time when an undesired adverse seismic event occurs. Whenever geotechnical activities identify any SRMP aspect that requires modification. Occasionally by a qualified person other than the Geotechnical Engineer. Each 12 months, by the Geotechnical Engineer, to review and reassess the design and operation parameters. The survey described by Hudyma & Heal (2007) can be adapted as the basis for a simple but effective internal audit, perhaps combined with relevant sections of the Department of Industry and Resources (DOIR) and Ground Control Group WA (GCGWA) audits. The following may trigger SRMP review: New information Continuous failures Continuous deficiencies Changed conditions Changed design parameters Shortfalls any shortfall should trigger the Continuous Improvement Process. List any memos, reports and publications that were consulted in updating the SRMP. One approach is to keep a paper master copy of the SRMP, and annotate by hand as often as necessary, capturing details of new work, new knowledge, changes and responses. In this sense the SRMP is never complete. This annotated paper copy SRMP is to be regarded as the current SRMP as it can be kept up to date daily. The electronic SRMP is updated based on the annotations, at least yearly, but preferably much more frequently. It must be easy for other personnel including auditors to locate the SRMP and its procedures, and to know that they are current. Australian Centre for Geomechanics 120

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