What Lies Beneath? A Safe Way of Surveying Mine Site Dams to Improve Water Management

Transcription

1 What Lies Beneath? A Safe Way of Surveying Mine Site Dams to Improve Water Management Jamie Hansen J.Hansen@aamgroup.com AAM Pty Ltd Abstract The importance of mine site water management is well recognised in the mining industry for a range of important operational and environmental reasons. However, the ability to accurately measure and report the volume of water contained in mine site dams can be a tricky business. Mine site Safety Officers shudder at the notion of putting people on dams in small boats, and specialised bathymetric surveying equipment is not normally found in the average mine surveyor s tool kit. This paper examines:- why conducting accurate surveys of mine site dams is important for mine site water management; problems with current techniques mine surveyors use to determine water volumes and dam capacities; and a remotely operated bathymetric survey solution which is now available to safely, accurately and regularly survey mine site dams. Some Case Studies demonstrating the results of mine site dam surveys using the remotely operated system are presented. Future developments of remotely operated survey vessels are discussed. ww.aamgroup.com

2 Introduction The tools available to mine surveyors for performing their measurement duties have changed dramatically in recent times. Laser Scanners have significantly improved the efficiency and safety of routine tasks such as determining end of month stockpile and pit volumes. A range of airborne systems are now commonly used for mine surveying tasks which are impractical to be undertaken by ground-based methods due to size, cost or safety reasons. With the mine surveyor s attention continuously focused on operational and production activities (i.e. where the action is) it s perhaps no surprise that there is little time or effort put towards examining new tools or methods for surveying mine site dams. The purpose of this paper is to examine the importance of mine site water management and to present a solution which is now available to mine surveyors to help survey dams safely, accurately and cost-effectively. Mine Site Water Management Who Cares? The importance of understanding and managing water is well recognised in the mining industry. Michael Royal states: The success or failure of a mining project is often linked to site water management, or the lack of it. Risks to a project s success include tailings dam failures during storm events, slope failures related to elevated pore pressures, water in blast holes leading to inefficient blasting (adding costs and environmental impacts), lack of process water, and excessive closure costs. (ref: Mining companies are required by environmental authorities to develop Water Management Plans and Water Balance Models that attempt to describe and predict in detail how water will be sourced, diverted, contained and released throughout the life of the mine and following mine closure. As show in Figure 1, Water Balance Models are complex and involve a combination of actual and predictive information. Therefore, regular monitoring and maintenance of the Water Balance Model is critical to ensure the water needs of the mining operation are being met, whilst complying with environmental regulations. Page 1

3 Figure 1 Sample Mine Site Water Balance Model (ref: The Role of the Mine Surveyor in Mine Site Water Management Although mine surveyors regularly measure and report important information such as volumes of ore stockpiles and the amount of overburden removed from the pit, their role in reporting the volume of water contained within the mine s various containment facilities is overlooked as a routine task. Currently, the only role most mine surveyors perform in the determination of water volumes (at least to some degree of regularity) is to measure the level of the dam waterline. The mine surveyor dutifully reports this information to the mine site s Environmental Officer, who cross-references the surveyed water level with a pre-defined storage curve to determine the current volume and remaining capacity of the dam. A simplified version of the process is illustrated in Figure 2. Page 2

4 1. Mine Surveyor measures RL at the water line 2. Environmental Officer cross references water level with volume from pre-defined Storage Curve Figure 2 Typical Method for Determining Mine Site Dam Volumes The problem with this method is it does not take into consideration any changes to the surface of the bed of the dam over time. In reality, the dam bed is continually changing and the available capacity of the dam is diminished due to factors such as sediment build-up, and, in the case of in-pit water storage facilities, material from the surrounding pit walls failing and falling into the pit. Figure 3 refers. With this in mind, the routine task of simply measuring the water level of the dam to accurately determine water volumes and dam capacities is not sufficient in a continually changing environment. Reporting inaccurate water volumes and making incorrect assumptions about available storage capacities have a direct impact on a mine site s Water Balance Model and Water Management Plan. Potential implications of an inaccurate Water Balance Model for a mine site include: water reserves for operational needs depleted earlier than anticipated; unbudgeted civil works required to increase holding capacity of water and tailings dams; and non-compliance with environmental regulations prior to (and during) wet season events. Page 3

5 1. Mine Surveyor measures RL at the water line 2. Volume error due to changing dam bed 3. Actual volume of water and available holding capacity diminishes over time Figure 3 Representation of changes in mine site dam holding capacity over time Page 4

6 Surveying Mine Site Dams A Challenge for Mine Surveyors So why is it that surveying mine site dams has presented such a challenge to mine surveyors? A few theories come to mind: Out of Site Out of Mind Like most problems human beings face, it s only when a problem manifests itself with some visual clues that we tend to take action. On a mine site, it s quite obvious if a dam has water in it. The water level rises and falls, either as a result of operational use or due to seasonal effects, and so the water level is diligently monitored. However, when changes are taking place beneath the water surface, as a result of sediment build-up for example, the perceived impact (problem) is less obvious and therefore tends not to attract the attention of regular measurement or monitoring. It should be noted that the out of sight out of mind theory extends to everyone and not just mine surveyors! Lack of Available Skills, Tools and/or Experience Although most mine surveyors complete at least one unit in hydrographic surveying during their studies, it is unreasonable to expect mine surveyors are competent hydrographic surveyors upon entering the workforce. Furthermore, it is unreasonable to expect that all mine surveyors should have in their store a suitable vessel and bathymetric survey equipment which may only be used once or twice during the year. This would explain why most mine surveyors experiences in conducting dam surveys have been limited to the use of primitive measurement devices such as long poles, weighted ropes or recreational fish-finders! In very shallow water environments (i.e. less than 1m in depth) good old fashioned guess-work is typically employed. Safety Above all though, safety is likely the main reason that has prevented mine surveyors conducting accurate surveys of mine site dams. Obtaining approval from mine site Safety Officers to put people on the water in a small dingy is an arduous process, taking weeks or months to obtain sign-off. A solution that overcomes each of these obstacles is needed Page 5

7 A Remotely Operated Survey System for Mine Site Dam Surveys Remotely operated hydrographic surveying systems have been used around the world for quite some time, but only recently adapted to suit the challenges of surveying mine site dams. One recent example is the Z-Boat 1800 (pictured), which is a battery operated, remotely operated hydrographic survey vessel. Key components of the system include: Navigation control signal from the shore to the boat: A 2.4 GHz frequency hopping spread spectrum radio system is used up to a range of about m. Control of the boat is achieved by a joystick operation on the shore. This system also provides telemetry of the boat's main battery power level to the shore unit. Page 6

8 Echosounder: The vessel can be fitted with either a single or dual frequency echosounder. For mine site applications, where dams are likely to have a high concentration of suspended particles in the water, a dual frequency echosounder that records a full echogram (or picture) of the sonar response is needed to allow the results to be analysed in detail. Data transmission from the boat to the shore operator: An on-board telemetry module accepts up to three inputs: GNSS position, depth soundings, and boat heading from a magnetic compass. These data streams are recorded on an internal memory card in the order they are received, and simultaneously transmitted to the shore via high power spread-spectrum radio. Bathymetric Survey data visible to operator during data acquisition Page 7

9 Data display and acquisition software on the shore PC: Hydrographic surveying software is used to manage the remotely-operated boat survey workflow, from designing predefined survey lines, through to processing, editing and output of final results. The vessel is constructed from acid-resistant Acrylonitrile Butadiene Styrene (ABS) plastic and stainless steel thus making it resistant to low or high ph water. The echosounder transducer is not exposed to the water outside the hull, but is instead sealed in a propylene-glycol filled cavity inside the hull. When equipped with a dual frequency echosounder, the vessel weighs approximately 60kg and is easily and safely carried to the shoreline and launched by two people. (Ref: How Much? Depending on the selection of individual components such as the GNSS receiver and echosounder type, the cost to purchase a system described above would typically be in the order of $70,000 to $80,000, plus training and ongoing maintenance costs. By comparison, the cost to engage a qualified surveying consultant equipped with a remotely operated survey vessel would typically be in the range of $10,000 to $30,000 per project, depending on the number and size of dams to be surveyed and the project deliverables. As with all purchasing decisions, whether to buy equipment outright, or to engage a specialist surveying consultant on an as-needs basis, a number of issues should be carefully considered: Will the equipment be used often enough to justify purchasing a system outright? Can training and ongoing maintenance costs be accommodated within existing budgets? Can the equipment be stored in a manner that it will operate when next required? Is there a risk of committing to a particular system and missing out on the benefits of future technology upgrades? Can existing survey staff afford to spend time: (a) learning how to operate a new survey system; and (b) undertake data collection and processing for dam surveys? Does in-house expertise include analysis of sounding data, identifying sediment from bed? Page 8

10 Case Studies Many dam surveys have been completed in Australian using a remotely operated survey vessel. The follow Case Studies provide a selection of typical project objectives and outcomes. Case Study #1 Volumetric Survey of a Tailings Storage Facilities, Mine Site, Northern Australia A remote mine site in northern Australia has an ongoing need to assess the remaining capacity of its Tailings Storage Facilities (TSFs). This is required to: monitor the storage capacity of the dams with respect to Design Storage Capacity; and report to regulatory authorities the quantity of water the dams can hold in the lead up to each wet season. In order to compute the remaining storage capacity of each dam both the submerged and exposed parts of the facilities need to be surveyed. Page 9

11 A remotely operated survey vessel was used to survey the submerged areas of the dams, while exposed areas were surveyed using a combination of terrestrial and airborne laser scanning. The bathymetric survey data was merged with the terrestrial and airborne LiDAR data to create seamless surfaces of the exposed and submerged area of the dams. Volumetric calculations and storage curves were derived from the combined surface to determine the current volume of tailings and water stored in the facility and their remaining capacities. Page 10

12 Case Study #2 Volumetric Survey of Sediment Dam, Coal Mine, Central QLD A coal mine was concerned that a sediment dam (specifically designed to contain sediment run-off from a nearby waste rock dump) was filling up with more sediment than antcipated, hence diminishing the total capacity of water it was designed to hold. The mine planned to excavate the sediment build-up to return the dam to its orginal designed water holding capacity. A bathymetric survey was required to determine how much sediment needed to be excavated before the remedial work commenced. Page 11

13 Being a relatively shallow dam (1.5m at its deepest point) the bathymetric survey required the use of a vessel capable of collecting accurate survey data in very shallow water. A remotely operated survey vessel was used to obtain depth measurements of the entire dam surface, right up to the shoreline. The bathymetric survey data was combined with existing airborne LiDAR data to allow a new storage curve to be computed to the dam s full supply level. The difference between design capacity and actual capacity was computed. The quantity of sediment to be excavated from the dam to return it to its original design holding capacity could then be determined prior to earthworks commencing. Page 12

14 Case Study #3 Investigation of Pump Intake Area of Raw Water Dam, Coal Mine, Central QLD In a curious twist, this case study involves a dam which was gradually increasing its holding capacity over time! A coal mine in central Queensland suspected that material from the bed of their raw water dam was being drawn into the pump supplying water to the wash plant. The mine was concerned that the designed dam bed surface was gradually being etched away and sediment in the pumped water affecting the productivity of the wash plant. A bathymetric survey was required to determine what secrets the dam was hiding beneath its surface. Page 13

15 Due to the dam being a Turkey s Nest design, the steep batters of the embankments made launching a small dingy into the dam impossible. A remotely operated survey vessel was therefore deployed which could be safely lowered to the waterline with the survey operator able operate the vessel from the embankment. Analysis of survey results confirmed the mine s suspicion that the bed of the dam was indeed being gradually etched away by the pump. The extent of the excavated material could be assessed and quantified and remedial action taken to reposition the water pump to prevent further sedimentcontaminated water being sent to the wash plant. Page 14

16 Learnings & Future Developments Although the specific reasons for undertaking bathymetric surveys of dams vary, some common themes have emerged in terms of: a) executing a successful mine site dam survey; and b) the direction future developments of remotely operated survey vessels may take. Executing a Successful Mine Site Dam Survey Planning: Experience has shown that site knowledge, coupled with existing spatial information such as georeferenced aerial photography, are key ingredients during the planning phase of a mine site dam survey. Input from mine site staff is essential to help provide guidance on logistical issues, such as site access and safety/induction requirements. Recent, georeferenced aerial photographs of the dam being surveyed help the operator pre-design survey lines to obtain maximum data coverage and avoid running into obstacles such as jetties and pumps. Site Support and Assistance: Mine site dam surveys are a two-person operation, even when using a remotely operated survey vessel. An experienced operator is required to ensure the system is configured correctly prior to data acquisition and to control the vessel during survey. A second person (the assistant) is needed to help lift the vessel to and from the water and to provide a second set of eyes during data acquisition to ensure the vessel is on track and following pre-defined survey lines. The second person does not necessarily need to have any technical experience, so costs for engaging a survey contractor to perform mine site dam surveys can be reduced if the mine site is prepared and able to provide a second set of hands. Survey Control: It is critical that the bathymetric survey operator liaises with the mine site surveyor to obtain correct survey control and to ensure the acquired data is on the correct datum. This is especially important if the bathymetric survey data is to be merged with above-water data for calculating storage capacity curves. Echosounder Selection: Although single frequency echosounders are adequate for some bathymetric survey applications, their performance can diminish as water becomes more turbid. In very turbid water (particularly active tailings dams) a single frequency echosounder may not be able to penetrate the entire water column and therefore not obtain accurate depth measurements. In such instances a dual frequency echosounder, which transmits both high and low frequency soundings is desirable. Low frequency soundings are more likely to return successful measurements from the dam floor even if there is a high concentration of suspended particles in the water. Page 15

17 Future Developments of Remotely Operated Survey Vessels As the use of remotely operated survey vessels grows, the technology will undoubtedly improve in functionality and performance to serve a range of applications. Based on recent project experience and the wish lists of both operators and clients, some future developments are envisaged: Operator Improvements: The range of remotely operated survey vessels is often stated in terms of the range of radio communications. In practice however, range is dictated by the visibility of the vessel. Depending on the vantage point on which the operator is standing (and the acuteness of their vision!) the vessel typically becomes difficult to see over 250m away. It is expected that remotely operated survey vessels will eventually come with a camera mounted to the front of the vessel to give the operator a first person view in real-time. This would allow the operator to take advantage of the full radio range and allow long lines to be surveyed even when the vessel is out of direct view. This would make a remotely operated survey vessel a viable option for surveying larger dams or for dams where direct line of sight vantage points may not always be possible. Sensor Improvements: It seems that one of the holy grails for mine site dam surveys is a system which can accurately measure sediment thickness. In theory, by using a dual frequency echosounder and comparing the high frequency depth value with the return obtained from the low frequency transmission, it is possible to obtain an idea of material thickness. However, the depth accuracy of low frequency soundings in very shallow water can be quite low, and the accuracy of calculated material thickness is dependent upon having a hard base on the dam, which is not always the case. Therefore, at present, the use of high and low frequency data to estimate sediment thickness should be used with some caution. Sediment thickness should ideally be measured using specialised equipment such as a sub-bottom profiler, which transmits sound pulses over a wide frequency range to penetrate multiple layers of sediment (penetration of up to 50m beneath the top layer of sediment is possible, depending on sediment type). Substituting a conventional single or dual frequency echosounder, for a sub-bottom profiler, is a logical next step for investigation to help improve the accuracy of determining mud and sediment thickness of mine site dams and tailings storage facilities. Page 16

18 Conclusion The importance of mine site water management and the reliance on finely tuned water balance models are not new concepts to the mining industry. Mining companies need to manage their water usage effectively to ensure there is adequate supply for mine production and processing. Environmental regulations dictate that mine sites have sufficient capacity to hold and contain contaminated waste water and run-off during major rain events. Without regular and accurate measurements of mine site water holding facilities, corporate objectives and regulatory requirements may not be met, resulting in significant financial loss and environmental damage. In the same way that mine surveyors are responsible for important measurement tasks such as reporting volumes of ore stockpiles and overburden removal, so too should a surveyor be entrusted with accurately measuring and monitoring the volumes and holding capacities of a mine site s water holding facilities. Mine surveyors have historically been restricted in regularly and accurately undertaking dam surveys due to insufficient tools, expertise or safety concerns. However, the technology now exists in the form of a remotely operated bathymetric survey vessel suited (and proven) for this task. With the emergence of any new surveying technology, the mine surveyor is required to undertake an evaluation with a number of key questions in mind, viz: Will this improve the quality of information provided to mine management? Will this improve the efficiency of the surveying task? Will this improve mine site safety? Can the cost/benefit of this be justified? Whilst it may be difficult to cost-justify the capital purchase and ongoing costs for a twice yearly survey, a remotely-operated survey boat service provides an on-demand, cost-effective hydrographic survey capability. Future imminent developments such as sensors to measure sediment thickness are also kept open to individual mine sites by utilising shared hydrographic resources, rather than locking in specific equipment by purchasing one s own kit. Whether buy or buy in, the ability to reliably define on-site water volumes is now available at a safe and cost effective level. Page 17