REPORT Joint Investment Holdings Ltd Kiwi Bacon - 317 to 319 New North Road, Kingsland Groundwater Drawdown and Settlement. Monitoring and Contingency Plan
REPORT Joint Investment Holdings Ltd Kiwi Bacon - 317 to 319 New North Road, Kingsland Groundwater Drawdown and Settlement. Monitoring and Contingency Plan Report prepared for: Joint Investment Holdings Ltd Report prepared by: Tonkin & Taylor Ltd Distribution: Joint Investment Holdings Ltd Auckland Council Tonkin & Taylor Ltd (FILE) 2 copies +pdf 1 copy + pdf 1 copy November 2014 T&T Ref: 23766.1 ver 2
Table of contents 1 Introduction 1 2 Project Description 2 3 Monitoring 3 3.1 General 3 3.2 Retention system 3 3.2.1 Retaining wall monitoring intervals 3 3.2.2 Retaining wall deflection Trigger Levels 3 3.2.3 Response procedure if the retaining wall deflection Action Trigger Levels are exceeded 4 3.3 Groundwater monitoring 5 3.3.1 Groundwater monitoring intervals 5 3.3.2 Groundwater drawdown Trigger Levels 5 3.3.3 Response procedure if the groundwater Alert Trigger levels are exceeded 5 3.3.4 Response procedure if the groundwater Action Trigger Levels are exceeded 5 3.4 Settlement monitoring 6 3.4.1 Settlement monitoring Intervals 6 3.4.2 Response procedure if the settlement Alert Trigger Levels are exceeded 6 3.4.3 Response procedure if the settlement Action Limits are exceeded 6 3.5 Potential contamination monitoring 7 4 Contingency Options 8 4.1 General 8 4.2 Groundwater drawdown contingency measures 8 4.2.1 Groundwater recharge 8 4.3 Retaining wall deflection contingency measures 9 4.4 Excessive ground settlement 9 5 Applicability 10 6 References 11 Appendix A: Drawings Kiwi Bacon - 317 to 319 New North Road, Kingsland Groundwater Drawdown and Settlement.
1 1 Introduction This report presents the Monitoring and Contingency Plan associated with the excavation and groundwater take for the development at 317 to 319, New North Road, Kingsland. The analysis was undertaken in accordance with an email of 23 October (ref 7). The multiple-storey building construction will include a four level basement, and is located at a site that extends from New North Road to Aitken Terrace, Kingsland. This report is based on the following reports previously carried out for the wider project: Geotechnical investigation report (ref 6). Groundwater seepage and retaining wall report (ref 8) The monitoring proposed will involve: inclinometers placed immediately behind or within piles of the southern and eastern retaining walls four groundwater monitoring wells to monitor the groundwater level within various geological units 23No ground settlement pins 11No deformation pins on the top of the retaining wall capping beams Six building deformation pins on the building immediately beyond the eastern boundary to the site The contingency will involve groundwater re-injection wells. Five are proposed to be installed along the southern and eastern boundaries. In addition consideration will be given to creating a drained basement if all of the following criteria are met: re-injection wells are not required during excavation of the basement, groundwater inflow is less than predicted from the seepage assessment settlement of adjacent ground and structures is less than 50% of that predicted by the groundwater seepage and retaining wall report (ref 8).
2 2 Project Description The site is situated in Kingsland at 317-319 New North Road and is made up of the legal titles Lot 1 DP 59489, Lots 2 and 3 DP 6092 and Lots 28 and 29 Blk 1 DP 383. The property covers an area of approximately 5,544 m2. The site is currently an open asphalt surfaced car parking area with a two level building to the western side. The carpark is level and on grade with New North Road on the southern half, and grades down to Aitken Terrace (a 3 m drop) on the northern boundary. A garage/shed is located at the northern end of the site next to Aitken Terrace. Regionally, the site is located near the crest of a north west facing slope which grades at approximately 1(vertically):8(horizontally) towards the north western motorway. To the west, the ground falls at a gentle slope (1:50) towards Kingsland. To the east, the ground rises (1:25) towards the Symonds Street ridge, with a local depression approximately 200 m to the east as New North Road passes beneath Dominion Road / Ian McKinnon Drive. To the south (and south east) the ground rises gradually (1:50) towards the flanks of Mt Eden. The proposed development consists of a 3 to 4 storey, split level basement carpark, with 4 storeys above ground. The building is likely to be a mix of residential and commercial use. Geotechnical investigations have identified the site as underlain by a deep sequence of alluvial sediments with East Coast Bays Formation (ECBF) rock at a depth of approximately 17 metres (5 to 7 metres below basement excavation level). The alluvial sediments tend to be weak and potentially compressible. The groundwater, as measured within nested piezometers, indicates a regional level of approximately 2 metres below the ground surface.
3 3 Monitoring 3.1 General Monitoring results will be collated by Tonkin & Taylor (T&T) and compared with the specified trigger levels. These records will be held by T&T and will be available for inspection as required. During construction, reporting of these results will be forwarded to Auckland Council (AC) at monthly intervals as detailed in this report. Once the basement is sealed, monitoring will continue for a period of 6 months and results will be reported bi-monthly. 3.2 Retention system The retention system will be monitored as follows; An inclinometer on the eastern wall, An inclinometer in the southern wall, survey monitoring points along the top of the retaining wall (approximately 11 pins). The results should be submitted to Holmes Consulting Group and Tonkin and Taylor within 3 working days of taking the readings. 3.2.1 Retaining wall monitoring intervals Measurements shall be accurate to ±2mm. At least two baseline surveys shall be undertaken before commencement of dewatering. Surveys of top of wall survey points and inclinometer readings shall be carried out weekly during excavation or when changes to the propping system are being carried out. During periods of little construction activity or when the inclinometers and survey points show little change, the monitoring interval may be reduced to fortnightly periods. Surveys shall continue until all floor levels have been installed and the temporary anchors have been removed. 3.2.2 Retaining wall deflection Trigger Levels Results of this monitoring are to be compared with the design assumptions. A level of 75% of the predicted maximum values has been adopted for increased monitoring. If the Alert Trigger levels in Table 3.1 are reached, monitoring frequency shall be increased to twice that presented in Section 3.2.1. The trigger for implementation of additional works (Action Trigger level) has been set at the likely expected maximum deflections and is presented in Table 3.1.
4 Table 3.1: Trigger Levels for retaining wall deflections Wall Location Depth below top of wall Alert Trigger Level - Increase Monitoring (75% of expected maximum deflection) Action Trigger Level Action Required (Expected maximum deflections) Eastern side of building (adjacent 18 Aitken St) Remaining 3 excavation walls Within 2 metres of the ground surface Below 2 m of ground surface Within 2 metres of the ground surface Below 2 m of ground surface 9 mm 12 mm 24mm 32 mm 9 mm 12 mm 27 mm 36 mm 3.2.3 Response procedure if the retaining wall deflection Action Trigger Levels are exceeded If the inclinometer deflection patterns are significantly dissimilar to predictions and the extent of these deflection patterns is significantly greater than expected or if the horizontal inclinometer deflections exceed Action Trigger Levels in Table 2.1 then the following should be carried out. Notify the Project Manager, Review the monitoring data, as-built details, geology, groundwater levels, surcharge loads and compare against the assumptions made in the modelling carried out for the design of the retaining wall, Report review response to the Project Manager. If considered necessary; Notify Auckland Council, Increase the monitoring frequency, Hold works within risk areas, Develop a detailed contingency plan and submit to The Consent and Consent Compliance Manager (AC).
5 3.3 Groundwater monitoring Groundwater levels will be monitored in four locations around and beyond the site. 3.3.1 Groundwater monitoring intervals From at least one month prior to the commencement of dewatering, the groundwater levels in all of the piezometers will be recorded at weekly intervals until the basement has been sealed. Following sealing of the basement, groundwater monitoring shall be carried out every two months for a period of 6 months. Sealing of the basement structure shall be considered when water inflow is less than 1 m 3 per day. 3.3.2 Groundwater drawdown Trigger Levels Tonkin and Taylor Ltd will review the results of this monitoring and compare with the trigger levels presented in Table 3.2 below. The baseline groundwater levels need to be set prior to commencement of dewatering and will generally consist of the depth below the lowest measured groundwater level within that borehole. This should take into account established seasonal groundwater variability. Recorded water level information should be provided in a tabular form and graphs which should note the established groundwater level trigger alert and alarm levels. Table 3.2: Groundwater drawdown Trigger Levels Borehole No Geological Unit Alert Level Action Level All boreholes Upper Alluvium 1.0 m 1.5 m Lower Alluvium 1.5 m 2.0 m ECBF rock 2.0 m 2.5 m 3.3.3 Response procedure if the groundwater Alert Trigger levels are exceeded If the Alert Trigger Levels are reached then the one or more of the contingency options described in Section 4 should be carried out together with: i. Notify The Consents and Consents Compliance Manager (AC) (or the nominated staff member acting on The Manager s behalf), in writing within 1 day of the trigger being monitored, with details of actions undertaken, ii. iii. iv. Submit to the Manager within 5 working days the survey monitoring results within a 50m radius of that monitoring bore, and compare them to settlement trigger levels, Increase groundwater level monitoring to daily intervals, Increase settlement monitoring within a 50 m radius to weekly intervals, v. Submit the results of iii and iv within 7 days of the settlement monitoring survey. These should be carried out until written approval from the Manager to revert to normal operation. 3.3.4 Response procedure if the groundwater Action Trigger Levels are exceeded If the groundwater Action Trigger Levels are reached then the following should be carried out:
6 i. Notify The Manager within 1 working day, ii. iii. Cease further lowering of the water tables or any other activity which has the potential to cause settlement until The Manager provides written notice to the Consent Holder that the Manager is satisfied that damage to buildings, structures and services is unlikely or that the Manager is satisfied that owners of potentially affected buildings, structures and services have given written approval for dewatering to continue. Instigate procedures in Section 4 and any other actions agreed with The Manager. 3.4 Settlement monitoring Settlement monitoring marks shall be established on the ground and buildings surrounding the excavation area. The monitoring points will be surveyed twice prior to any excavation with a precise level; i.e. to an accuracy of ±1mm. 3.4.1 Settlement monitoring Intervals Settlements will be monitored at the locations shown on Drawing 29253.001-F3. Monitoring will be carried out at monthly intervals from the commencement of dewatering. Following sealing of the basement, groundwater monitoring shall be carried out every two months for a period of 6 months. Settlement Trigger Levels Tonkin and Taylor Ltd will review the results of this monitoring and compare with the trigger levels presented in Table 3.3 below. Table 3.3: Settlement Trigger Levels Alert Level Action Level Building Settlement 7 10 Ground Settlement 15 25 Differential settlement between any two building marks 1 in 750 1 in 500 Differential settlement between any two ground marks 1 in 500 1 in 250 3.4.2 Response procedure if the settlement Alert Trigger Levels are exceeded If the alert trigger levels are reached then one or more of the contingency options described in section three of this plan will be carried out together with the following: i. Alert the Manager in writing within 24 hours of exceedance, ii. iii. iv. Increase the survey monitoring to weekly intervals, Revise settlement modelling, Report within two weeks to The Manager of the settlement review. 3.4.3 Response procedure if the settlement Action Limits are exceeded Alert the Manager in writing within 24 hours of exceedance.
7 The Manager may require all work that has potential to cause settlement to cease until The Manager is satisfied. 3.5 Potential contamination monitoring We understand that a detailed site investigation and analysis of soils in accordance with the Contaminated Land Management Guidelines (ref 5) is proposed prior to construction to ascertain the presence, if any, of contamination on the site along with any remediation required. Any potential for groundwater discharges to be contaminated should be considered as part of these investigations.
8 4 Contingency Options 4.1 General The monitoring data will be gathered and compared with trigger levels. If any of the specified trigger levels are exceeded the general response will be to; Alert the Consent and Consent Compliance Manager (AC), Activate re-charge wells Revise the model of the expected response, Revise predictions of final response, Select remedial action, Carry out remedial works and review results. Specific actions will be selected depending on the exact nature of the problem. The available contingency actions are detailed in the following sections. 4.2 Groundwater drawdown contingency measures The retaining wall is to be constructed as an essentially watertight structure. Some seepage is expected through the base of the excavation prior to the construction of the basement floor. In the event of groundwater drawdown exceeding the monitoring trigger level limits, the following actions may be taken: Activate the re-injection wells Install additional re-injection wells Grout behind the wall in areas of excessive seepage. Seal (grout) any seepage paths which become evident in the base of the excavation. Construct a grout curtain to reduce inflow. Backfill the excavation. 4.2.1 Groundwater recharge Groundwater recharge has been identified as a contingency measure to mitigate groundwater drawdown and potential settlement. The following monitoring requirements have been adopted for when shallow reinjection wells are in place. Reporting shall be in accordance with Section 3.3 above. The groundwater recharge wells will be used to inject potable water into the underlying alluvial and ECBF around the excavation in order to mitigate changes in groundwater levels. The reinjection wells will be put in place at the commencement of construction and will consist of the following: 20 m deep 150mm diameter wells spaced at approximately 20 m centres (depending on services) along the southern and eastern boundary. Well to be constructed of Class C upvc pressure pipe of 80NB to 100NB with 15 m long screen with the top of the screen at 5 m below the ground level. A water supply manifold connected to a header tank with float valve and flow meter. Groundwater recharge must cease if groundwater rises above a certain maximum level, as addressed within Table 4.1.
9 Table 4.1: Recharge Trigger Levels Trigger level Alert Level +0.2m above maximum recorded groundwater level Action Level +0.5m above maximum recorded groundwater level Action Increase monitoring to daily Cease water injection until groundwater levels decrease below action levels. 4.3 Retaining wall deflection contingency measures If the retaining wall deflections exceed the monitoring limits, a review of the retaining design model will be carried out to assess the increased load in the piles, anchors and props. If required the following action may be taken: Install additional props or ground anchors, Reduce load on the wall by allowing water to drain from behind the wall. This would be temporary while one of the other mitigation measures is undertaken. Groundwater levels will also be monitored as detailed in Section 3.3. Place a berm of soil in front of the wall 4.4 Excessive ground settlement Should excessive settlement occur adjacent to the site, one or more of the following actions will be carried out: Discussions on the situation with the property/services owner that may be affected, Monitor the rate of settlement (assuming that other steps have been undertaken to address the cause), Check public safety is maintained.
11 6 References 1 Auckland Council. http://unitaryplan.aucklandcouncil.govt.nz/pages/plan/book.aspx. Part 3. Chapter H. Section 4.17 2 Auckland Council (6 October 2014). Letter to Mt Hobson Group. Resource Management Act 1991. Resource Consent Application at 317-319 New North Road, Kingsland. R/LUC/2014/2425. Request for further information pursuant to Section 92. 3 Department of Building and Housing (1 December 2008) Compliance document for New Zealand Building Code. Clause B1. Structure. 4 Holmes Consulting Group Ltd (26 August 2014 @ 9:53am). Email from Sam Cooper to Andrew Langbein. 18 Aitken Property file. 5 Ministry for the Environment (October 2011). Contaminated Land Management Guidelines No. 5. Site Investigation and Analysis of Soils 6 Tonkin & Taylor Ltd (August 2014). Report to Joint Investment Holdings Ltd. Kiwi Bacon 317 to 319 New North Road, Kingsland. Supplementary Geotechnical Investigation. Ref 23766.1 7 Tonkin & Taylor Ltd (23 October 2014). Email to Mark French. RE: Kiwi Bacon Geotechnical. 8 Tonkin & Taylor Ltd (November 2014). Report to Joint Investment Holdings Ltd. Kiwi Bacon 317 to 319 New North Road, Kingsland. Groundwater Seepage and Preliminary Retaining Wall Analysis Report. Ref 23766.1
Appendix A: Drawings Groundwater monitoring and settlement monitoring location plan