Earthquake Commission Darfield Earthquake Recovery Geotechnical Factual Report New Brighton

Transcription

1 REPORT Earthquake Commission Darfield Earthquake Recovery Geotechnical Factual Report New Brighton

2 REPORT Earthquake Commission Darfield Earthquake Recovery Geotechnical Factual Report New Brighton Report prepared for: EARTHQUAKE COMMISSION Report prepared by: TONKIN & TAYLOR LTD Distribution: EARTHQUAKE COMMISSION TONKIN & TAYLOR LTD (FILE) 3 copies 1 copy February 2011 T&T Ref: REP-NBT-FAC

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4 Table of contents 1 Introduction 1 2 Project background 1 3 Site description General Regional geology 2 4 Geotechnical investigations General Cone penetration testing (CPT) Machine boreholes Geophysical testing 4 5 Groundwater observations 4 6 Laboratory testing 4 7 Environment Canterbury information 4 8 References 4 Appendix A: Appendix B: Appendix C: Appendix D: Investigation Location Plans Cone Penetration Testing Geophysical Testing Environment Canterbury Boreholes

5 1 1 Introduction The Earthquake Commission (EQC) engaged Tonkin & Taylor (T&T) to undertake site investigations around selected suburbs of Canterbury following the Darfield Earthquake sequence that started on 4 September These site investigations comprise: a) Cone penetration testing b) Machine boreholes c) Geophysical testing d) Groundwater observations e) Laboratory testing Site investigation works and factual reporting is being undertaken for a number of suburbs that were affected by liquefaction induced land damage. The report contains factual data to provide a geotechnical source for council consent applications for the suburb. This report presents all currently available geotechnical and engineering geological investigations that were commissioned by EQC after the 2010 Darfield Earthquake and all readily available data that Environment Canterbury and Christchurch City Council hold for the suburb. This is a factual report of field investigations. The field investigations have been undertaken at discrete locations and no inferences about the nature and continuity of ground conditions away from the investigations locations are made. No interpretation of investigation results has been made in this report. 2 Project background The magnitude (M) 7.1 Darfield Earthquake occurred on 4 September The epicentre was close to the town of Darfield, approximately 40 km west of Christchurch. The earthquake caused extensive ground liquefaction, in localised areas of Canterbury. The liquefaction resulted in major ground settlement, lateral spreading and, to a lesser extent, foundation support failure, with consequential building damage. T&T carried out a broad geotechnical land damage assessment and mapping of the main urban areas in the Canterbury region that have been significantly affected by strong ground motion arising from the earthquake. The results of that assessment are presented in the T&T Stage 1 report (Ref 1) dated October 2010 and released to the general public by EQC. The Stage 1 report presented the land damage categorisation, mapping methodology, information and results generated to 1 October Land remediation concepts and generalised land remediation options were also presented. The T&T Stage 2 report (Ref 2) dated November 2010 and released to the general public by EQC is based on a detailed suburb-by-suburb engineering assessment into the nature and extent of land damage. It also identifies land repair strategies for each suburb and areas most affected by land damage. The report presents 22,500 mapped properties as at 5 November 2010 in Canterbury and separates them into three recovery zones; A, B and C. These recovery zones relate to the extent of land repair required and allow for a coordinated recovery plan.

6 2 3 Site description 3.1 General New Brighton subject area is located in the north of Christchurch approximately 6 km from Christchurch CBD (Drawing LOC-NBT-01, Appendix A). The subject area is within the Christchurch City Council jurisdiction. The Avon River runs through the south of the subject area. The area is low lying and topographically is typically flat. 3.2 Regional geology Geology of the Christchurch Urban Area (Ref 3) shows the subject area of New Brighton to be underlain by three different geologies, sand, silt and occasional peat deposits of drained lagoons and estuaries, sand of fixed and semi-fixed dunes and beach deposits, and alluvial sand and silt overbank deposits. All three deposits are of Holocene age. 4 Geotechnical investigations 4.1 General The geotechnical investigations commissioned by EQC from September 2010 were determined and located under direction from T&T. The locations were selected taking into account access for the site investigation plant, the presence of services and any traffic management requirements. The current investigations comprise: (a) 24 Cone penetration tests (CPTs) (b) 480 m of MASW geophysical testing The location of the site investigations is shown on Drawing GIP-NBT-01 (Appendix A) and each of the above investigation techniques is discussed below. 4.2 Cone penetration testing (CPT) Five companies were commissioned to provide CPT probing services on this project and information on the CPT rig capability is summarised briefly below. The output of each CPT rig varies and therefore the testing data has been produced on a standard T&T template. The company which carried out each CPT test is identified on the CPT log. The companies that carried out the testing were: a) Opus International Consultants Ltd (Opus) - 18T truck-mounted rig b) Perry Drilling Ltd (Perry) - 20T rubber tracked truck-mounted rig c) McMillan Drilling Services (McMillan) - 14T truck-mounted rig d) Geotech Drilling (Geotech) - 16T truck-mounted rig e) Ground Investigations Ltd (GI) - self-augering Pagani CPT rig. The rig augers reaction anchors into place in the ground in order to drive a standard CPTu cone with up to 20T of force. Selected CPT probes had down-hole seismic shear wave velocity testing carried out. These are identified on each CPT log and summarised in Table 4-1 below.

7 3 Each CPT probe location was pre-dug by hand to between 800 mm and 1200 mm to mitigate the risk of unforeseen underground services being present at the selected investigation locations. The results of the CPT probing are presented in Appendix B. The CPT testing undertaken is summarised in Table 4-1. Table 4-1 Cone penetration testing summary CPT ID Maximum depth (m) Location (me) Location (mn) Ground Surface R.L CPT-NBT CPT-NBT Comments CPT-NBT Seismic CPT CPT-NBT CPT-NBT CPT-NBT Seismic CPT CPT-NBT CPT-NBT CPT-BUR CPT-BUR CPT-BUR Seismic CPT CPT-BUR CPT-BUR CPT-BUR CPT-BUR CPT-BUR Seismic CPT CPT-BUR CPT-BUR CPT-BUR CPT-BUR CPT-BUR CPT-AVD CPT-AVD CPT-AVD Machine boreholes No machine boreholes have been carried out in the suburb of New Brighton.

8 4 4.4 Geophysical testing Southern Geophysical Ltd completed a series of geophysics tests at specified locations within the suburb. The testing comprised Multichannel Analysis of Surface Waves (MASW) and was used to provide a shear wave velocity profile with depth. The geophones used were typically spaced at 1.0 m centres, which have produced a vertical profile extending around 13.0 m below ground level. This technique was carried out at approximately 5.0 m centres to provide a two dimensional profile of subsurface shear wave velocities. Copies of the geophysical testing results are presented in Appendix C. 5 Groundwater observations In New Brighton, during the investigations, groundwater depths ranged 3.1 to 0.5 m below existing ground level. These identified levels were based on dynamic pore water pressure results in CPT testing and field observations of waterways (rivers and streams). 6 Laboratory testing No laboratory testing has been carried out for the subject area. 7 Environment Canterbury information Additional borehole data, which is held by Environment Canterbury for the subject area, has been included in Appendix D. This data has been reproduced with the permission of Environment Canterbury and forms part of previous investigations undertaken by a variety of geotechnical consultants. Records for boreholes outside of the area covered by this report may be obtained from Environment Canterbury. 8 References 1. Tonkin & Taylor Ltd (October 2010), prepared for the Earthquake Commission Darfield Earthquake 4 September 2010 Geotechnical Land Damage Assessment & Reinstatement Report STAGE 1 REPORT. 2. Tonkin & Taylor Ltd (November 2010), prepared for the Earthquake Commission Darfield Earthquake 4 September 2010 Geotechnical Land Damage Assessment & Reinstatement Report STAGE 2 REPORT. 3. Brown, L.J.; Weeber, J.H. (1992), Geology of the Christchurch urban area. Institute of Geological & Nuclear Sciences 1:25,000 geological map 1. 1 sheet + 104p. Lower Hutt, New Zealand, Institute of Geological and Nuclear Sciences Limited.

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