Median water table elevation in Christchurch and surrounding area after the 4 September 2010 Darfield Earthquake

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van Ballegooy, S.; Cox, S.C.; Agnihotri, R.; Reynolds, T.; Thurlow, C.; Rutter, H.K.; Scott, D.M.; Begg, J.G.; McCahon, I. 2013 Median water table elevation in Christchurch and surrounding area after the 4 September 2010 Darfield Earthquake. Lower Hutt, N.Z.: GNS Science. GNS Science report 2013/01 66 p. + 8 appendices

Abstract: The sequence of Canterbury earthquakes during 2010 and 2011 caused substantial changes to land in Christchurch City and surrounding areas, including widespread uplift and subsidence, liquefaction, ground surface deformation and lateral spreading. Liquefaction and liquefaction risk is now a major engineering concern for land remediation and assessment in the city. The depth to the water table is a fundamental parameter when undertaking liquefaction assessments which aim to determine the potential future damaging effects of liquefaction for the purposes of building foundation design. Maps of the median water table elevation and depth below ground have been derived for Christchurch City and surrounding area, for the period since the 4 September 2010 Mw7.1 Darfield Earthquake. The study covers an area between Prebbleton in the southwest, Swannanoa in the northwest, and the coastline in the east. Monitoring well contour and grid data sets will be provided via the Canterbury Earthquake Recovery Authority (CERA) Canterbury Geotechnical Database (CGD) website, then updated periodically as new data become available. Users will need to understand the assumptions and limitations of the calculated median water table. This report provides documentation of the methodology, data sets, assumptions used to derive the water table maps, confidence levels, as well as new observations, results and recommendations for updates in the future. Data from 806 shallow monitoring wells were obtained from Christchurch City Council (CCC), Environment Canterbury (ECan) and geotechnical investigations carried out for the Earthquake Commission (EQC). 550 monitoring wells had records of nine months or longer from which a representative median water table elevation was determined for each site. 256 wells had short-term records, from which surrogate medians were estimated using data from nearby wells. LiDAR Digital Elevation Models and physical survey data were used to correct the monitoring well measuring points for ground elevation changes caused by the earthquakes. River and coastline data were used as breaklines to help shape and position the water table contour maps at places of significant groundwater-surface water interaction. Differences between the (preferred) median water table derived from post-Darfield Earthquake longer-term data ( 9 month records), and a surface derived using surrogate medians derived from short-term (< 9 month) data, are subtle and in most places ± 0.5 m. There are strengths and weaknesses in each surface, so end users should decide which is more appropriate depending on their specific engineering requirements and local site observations. (auth)