Median water table elevation in Christchurch and surrounding area after the 4 September 2010 Darfield Earthquake. Version 2

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van Ballegooy, S.; Cox, S.C.; Thurlow, C.; Rutter, H.K.; Reynolds, T.; Harrington, G.; Fraser, J.; Smith, T. 2014 Median water table elevation in Christchurch and surrounding area after the 4 September 2010 Darfield Earthquake. Version 2 . Lower Hutt, N.Z.: GNS Science. GNS Science report 2014/18 79 p. + 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. Understanding the future liquefaction vulnerability is a major engineering consideration when determining how to rebuild on liquefaction prone land. The depth to the water table is a fundamental parameter when undertaking liquefaction vulnerability assessments that aim to determine the potential future damaging effects of liquefaction for the purposes of building foundation design. Maps of the median water table elevation, percentile statistics of its fluctuation, and depth below ground were derived for Christchurch City and surrounding area, for the period from 4 September 2010 (Mw7.1 Darfield Earthquake) to 30 November 2013. The study includes data covering an area between Prebbleton in the southwest, Swannanoa in the northwest, and the coastline in the east, but is specifically intended for use in Christchurch residential land (zoned TC1, TC2, TC3, and Red Zone) within this area. Monitoring well, contour and grid data sets will be provided via the Canterbury Earthquake Recovery Authority (CERA) Canterbury Geotechnical Database (CGD) website. Map development necessitated a number of assumptions and interpretations, not immediately evident in the final product. This report provides documentation of the methodology, data sets, assumptions used to derive the water table maps, confidence levels and caveats, as well as new observations, results and recommendations for future work. Data from 967 shallow monitoring wells were obtained from Christchurch City Council (CCC), Environment Canterbury (ECan) and geotechnical investigations carried out for the Earthquake Commission (EQC). 657 monitoring wells had records of twelve months or longer from which a representative median water table elevation was determined for each site. 310 wells had short-term records, from which surrogate medians were estimated by using longer-term data from nearby wells to account for seasonal variation. 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 help shape and position the water table contour maps at places of significant groundwater-surface water interaction. (auth)