Layer: Soil Classification (ID: 0)

Name: Soil Classification

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Description: It has long been established that earthquake ground motions may be amplified by weak deposits such as soils and other unconsolidated sediments. Seismic microzonation aims to identify these amplification effects through the classification of subsoil classes or local ground shaking properties and appropriate engineering design. In New Zealand, the current building code NZ1170.5 (Standards New Zealand 2004) specifies discrete subsoil classes ranging from A (hard rock) to E (very soft sediment).This project has developed new subsoil class maps for the Wellington City Territorial Authority area managed by the Wellington City Council. The maps were generated from new geological mapping available for the area, an updated borehole database, a database of site period measurements and a 3D geological model constructed for this study. The updated mapsuse the 2004 NZS1170.5 standard subsoil classification.The area mapped in this study extends from Tawa southwards, encapsulating Johnsonville, Newlands, Khandallah, Wilton, Karori, Ngauranga, Horokiwi, Thorndon, Te Aro, Newtown, Island Bay, Kilbirnie, Miramar and Seatoun. These areas all have accumulations of looseto dense Quaternary sediment deposited on indurated and variably weathered Rakaiaterrane basement rock. The sediment in these areas varies in thickness from a few metresto several hundred metres. The soft sediments are prone to amplification of ground motion during earthquakes, particularly where significant thicknesses are preserved.The subsoil classifications mapped in Wellington City are NZS1170.5: B, C, D and E. To create the boundaries and subsoil class maps, a 3D geological model has been constructed,drawing on databases of site period measurements and borehole data. In total, 931 site period measurements from previous studies are compiled in the database, with some reprocessed for this study. Records from 2422 boreholes are also compiled into another database from sources such as the New Zealand Geotechnical Database (NZGD), GNS Science and private records available to GNS Science for research purposes. A 3D geological model was created from these data and from existing geological mapping to model the sub-surface geometry of11 geological units and create depth to basement and thickness maps. All of these data were utilised to create a map of the subsoil classification in Wellington City and improve on previous studies through the integration of new data compiled and advanced modelling techniques.Several areas around Wellington are data-poor and lack borehole information or geophysical measurements to constrain the subsoil classes and their boundaries. We recommend that additional data is collected in important zones, such as those in Kilbirnie and Miramar where subsoil classes D and E are modelled, allowing their extent to be better determined.The derivative maps are intended only to be used as a general guide to site conditions and site subsoil class. They should not be used to make site-specific assessments for geotechnical design purposes. The derived maps are intended to provide a set of likely baseline conditions that can be refined with the results of more detailed ground investigation. This study is based on the best information available at the time of compilation. It is important that the end-user of these data understands the limitations and uncertainties in the primary and derived data sets. The conclusions drawn from this work may be subject to change as more information is made available in the future.

Copyright Text: Hill MP, Kaiser AE, Bourguignon S, Bruce ZR. 2021. Seismic microzonation map for Wellington City – adoption of NZS1170.5 subsoil classifications. Lower Hutt (NZ): GNS Science. 65 p. + 1 map. Consultancy Report 2020/119.

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