DISCLAIMER This report has been prepared by the Institute of Geological and Nuclear Sciences Limited (GNS Science) exclusively for and under contract to Earthquake Commission, Wellington City Council, ACC and Wellington Regional Civil Defence Emergency Management Group. Unless otherwise agreed in writing by GNS Science, GNS Science accepts no responsibility for any use of, or reliance on any of the contents of this Report by any person other than Earthquake Commission, Wellington City Council, ACC and Wellington Regional Civil Defence Emergency Management Group and shall not be liable to any person other than Earthquake Commission, Wellington City Council, ACC and Wellington Regional Civil Defence Emergency Management Group, on any ground, for any loss, damage or expense arising from such use or reliance. The data presented in this Report are available to GNS Science for other use from June 2010 BIBLIOGRAPHIC REFERENCE Boon, D. P., Perrin, N. D., Dellow, G. D., Lukovic, B. 2010. It’s Our Fault – Geological and Geotechnical Characterisation and Site Subsoil Class revision of the Lower Hutt Valley, GNS Science Consultancy Report 2010/163. 60 p. Project Number: IOF430W3510 Confidential 2010 CONTENTS EXECUTIVE SUMMARY ........................................................................................................III 1.0 INTRODUCTION ..........................................................................................................1 1.1 Site class determination in New Zealand ...................................................................... 2 1.2 Report overview............................................................................................................. 2 2.0 GEOLOGICAL AND GEOTECHNICAL CHARACTERISATION.................................2 2.1 Introduction....................................................................................................................2 2.2 Previous characterisation studies.................................................................................. 3 2.3 Topographic data sources for the digital terrain model................................................. 3 2.4 Hutt Valley Drillhole Database....................................................................................... 4 2.5 Geology .........................................................................................................................5 2.6 Geotechnical Characterisation .................................................................................... 14 2.7 Engineering geological model ..................................................................................... 19 3.0 SHEAR-WAVE VELOCITY CHARACTERISATION..................................................27 3.1 Introduction.................................................................................................................. 27 3.2 Methodology................................................................................................................ 27 3.3 Available shear-wave velocity profiles......................................................................... 27 3.4 Results of Vs characterisation..................................................................................... 28 3.5 Discussion of Vs characterisation ............................................................................... 31 3.6 Comparison of Vs estimates with Californian analogues............................................ 33 3.7 Is there a shear-wave velocity gradient in the basin? ................................................. 34 3.8 Evaluation of the new noise interferometry technique ................................................ 34 3.9 Comparison of geological and geophysical model results .......................................... 35 3.10 Limitations of the characterisation results ................................................................... 36 4.0 THEMATIC MAPS......................................................................................................37 4.1 Depth to bedrock map ................................................................................................. 37 4.2 Low-amplitude natural period (or site period) map...................................................... 39 4.3 Vs30 map .................................................................................................................... 41 4.4 Site subsoil class map................................................................................................. 43 5.0 CONCLUSIONS .........................................................................................................45 6.0 RECOMMENDATIONS ..............................................................................................47 7.0 ACKNOWLEDGEMENTS ..........................................................................................47 8.0 REFERENCES ...........................................................................................................47 GNS Science Consultancy Report 2010/163 i Confidential 2010 FIGURES Figure 1 Location of Lower Hutt Valley study area...................................................................................1 Figure 2 Geological map of the study area............................................................................................11 Figure 3 Plan showing type and distribution of data available for this study..............................................13 Figure 4 Plot showing all SPT results in Lower Hutt. Depth of N-value plotted is at the base of the test depth interval........................................................................................................................15 Figure 5 SPT 'N' value distributions for mapped Recent (Taita) alluvium (fa) and marginal marine deposits (fm). The peaks for N=50 and N=60 result from inconsistent recording methods by drillers who have different criteria for test “refusal”....................................................................16 Figure 6 An exploded view of the Lower Hutt valley engineering geological model (viewed obliquely, looking north)........................................................................................................................20 Figure 7 The GSI3D software window illustrates the 3D modelling workflow. Map window (top left) shows drillhole data, maps, geological maps, and location of cross-sections; the cross- section window (bottom left) is where the cross-sections are manually drawn. Note how the sections are capped by the DTM surface and drillhole records are visible as graphic logs. The modeller digitises unit basal boundaries to construct each cross-section using the drillhole sticks to build correlations. The drillhole log window (bottom right) displays drillhole database data in the desired graphic log format (e.g. depth, collar height, drillhole id, descriptions). The 3D window (top right) displays the calculated TIN surface, here the reclaimed land at Seaview (defined by correlation points and nodes). Here the modeller visually checks the outcome of the volume calculation to ensure the TIN calculation is geologically realistic. .............................................................................................................22 Figure 8 Oblique view, looking north, of the cross-section fence diagram that forms the skeleton of the 3D model. .......................................................................................................................23 Figure 9 A series of vertical synthetic cross-sections (A to F) generated by slicing the 600 m thick 3D engineering geological model reveal the basin structure at various locations across the Valley. No vertical exaggeration..............................................................................................24 Figure 10 Summary of final shear-wave velocity ranges (top) for engineering geological model units 2– 7 (no data for unit 1 in Lower Hutt Valley). Original data values summarised into 15 m/s bins and displayed as histograms (centre) distinguished by colour to indicate original data source (bottom). ..............................................................................................................................30 Figure 11 Histogram showing frequency distribution of shear-wave velocity estimations for Unit 2. Values coloured by technique and assigned to 15 m/s bins for clarity. ........................................32 TABLES Table 1 Summary of Hutt Valley drillhole database fields. The seq_no (#) is the primary key for all tables.....................................................................................................................................5 Table 2 Engineering geological unit definitions used in the 3D model (numbered 1–7).............................18 Table 3 Shear-wave velocities summarised for each engineering geological unit in the Lower Hutt basin. Vs values are rounded to the nearest 5 m/s. ..................................................................29 Table 4 Comparison of the unit boundary depths between the 3D engineering geological model and the geophysical Vs model. Depth values given as an average (rounded to the nearest metre) with one standard deviation given in square brackets. *indicates only 1 comparison available. .............................................................................................................................35 MAPS Map 1 Depth to bedrock...................................................................................................................38