Dynamic site characterization of central Auckland reclamation zones

Abstract

The original coastline along the central Auckland City waterfront has been progressively modified since 1840, through a series of reclamation fills. These fills are highly variable, comprising a range of materials including Excavated Rock Fill, Hydraulic Fill and discrete lenses of Industrial and Domestic Waste. Traditional methods of characterising the seismic behaviour of ground are based on an assessment of Cone Penetration Tests (CPT’s) and Standard Penetration Tests (SPT’s). However, these techniques fail to adequately capture heterogeneity of reclamation fills. Geophysical techniques, such as horizontalto- vertical spectral ratio (HVSR) and multi-channel analysis of surface waves (MASW) have the potential to provide overall profile characteristics of the variability of these reclamation fills and lead to an improved understanding of their dynamic response during earthquakes. The aim of this research is to characterise the dynamic properties of the soils in the reclaimed zones along the central Auckland City waterfront. To achieve this, historic documentation was collated to produce a comprehensive reclamation zone map and historic timeline. Additionally, past subsurface investigations were compiled to produce a depth to ECBF map and representative geologic profiles for each reclamation zone. MASW was used to generate Vs profiles at eight locations and HVSR was used to determine fundamental period at 156 locations. These were conducted with in-situ dynamic soil properties which are considered largely undisturbed and then calibrated using past subsurface investigations. Geologic profiles for each reclamation zone was made using borehole data along with associated SPT and CPT data collated from past geotechnical investigations. These profiles were used to constrain MASW and HVSR to determine the characteristics of each reclamation zone and produce maps illustrating the fundamental period and site subsoil in accordance with NZS1170.5:2004. Site subsoil classes C and D were identified. Site subsoil D was located proximal to paleo-channels with rapid change in depth to ECBF and containing thickened deposits of reclamation fills overlying young, unconsolidated alluvial sediments. The complexity of the site is compounded with the heterogeneous nature of reclamation fill deposits with the presence of paleo-channels. Results indicate highly variable Vs averages across zones and within the same zone therefore site specific assessment is necessary. Further, results indicate that HVSR is a valuable technique and can be considered a good alternative to deriving depth to bedrock and fundamental period at locations which MASW is not logistically possible. Findings gained from this research will allow more accurate hazard and risk estimates for Auckland City.