Abstract
This paper presents an overview of the soil profile characteristics at strong motion station (SMS) locations in the Christchurch Central Business District (CBD) based on recently completed geotechnical site investigations. Given the variability of Christchurch soils, detailed investigations were needed in close vicinity to each SMS. In this regard, CPT, SPT and borehole data, and shear wave velocity (Vs) profiles from surface wave dispersion data in close vicinity to the SMSs have been used to develop detailed representative soil profiles at each site and to determine site classes according to the New Zealand standard NZS1170.5. A disparity between the NZS1170.5 site classes based on Vs and SPT N60 investigation techniques is highlighted, and additional studies are needed to harmonize site classification based on these techniques. The short period mode of vibration of soft deposits above gravels, which are found throughout Christchurch, are compared to the long period mode of vibration of the entire soil profile to bedrock. These two distinct modes of vibration require further investigation to determine their impact on the site response. According to current American and European approaches to seismic site classification, all SMSs were classified as problematic soil sites due to the presence of liquefiable strata, soils which are not directly accounted for by the NZS1170.5 approach.
Highlights
STRONG MOTION NETWORK OVERVIEWAn extensive database of strong ground motion records has been captured by the strong motion station (SMS) network in the Canterbury region during the 2010-2011 Canterbury earthquake sequence [1,2]
This paper provides a detailed update of the soil profile characteristics at strong motion stations in the Christchurch Central Business District (CBD) based on recently completed geotechnical site investigations
These characterization efforts provide a basis for the development of site response analysis models to better understand the observed ground motions in the 2010-2011 Canterbury Earthquake Sequence
Summary
STRONG MOTION NETWORK OVERVIEWAn extensive database of strong ground motion records has been captured by the strong motion station (SMS) network in the Canterbury region during the 2010-2011 Canterbury earthquake sequence [1,2]. Representative soil profiles were developed for each of the four CBD SMSs using cone penetrometer testing (CPT), boreholes and standard penetration testing (SPT), shear wave velocity (Vs) profiling via surface wave methods, and horizontal to vertical spectral ratio (H/V) data performed/obtained in close vicinity to the station location [3]. This data was used to 1) assess the liquefaction potential of each site, which was compared to the surface manifestation and ground motion characteristics recorded during the Darfield and Christchurch earthquakes, and 2) define the NZS1170.5 [4] site classes with more confidence than the previously assumed classifications. To compare to New Zealand practice, sites were classified using the National Earthquake Hazards Reduction Program (NEHRP) [5] and Eurocode 8 (EC8) guidelines [6] following American and European practice, respectively
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