Seismic Ground Response at a Deep Soft Soil Site: Comparing 1D Numerical Solutions with Observed Data from the Christchurch Earthquakes

The earthquake hazard for an engineered structure is influenced by the dynamic response of the soils that underlie the site. During seismic shaking, bedrock motions are altered when travelling through surficial soils. Analytical methods to model these effects have developed over the last few decades. This paper compares the observed ground response at a deep soil site at Lyttelton Harbour, Christchurch with the calculated response using both equivalent-linear and non-linear 1D analytical methods. Ground motion records from two seismographs were utilised; one located on a nearby rock outcrop and the other at the surface of a 60m deep soft soil profile. The observed ground response indicates the presence of non-linear, hysteretic and 3-dimensional effects at the site. Comparisons between the observed and calculated acceleration response spectra, peak ground accelerations and site periods are made. The non-linear analysis demonstrates an overall good fit to the observed data, and is shown to outperform the NZS1170.5 subsoil class method for most periods, emphasising the importance of carrying out a ground response analysis for important structures.