Performance of building foundations on gravel during the Christchurch earthquakes

Abstract

A dramatic consequence of the Christchurch earthquakes of 2010 and 2011 was the widespread liquefaction in the city. However, large volumes of ejecta were not evident for those parts of the CBD where the upper layers in the soil profile are sandy gravel and gravelly sand and the shallow foundations for mid to high-rise buildings were found to have performed well. The paper summarises two threads of our current investigation into the satisfactory performance of these foundations. First, our work on the effect of the gravel permeability on the rise and dissipation of excess pore water pressure during cyclic loading of a soil profile idealised from Christchurch geotechnical data is summarised. The Cyclic1D software, which performs one-dimensional nonlinear effective stress site response analysis, was used. Permeability values associated with gravel were found to suppress the cyclic accumulation of excess pore water pressure in the gravel layers. Thus the gravels will not have experienced a loss in stiffness and shear strength during the earthquake shaking. Second, consideration of the effect of the rocking of the shallow foundations on the seismic response of the structures is also summarised. We report briefly on the centrifuge modelling of shallow foundation rocking on dense dry sand and associated spring bed modelling using nonlinear detachable and re-attachable springs. A surprising conclusion from this modelling is that small amounts of rocking have a large beneficial effect on the building response.