The seismic response of volcanic sites

Abstract

This report presents both an analytical and experimental investigation of the seismic response of volcanic soil sites. This important area of investigation has very little published research data. Volcanic soils are commonly found in the Auckland and central North Island areas of New Zealand.

A numerical two dimensional effective stress method of analysis was developed that allowed the inclusion of the in-plane geometry of a site when determining its site response and liquefaction potential. Liquefaction of volcanic sands was observed during the 1987 Edgecumbe earthquake, and the effective stress analysis provides a means of gaining an insight into this behaviour.

A cyclic triaxial testing programme on a central North Island pumiceous sand was undertaken to evaluate its dynamic properties. These results indicated that the pumice sand behaved similarly to other sands when subjected to the relatively low stress dynamic motions. The induced stresses were found to be of insufficient magnitude to induce grain crushing. This testing programme allowed an indepth investigation of two numerical pore pressure models to be carried out. These results showed that the models were capable of simulating the dynamic behaviour of the volcanic sand with reasonable accuracy.

The influence of permeability on the liquefaction process was investigated using a one dimensional effective stress solution. It was found that the relatively high permeabilities of volcanic sands significantly reduced the liquefaction response due to excess pore water drainage.

Low strain analyses suggested that the influence of the boundaries in the two dimensional solution was to stiffen the overall properties of the basin when the aspect ratio is decreased. High magnitude (non linear) analyses showed a more complex behaviour in which there were no clear trends as to the influence of basin geometry. Effective stress analyses showed that the lateral boundaries can also have a significant effect on the liquefaction response of a site.