Performance of Taranaki Volcanic Silts under Cyclic Loading

Abstract

WSP has been involved in a number of building projects in New Plymouth. Some of the buildings are founded on volcanic silts (Taranaki ash). Field and simplified laboratory tests often indicate that these materials are sensitive in nature. Most of cohesive soils, even if they are non-liquefiable, can be prone to 10-30% strength loss due to cyclic softening under seismic load. However, strength loss in sensitive materials can be 4 to more than 50 times in some cases. There are no commonly accepted methods to predict strength loss for sensitive cohesive soils under cyclic loading. It was therefore critical to carry out a study of dynamic response of the sensitive soils for one of building sites in New Plymouth. Triaxial tests (cyclic and monotonic) of undisturbed soil samples were carried out to confirm dynamic response of the sensitive soils. The tests provided valuable information on the dynamic behaviour of the soils and enabled the designers to assess stability of the building platform and develop reliable structural foundation options with higher level of reliability.