Sensitive pyroclastic soils in the Bay of Plenty, New Zealand: microstructure to failure mechanisms

Abstract

Sensitive soils derived from weathered rhyolitic pyroclastic materials are associated with many landslides in the Bay of Plenty. Undrained, consolidated static triaxial tests show contractive p’-q’ plots, strain-softening stress-strain behaviour coupled with rising pore water pressures, shear band formation after peak strength, and considerable strain softening. Cyclic triaxial tests confirm brittle failure and extensive softening of the soil. Pore pressure gradients developed during shearing initiate collapse of clay microstructures into shear zones; further excess pore pressure generation in the shear zone leads to progressive failure. Halloysite, a low-activity clay mineral, is associated with sensitive layers within the pyroclastic sequences. Mushroom cap–shaped spheroidal halloysite results in weak short-range interactions between exposed clay surfaces on incomplete spheroids. This weak bonding allows disassociation of clay aggregates during slope failure, leading to strain softening and the development of flow post-failure.