Static failure mechanisms in sensitive volcanic soils in the Tauranga Region, New Zealand

Sensitive soils derived from weathered rhyolitic volcanic materials have contributed to major landslides in the Bay of Plenty. Soil was sampled from sensitive layers near the failure surface of two coastal landslides bordering Tauranga Harbour. Methods were adapted from Gylland et al. (2014) including undrained, consolidated static triaxial tests at a high compression rate of 0.5 mm/min in order to replicate rapid loading during landsliding. Like sensitive materials derived from glacial till, sensitive volcanic derived material showed contractive p-q plots, strain softening stress- strain behaviour, coupled with rising pore pressures, and single or double shear band formation after peak strength was reached. Little to no cohesion or friction softening occurred between peak and residual states. This evidence indicates that like sensitive soils derived from glacial till, the low permeability of the clay allows pore pressure gradients to evolve, eventually initiating collapse of clay microstructures into shear zones, where further excess pore pressure generation within the shear zone instigates progressive failure. Shear zone development causes a rapid loss of global resistance, expressed as strain softening behaviour.