On the role of volcanic ash deposits as preferential submarine slope failure planes

Abstract

Volcanic ash deposits have been proposed to potentially liquefy and act as preferential failure planes during submarine landslides along active margins. However, particle angularity and surface roughness of volcanic material significantly influence its shear behavior and hence do not unambiguously support this contention. This manuscript challenges the hypothesis of preferential failure of ash layers by conducting drained and undrained monotonic and undrained cyclic triaxial shear experiments (σ’n < 1 MPa) on volcanic silts/sands and quartz sands as a reference material. Results attest the key role of particle geometry and particle strength in sediment stability. Decreased failure susceptibility of volcanic material compared to smoothed, rounded, and hard-grained quartz sand was attributed to roughness, angularity, and low crushability at low effective confining stresses in drained monotonic and undrained cyclic shear experiments. However, in undrained monotonic shear experiments on soft-grained and porous volcanic material, we observe major weakening due to crushability and excess pore pressure buildup related to particle internal water expulsion. In contrast, hard-grained angular volcanic particles are less susceptible to liquefaction than grain-size equivalent-rounded quartz sand, and may in fact be favorable for seismic strengthening, in which ground shaking causes enhanced settling rather than failure. Our data suggest that ash layers may not serve as preferential failure planes in the majority of earthquake-triggered submarine landslides along active margins.