A Conceptual Model of Pore Pressure as Trigger of Subaqueous Landslides based on in situ Measurements

In addition to mineralogical composition, pore fluid pressure is a crucial factor controlling the effective strength and mechanical behaviour of water-saturated sediments. A large number of research projects on the stability of submarine sediments exists, however, most of the previous work has not been measured in situ, but has been based on geophysical or sedimentological data. Here, we present a study on earthquake-triggered slope failures in Lake Lucerne (Switzerland) where detailed geotechnical in situ measurements (CPT, Vane Shear) shed light on the landslide-initiation mechanisms. In situ shear strength and pore pressure are complemented by shear and consolidation behaviour measured in the laboratory. We identify two lithological units: soft post-glacial clays with a mean undrained shear strength cu of 6 kPa overlying more competent (max. cu= 16 kPa), fine-grained glacial deposits with coarser components and excess fluid pressures (in places near lithostatic). The boundary between the two units with different consolidation histories (OCR = 0.86 in post-glacial vs. OCR = 1.58 in glacial sediments) represents the sliding surface of the landslide. Based on the principle of effective stress, we further discuss a conceptual model for earthquake-triggered failure initiation along the lithological boundary between the two units. Co-seismic stress fluctuations cause small-scale hydrofracturing in the lower, over-consolidated section. Hence, stress is transferred upward to the base of the softer, less stable unit. Here, excess pore pressures initiates sliding along a failure plane developing at this boundary, causing the entire post-glacial sedimentary section to slip downslope. We propose that many submarine landslides may follow a similar failure mechanism induced by pore pressure transients from depth.