Landslide mobility enhanced by dynamic basal liquefaction of underlying sediments

Far-traveled landslides greatly increase hazard and risk. Although pervasive liquefaction in debris flows and flow slides can dramatically boost their mobility, the effects of liquefaction on the mobility of coherent landslides is more difficult to forecast. In 2014, the Oso landslide in Washington State, USA failed rapidly and swept across more than 1 km of the adjacent flat alluvial valley, killing 43 people. We mapped over 350 sand boils that emanated from the alluvium under the debris-avalanche hummock deposit. Although transient, these sand boils represent definitive evidence of sub-bottom (basal) liquefaction of the alluvium beneath the overriding slide. The hummocks in the slide mass were not liquefied and they commonly rafted upright vegetation, including coniferous trees, and intact layered glacial sediments across the valley floor. A liquefied base provides little shear resistance, greatly enhancing slide mobility. Our extensive laboratory testing and numerical modeling revealed that several mechanisms may have enhanced basal liquefaction at Oso: rapid undrained loading, shearing of contractive alluvial sediments, and cyclical loading from ground shaking associated with rapid emplacement. 

Here we further investigate the potential for a rapidly moving slide mass to dynamically liquefy underlying alluvial sediments through undrained loading. We use a fully coupled poro-elastic numerical model with parameters determined by laboratory tests of the valley alluvium at the Oso landslide site. Given a landslide speed of 10 m/s, estimated from seismic records of the event, our modeling demonstrates that rapid loading induces transiently elevated pore-fluid pressures nearly equal to the overriding landslide load. These pore-fluid pressures are capable of liquefying the saturated alluvium, reducing its shear strength, and enhancing mobility. Both landslide speed and the hydraulic conductivity of the underlying alluvium strongly modulate the potential for liquefaction. Slower landslide speeds and/or greater alluvial hydraulic conductivity allow simulated pore pressures from loading to dissipate before reaching liquefaction levels. Only specific combinations of these parameters promote basal liquefaction. Such basal liquefaction effects may enhance the mobility of other slides traveling rapidly across saturated alluvium in adjacent valley floors.