Exploring the entrainment of liquefied bed material in landslides

Landslide runout analyses are conducted to predict the motion and distal reach of potential future landslides to inform landside hazard zonation, risk management, and the design of the optimal location and height of mitigation strategies such as barrier systems. A key uncertainty in these analyses relates to the erodibility and entrainment of sediment which may unexpectedly increase the volume of the landslide and affect travel distance. In this study we explore the case of a landslide overriding and entraining loose saturated valley floor sediments; in particular, whether such a scenario may cause the overridden sediments to liquefy, and if so, the extent to which that liquefaction affects the depth of erosion of the bed material and mobility of the slide.

The presence of a loose saturated layer of soil at the base of a slope inherently introduces a soil region potentially prone to static liquefaction. This scenario fulfills the criteria required for instability: a) loose contractive soil, which happens to be co-located in an area that is both b) fully saturated, and c) within reach of a shear trigger (i.e., being overridden by the landslide). This scenario was reproduced in the Queen’s landslide flume, using a horizontal liquefiable bed of saturated fine sand 2 m in width, 7 m in length, and 0.3 m in height located at the bottom of the inclined portion of the flume. Landslides of up to 1,200 kg of granular material were then released from the top of a 6.5 m long slope inclined at 30 degrees to impact the bed at speeds of up to 6 m/s. Behaviour of the sand bed upon impact was captured using ultrahigh speed imaging of the landslide and bed profiles, a Blickfeld LiDAR sensor positioned opposite to the landslide to capture point cloud scans of the slide, and a linear array of porewater pressure sensors within the sand bed. Preliminary results of experiments attempting to liquefy the valley floor sediments are presented as we explore the requisite conditions for different rates of entrainment, and in extreme cases, liquefaction, of the sand bed, as well as the effect of liquefaction and entrainment on the speed and volume of the slide.