FRICTION-REDUCTION MECHANISMS THAT AFFECTED THE MOBILITY OF THE MAY 2014 WEST SALT CREEK ROCK AVALANCHE, MESA COUNTY, COLORADO

The 54.5 Mm³, highly mobile (height/length = 0.14) West Salt Creek rock avalanche had features consistent with two primary friction-reduction mechanisms: (1) dynamic fragmentation and (2) basal liquefaction, slip, and entrainment. Isolated crazed boulders, gradual downslope fining of the debris with rounding of clasts and decreasing abundance of brittle mudstone boulders are consistent with dynamic fragmentation. A May 2016 outburst flood cut downward as much as 32 m into the avalanche deposit, revealing the following features that are consistent with the avalanche gliding on a low­resistance bed: A slump block at the head of the rock avalanche contains large, tilted blocks of shale bedrock and dilated rockslide debris resting in a sharp, northward dipping contact on a water perching, clay­rich diamicton. In the rock avalanche deposit’s central core, cuts through hummocks exposed dilated rock-slide debris with no discernable internal structures or grading. In the toe area, the rock-avalanche deposits consist of loose, unsorted shale fragments with a few basalt boulders and logs. A laterally extensive layer of clay­rich material containing angular shale fragments with flow bands containing entrained organic debris, and a very irregular upper contact forms the base of the rock-avalanche deposits. This layer rests on a sequence of sediments topped by a well-developed, clay-rich soil.

Avalanche simulations using the numerical model DAN (Hungr, 1995) yielded estimates of travel distance as a function of strength parameters for models representing dynamic fragmentation and basal slip to determine whether either model yields travel distances consistent with observed distance. Due to thin distal deposits, observed travel distance of the avalanche by dynamic fragmentation alone is achievable only if shale strength is lower than measured by others or the internal friction coefficient of the fragmented rock is unusually low. In contrast, the travel distance is easily achievable by basal slip given the low residual friction angle of the clay (9°–15°) and 33% – 60% strength reduction induced by pore pressure rise from liquefaction during rapid shear.