Mobilization of debris flows from soil slips, San Francisco Bayregion, California
The thousands of debris flows that mobilized from shallow slides in the San Francisco Bay region during the rainstorm of January 3-5,1982, left evidence of the range in soil textures susceptible to mobilization and of differences in completeness and speed of mobilization. These differences in mobilization are related to a broad range in the ratio of saturated water content to liquid limit, which we have used as an approximate index of mobilization potential. To understand such differences in mobilization, we have explored the transformation from slide to flow, using relations among inplace void ratio, void ratio needed for flow from the slide scar, and the steady-state line. These relations define two principal means of direct transformation from slide to flow: contractive soil behavior, which commonly results in liquefaction, and dilative soil behavior, which in many cases probably results in partial mobilization of the slide mass. These means of mobilization determine the completeness of mobilization of slides and the time required for mobilization; they also influence the thickness and lumpiness of deposits, as well as the travel distance of debris flows. These relations permit means of mobilization to be predicted in both an approximate and a precise manner through soil testing.