Concepts of critical-state soil mechanics provide a foundation for understanding the mechanisms by which static masses of soil and rock mobilize into rapidly deforming debris flows. Critical-state principles indicate that soil initial bulk density, effective stress, and hydraulic diffusivity determine changes in bulk density and pore pressure that occur during failure. These changes can cause a transition from slow, creeping failure to widespread, rapid failure and flow (NRC, 1985). However, virtually all attempts to assess critical-state soil behavior have been limited to standard engineering methodologies, primarily laboratory triaxial tests under undrained (sealed) conditions. Such tests emphasize the small strains that can damage structures, but can replicate neither the drainage conditions nor the very large shear strains (> 1) and strain rates (> s-1) that characterize rapid landslides and debris flows. Thus, while critical-state concepts are widely accepted in principle, little or no data exist to test their applicability to large, rapid deformations characteristic of debris flows (Stark et al., 1997).
We propose to collect the experimental data necessary to extend and apply the principles of critical-state soil behavior to debris-flow mobilization. Using two unique facilities, two sets of complementary experiments will be conducted: large-scale debris-flow initiation experiments at the U.S. Geological Survey debris-flow flume and high-strain laboratory tests using a ring-shear device at Iowa State University. Previous debris-flow initiation experiments used the USGS flume to measure changes in pore pressure preceding and during slope failure and trigger debris-flow mobilization. We will explore the causes of these pore-pressure increases and the range of initial conditions under which they occur by conducting similar tests at the USGS flume in which the initial bulk density is varied systematically and subsurface soil deformation is measured continuously. Ancillary tests will provide a second means of studying the coupled changes in bulk density and pore pressure that accompany failure and that may either instigate or suppress the mobilization of debris flows.
