Field observations of basal forces and fluid pore pressure in a debris flow

Using results from an 8 m(2) instrumented force plate we describe field measurements of normal and shear stresses, and fluid pore pressure for a debris flow. The flow depth increased from 0.1 to 1 m within the first 12 s of flow front arrival, remained relatively constant until 100 s, and then gradually decreased to 0.5 m by 600 s. Normal and shear stresses and pore fluid pressure varied in-phase with the flow depth. Calculated bulk densities are rho(b) = 2000-2250 kg m(-3) for the bulk flow and rho(f) = 1600-1750 kg m(-3) for the fluid phase. The ratio of effective normal stress to shear stress yields a Coulomb basal friction angle of phi = 26 degrees at the flow front. We did not find a strong correlation between the degree of agitation in the flow, estimated using the signal from a geophone on the force plate, and an assumed dynamic pore fluid pressure. Our data support the idea that excess pore-fluid pressures are long lived in debris flows and therefore contribute to their unusual mobility.