Dominant particle support mechanisms in debris flows at Mt Thomas, New Zealand, and implications for flow mobility

ABSTRACT Within zones of little or no deformation by internal shearing in debris flows at Mt Thomas, about two‐thirds of the weight of large particles is supported by buoyancy and about one‐third by static grain to‐grain contact. In boundary shear zones of low velocity flows and in high velocity, turbulent debris flow, grain‐to grain contact is replaced by turbulence and dispersive pressure. Cohesive strength of the clay + silt + water interstitial fluid provides less than 2 % of the force keeping particles larger than 1 cm gravel in suspension. Excess pore pressure is generated in the interstitial fluid by the weight of coarse particles suspended in the slurry. According to Coulomb strength theory, pore pressures measured in these debris‐flow slurries reduce the shear strength of the material to less than 10 % of what it is in the unsaturated state. The excess pore pressures are slow to dissipate because of the small connections between pore spaces that result from the extremely poor sorting of the debris and the presence of silt and clay in the pore fluid. Maintenance of sufficient pore space to trap fluid and facilitate flow on low‐gradient slopes may be accomplished by dilatancy and subsequent partial liquefaction of the debris during shear.