A2 Rapid Landslides and Mass Flows

Title

Hillslides occur when the mobilizing shear stress exceeds the shear strength of the soil or rock. The safety factor F_s summarizes this competence: F_s < 1 implies breakage. Heavy rain is the most frequent trigger: it raises the interstitial pore pressure (u_w), reducing the effective Mohr-Coulomb resistance. On saturated slopes, pore pressure can completely nullify apparent cohesion, triggering planar or translational slides. When the mobilized material is mixed with water, the concentration of solids determines whether the movement is a debris flow (debris flow) or a mudflow (mudflow). Liquefaction occurs when the pore pressure exceeds the total stress: the soil completely loses its resistance and behaves like a fluid. Physical models range from infinite slope analysis (TRIGRS for rainfall instability) to rheological flow models (Bingham, Herschel-Bulkley) for mass flows.

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