On pore water pressure variations causing slide velocities and accelerations observed in a seasonally active landslide

Individual block displacements, continuously recorded in spring 1987 in a landslide in the French Alps, are strongly affected by rain and dry spells. Although a viscous component is present in the sliding mechanism, variation of the plastic strength characteristics is necessary to explain the controlled sliding. A stick‐slip movement behaviour and a diurnal variation in slide rate, attributed to the diurnal variation in evapotranspiration, were recorded. The observed numerous accelerations and decelerations of the blocks movements are attributed to small variations in pore pressure. These variations are explained and their magnitude is calculated: starting from limit equilibrium increases of pore water pressure smaller than 0‐1 k Pa are sufficient to provoke the maximum recorded acceleration and velocity for a block of 2000 tons in weight.