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The behavior of reactivated and first-failure landslides after large displacements is controlled by the available shear resistance in a shear zone and/or along slip surfaces, such as a soil-bedrock interface. Among the factors influencing the resistance parameter, the dependence on the shear rate can trigger catastrophic evolution (rate-weakening) or exert a slow-down feedback (rate-strengthening) upon stress perturbation. We present ring-shear test results, performed under various normal stresses and shear rates, on clayey soils from a landslide shear zone, on its parent lithology and other lithologies, and on clay-rock interface samples. We find that depending on the materials in contact, the normal stress, and the stress history, the shear-rate-dependent behaviors differ. We discuss possible models and underlying mechanisms for the time-dependent behavior of landslides in clay soils.

Plain Language Summary The occurrence and behavior of landslides depend on a number of geologic, climatic, and anthropic factors. Among them, the resistance of the deforming material plays a fundamental role. It is controlled by various parameters, such as the mineral composition of the soil or rock and its structure, the pressure and chemistry of the water filling the pores, the temperature and deformation rate at which the movement occurs. We present the results of laboratory experiments in which we simulate the landslide condition by shearing, one against the other, samples of a clayey soil, of the rock from which the soil derives and of other rocks, or samples made for one half by soil and for the other half by rock. We use samples from the shear zone of a real landslide, and we show that the behaviors of various material assemblies, when sheared at different velocities, are different: the strength can both increase or decrease with the shear velocity. We also show that the pressure we apply to the samples-to simulate landslides of different depths, or different water pressures within the pores-and the compression history can affect the behavior.