7.20 Mass-Movement Causes: Water

Water plays an important role in causing mass movement processes. The plasticity of soils is caused by water absorption on particle surfaces resulting in specific plasticity characteristics and swelling and shrinking processes. In the saturated soils and rocks, the groundwater body produces hydrostatic pressure, which increases linearly with depth, and buoyant stress, which creates an upthrust effect that is opposed to the normal stress of the material system. In dynamic groundwater systems, the water flow creates a drag force on the soil particles resulting in seepage pressure on these particles. The principle of the effective stress theory is related to the effects of water fillings in the void system and the development of negative and positive pore-water pressures affecting change of shear strength behavior of the soil and rock material. Water plays a significant role in the development of different mass movement processes in relation to the long-term influence on the underground material and to short-term triggering mechanism by precipitation and infiltration. These effects are discussed for sliding processes, debris flows, sand liquefaction, rock mass strength, and stability of ice-filled fractures of permafrost rocks.