The perceptible downward sliding, falling, or flowing of masses of soil, rock, and debris (mixtures of soil and weathered rock fragments). Landslides range in size from a few cubic meters to over 109 m3 (3.5 × 1010 ft3), their velocities range from a few centimeters per day to over 100 m/s (330 ft/s), and their displacements may be several centimeters to several kilometers. Mass wasting
The U.S. Highway Research Board classification divides landsliding of rock, soil, and debris, on the basis of the types of movement, into falls, slides, and flows. Other classifications consider flows, along with creep and other kinds of landslides, as general forms of mass wasting.
Falls occur when soil or rock masses free-fall through air. Falls are usually the result of collapse of cliff overhangs which result from undercutting by rivers or simply from differential erosion. Slides invariably involve shear displacement or failure along one or more narrow zones or planes. Internal deformation of the sliding mass after initial failure depends on the kinetic energy of the moving mass (size and velocity), the distance traveled, and the internal strength of the mass. Flows have internal displacement and a shape that resemble those of viscous fluids. Relatively weak and wet masses of shale, weathered rock, and soil may move in the form of debris flows and earthflows; water-soaked soils or weathered rock may displace as mudflows.
Mining and civil engineering works have induced myriads of landslides, a few of them of a catastrophic nature. Open-pitmines and road cuts create very high and steep slopes, often quite close to their stability limit. Local factors (weak joints, fault planes) or temporary ones (surges of water pressure inside the slopes, earthquake shocks) induce the failure of some of these slopes. The filling of reservoirs submerges the lower portion of natural, marginally stable slopes or old landslides. Water lowers slope stability by softening clays and by buoying the lowermost portion, or toe, of the slope.
Advances in soil and rock engineering have improved the knowledge of slope stability and the mechanics of landsliding. Small and medium-sized slopes in soil and rock can be made more stable. Remedial measures include lowering the slope angle, draining the slope, using retaining structures, compressing the slope with rock bolts or steel tendons, and grouting. Engineering geology Erosion Soil mechanics