LANDSLIDES
The landslide is used to denote the downward and outward movement of slope forming materials along the surface of separation. The landslides are classified on the basis of movement and types of material. The types of movement are divided into the five main groups: falls, topples, slides, spreads, and flows. A sixth group, complex slope movement includes a combination of two or more of the other types of the movement. Similarly, the types of material are divided into two classes; rock and engineering soil. Soil is further subdivided into the debris and earth based on the grain size. A short description of various landslide types is given below.
Falls
Falls are abrupt movement of slope materials that became detached from steep slope or cliffs, movements occur by free fall or a series of leaps and bound down the steep slope. The relatively free character and lack of slide plane differentiate the rock fall and rockslide. Depending upon the type of slope materials involved, it may be a rock fall, debris fall, and earth fall.
Topples
A topple is a block of rock that tilts or rotates forward on pivot or hinge and then separates from the main mass falling on the slope and subsequently bouncing or rolling down the slope.
Slides
The term slides refer to the mass movements with a distinct surface of rupture or zone of weakness separating the slide material from the more stable underlying materials. The major types of slides are rotational and translational slides. Rotational Slides occur on the slopes of homogeneous clay or shale and soil slopes. The slide movement is more or less rotational about an axis that is parallel to the contour of the slope. The scarp at the head may be almost vertical, while the toe bulges upwards and flows out. Translational slides are mass movements on a more or less planar surface. The movement of translational slide is controlled by surface of weakness such as bedding planes, joints, and faults. Slide materials may range from loose unconsolidated soils to slab of rock. Block slides are translational slides in which the moving mass consists of a single unit of rock block that moves down-slope.
Spreads
The failure in this case is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments are transformed into a liquefied state. Rapid ground motions such as earthquakes usually trigger the failure. Flows Creep is an imperceptibly slow, steady, downward movement of slope forming of soil or rock. The movement is essentially viscous enough to produce permanent deformation but too small to produce failures as in landslides.
A creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles and small soil ripples or ridges.
Debris Flow is form of rapid mass movement involving loose soil, rocks and organic materials along with entrained air and water to form slurry that flows down-slope. Debris Avalanche is a type of very rapid to extremely rapid debris flow. Earth Flows have a characteristics bowl-like depression at the head where the slope material becomes liquefied and flows out.
Mud Flows is a type of earth flow consisting of material containing about 50 per cent of sand, silt, and clay-sized particles and flow rapidity.
Complex Movements
Slope movement involving two or more principal types of movement are called complex movements. For example, rolling rock blocks from higher elevations due to rock falls may cause debris slide at lower elevations.
Prevention of Landslides
Prevention of large, natural landslides is nearly impossible, but common sense and good engineering practices can do minimize the hazard. Common engineering techniques for landslide prevention include the provisions for surface and subsurface drainage, removal of unstable slope materials (grading), construction of retaining wall, or some combination of these. Drainage control is usually an effective way to stabilize a slope. The basic idea is to keep water from running across or infiltrating the slope. Surface water may be diverted around the slope by a series of gutters. This practice is common for road cuts. The amount of water infiltrating a slope may also be controlled by covering the slope with an impermeable layer such as soil cement, asphalt, or even plastic. Groundwater may be inhibited from entering a slope by excavating a cutoff trench. The trench is filled with gravel or crushed rock and placed in a position to intercept and divert groundwater away from a potentially unstable slope. Grading slopes is another way to increase slope stability. Two common techniques are reducing the gradient of a slope by a single cut-and -fill operation and benching. In the first case, material from the upper part of a slope is removed and placed near the base. The overall gradient is thus reduced and material is removed from an area where it contributes to the driving force, and placed at the toe of the slope, where it increases the resisting forces. This method is not practical on very steep, high slopes. As an alternative, the slope may be cut into a series of benches or steps. The benches are designed with surface drains to divert runoff. The benches do reduce the slope and, in addition, are good collection sites for falling rock and small slides. Retaining walls, constructed from concrete cribbing, gabions (stone-filled wire baskets), or piles (long concrete, steel or wooden beams driven into the ground) are designed to provide support at the base of a slope. They should be keyed in well below the base of the slope, backfilled with permeable gravel or crushed rock, and provided with drain holes to reduce the chances of water pressure building up in the slope.
