landslides Landslides are phenomena in which soil or rock moves down a slope under the action of gravity. The nature of movement may be falling, flow, sliding, or toppling. Landslides generally have distinct lateral boundaries (Fig. 1), and are therefore readily identifiable as such. They are distinct from soil creep, which, while satisfying the conditions of downslope movement, does not entail motion on a discrete shear surface. All landslides are therefore forms of mass movement, but not all mass movements are landslides.

When the strength of a soil or a rock is insufficient to support the stresses acting on the slope, a landslide will develop. The strength of soils and rocks can be described in terms of the cement and the bonds between grains (cohesion) and the interlocking of the grains through their surface roughness (friction). Clays have high cohesion but low frictional strength, whereas sands have low cohesion but higher frictional strengths. Rocks can be high in both characteristics and tend to slide along pre-existing failure planes such as joints. The biggest single influence on the occurrence of landslides is the presence of water. Landslides tend to happen as a result of high moisture contents which result in high water pressures in the soil pores. These high pore-water pressures reduce the contact between grains, thus reducing the frictional strength of the soil.

Falls

The simplest type of landslide is a fall, which can occur either in soils or in rocks. Falls are generally thought of as rock falls. They happen when steep slopes and inherent weaknesses in the rock or soil allow a block of the material to detach and fall to the ground. There may be some degree of bouncing and rolling, but the initial movement is that of a falling object. Very large rock falls can trap enough air to create a cushion, enabling a landslide to travel great distances at high speeds. This type of landslide, known as a rock avalanche, has resulted in large numbers of casualties. The 1970 Huascaran rock avalanche in Peru caused the deaths of at least 17 000 people in the towns of Yungay and Ranrahirca. It is believed that this landslide moved at speeds of over 320 km h⁻¹ (200 miles per hour). If a mix of soil and rock is involved, the movement is referred to as a debris fall. Rock avalanches can be recognized by the long distances they travel, their great lengths compared to their breadths, and the fact that they may move so fast that they can even travel over small hills and up valley sides. Rock falls range in size from single blocks of less than 1 m in diameter to multiple blocks with volumes of many thousands of cubic metres.

Slides

Those landslides that occur through sliding on a discrete surface are known as rock slides, debris slides, or earth slides. Mudslides, the third of these groups, are a specific form of slide in which a fluidized mass moves on a shear plane. Unlike falls, slides move through shearing on a discrete surface. The movement can be either simple translation, in which the moving mass slides on a planar shear surface, or rotation, in which the mass slides on a semicircular shear plane. Rotational movement results in the sliding block becoming tilted as well as rotated. Slides are probably the most complex and varied group of landslides that can involve both soil or rocks. In soils there will not usually be a pre-existing weakness, but in rocks there normally will be. Such weaknesses can be present as fractures in the mass of the rock, or as weaker beds, such as mudstones.

Flows

In earth, debris, or rock flows there is movement of saturated soil material, often at very rapid rates (see debris flows). Earth or debris flows occur where soil material becomes saturated and oozes, sometimes at high speeds, downslope. Rock flows tend to occur through the saturation of weak, often clay-rich, layers in the ground. When these begin to move, they carry the overlying, more rigid, rocks with them. Flow slides of any variety tend to be the result of high fluid pressures. While these fluid pressures are normally water pressures, flow slides can be triggered by high air pressures. During the Kanto Province earthquake in China in 1920 large flow slides occurred in loess, a silty soil deposit. These landslides are believed to have resulted from the collapse of the delicate loess soil structure, which was caused in turn by earthquake shaking. When this occurred the air could not escape from the open pore structure, and the air pressure in the silt reduced the frictional contact. Huge flow slides then took place which resulted in over 20 000 fatalities.

Topples

Topples represent a variety of landslide that occurs when rock or soil material becomes detached from an exposed face. Rather than a simple falling motion, a topple will involve some form of outward rotational motion from the rock or soil mass. Toppling generally occurs when blocks are tall in comparison with their width. Rock topples, for example, are common in rock masses that show columnar jointing. Small toppling failures have occurred from the crags on which Stirling and Edinburgh castles sit, and they posed a hazard to the general public before work was undertaken to reduce the risk.

Recognition of landslides

Landslides can be recognized by their geomorphological features, both from satellite images and aerial photographs as well as from detailed ground investigation work. Active landslides tend to show exposed soil or rock, well-defined boundaries, egress of water from various points of contact, and a readily identifiable source and accumulation zone for the sliding material. In relic landslides, the shape of the source and accumulation zones may be visible, but these will normally be muted by vegetation cover. Relic landslides can, however, be differentiated from the surrounding area because of their shape and their vegetation, which is often found to contrast with that of stable ground. This is due to frequent differences in soil moisture conditions on and off the sliding mass. The geomorphological features associated with landslides in soils are shown in Fig. 1. The sides of landslides will be marked by lateral shear surfaces, while tension cracks and minor scarps may occur near the head of the landslide and along the body of the sliding mass. The soil or rock from the landslide accumulates in what is called the ‘toe’ of thelandslide.

Landslides are three-dimensional features. In addition to the geomorphology that can be used to define a landslide in plan view, landslides can also be clearly defined in cross-section. The basal shear surfaces can be defined through drilling programmes designed to assess the three-dimensional shape of the landslide. An understanding of the geometry of the sliding mass, a knowledge of the strength of the materials, and a good assessment of the groundwater conditions can be used to form an estimate of the hazard posed by a landslide.

Landslides as extreme events

Landslides can be triggered by a variety of events. Coastal landslides, such as Black Ven in Dorset, can be triggered, or speeded up, by changing rates of coastal erosion. Landslides may also be initiated by extreme events. Cyclone Bola resulted in thousands of landslides in New Zealand in 1986, and the Loma Prieta earthquake in California in 1989 caused many large landslides. In large-magnitude earthquakes (magnitude 6.7 or greater) more than half the fatalities are the result of landslides triggered by ground shaking. Volcanic activity can also make slopes unstable. An example was the collapse of the east flank of Mount St Helens in 1980. After a period of volcanic inflation, the slopes became so steep that they were no longer stable. An earthquake triggered the landslide from the side of the mountain at 8.32 a.m. on 18 May. Once this slope had collapsed it allowed the release of the magma from the volcano, and the famous eruption followed.

These large-magnitude events are normally infrequent, but when they do occur they can be very damaging. It is such catastrophic slope failures which are generally thought of when landslides are mentioned, although the smaller, less newsworthy types are much more common.

W. Murphy

Bibliography

Crozier, M. J. (1986) Landslides: causes, consequences and environment. Croom Helm, London.

Landslides

Landslides are natural hazards that cause millions of dollars of damage each year and also cause many deaths. They are defined as downslope movements of soil and rock under the influence of gravity. They are triggered primarily by water, but sometimes earthquakes can lead to some spectacular landslides. The water comes mainly from high-rainfall storms, but also can come from rapid snowmelt.

Factors

The stability of a slope can be described as two forces working against one another. Driving forces work to cause slope materials to move downslope, whereas resisting forces act to keep the materials on the slope. When the ratio of resisting forces to driving forces (called the factor of safety) is greater than 1, the slope is stable. When it is less than 1, the slope usually fails.

Water and Vegetation.

Water can increase the driving forces and reduce the resisting forces. Saturation from rainfall can increase the slope mass, thereby increasing the driving forces. Filling the slope soil pores with water also reduces soil cohesion by allowing particles to pass by one another, thereby reducing the internal resistance of the soil to sliding. To reduce landslide danger on a slope, the first thing done is to remove the water.

Vegetation also is important to slope stability because it increases resisting forces through its roots, especially tree roots, that bind the soil. Trees also act as natural pumps that remove water from the soils through evapotranspiration , thereby increasing slope stability.

Slope and Materials.

As the slope angle increases, the driving forces also increase. Few landslides occur on slopes less than 15 degrees. Cutting a road into a slope will create an oversteepened slope prone to landslides if a wall is not built. Certain slope materials also have weak strengths and low resisting forces to landslides. For example, clay, shale, serpentine, and uncompacted fill are prone to failure.

If the slope bedrock is inclined and is somewhat parallel to the slope, it is called a dip slope. Landslides are prone along failure planes (clay beds and old soils) on these dip slopes. Examples of dip-slope failure are Italy's Vaiont Dam disaster in 1963 that killed 2,600 people, and the Gros Ventre landslide that dammed Wyoming's Gros Ventre River in 1925.

To stabilize a slope or prevent landslides, one needs to lower the slope angle, drain the slope of water, build retaining walls, plant vegetation, and avoid building on old landslides. If the slope has moved once, it has a high chance of moving again.

Classification

Names are given to different landslides depending upon the process that brings the soil and rock down the slope. Falls are the free fall of detached materials, usually rocks, which descend down a steep slope. Translational slides occur along a failure surface in the bedrock and move parallel to the surface. If the sliding mass occurs along a curved plane, it usually is called a slump.

Flows.

Flows occur when material moves downslope as a viscous (thick) fluid. Most of these flows are saturated with water. Fast-moving ones move as a slurry that can be as much as 70 percent water and 30 percent sediment, and these are called debris flows (see figure). If the flow contains mostly fine-grained particles like sand, silt and clay, it is called a mudflow. They frequently follow stream canyons and pose significant hazard to life and property. Velocities can reach 55 kilometers (34 miles) per hour. Earthflows are slower flows that generally originate on hillslopes as large tongues that break away from scarps (arc-shaped steep slopes cut into a hill).

Debris Avalanches.

The fastest and largest landslides are called debris avalanches, because they travel at speeds up to 300 kilometers (186 miles) per hour, travel long distances sometimes in excess of 30 kilometers (18.6 miles), and are very large. The world's largest historical landslide occurred in Washington state when Mount St. Helens erupted in 1980.^* In Yungay, Peru, about 22,000 people were killed in 1970 when a debris avalanche descended from the volcano Nevado Huascaran, traveling a distance of 14 kilometers (8.7 miles) in only a few minutes.

Creep.

Some movement on slopes can be imperceptible, called creep. Creep is caused by gravity, and is assisted by freeze–thaw or shrink–swell processes operating on the soil sediments. Rates range from 0.1 millimeter (0.004 inch) per year to 10 centimeters (about 4 inches) per year. Evidence of movement is found in cracked walls, leaning telephone poles, and leaning trees that adjust (straighten) their direction of growth, creating so-called "trees with knees," also called pistol-butt trees.

Recognizing Landslide Terrains

Scientists "read" the landscape for signs that tell of past processes. Several diagnostic landslide features tell of past movement on the terrain. First, scarps are arcuate, steep slopes cut into the hill where the landslide has torn away. Second, hummocky topography is "bumpy" ground that has been produced by the landslide mass-weathering over time. A hummocky slope with a steep, half-moon slope at the top usually means an old landslide occurred there.

Younger landslides generally will have vegetation on the slide mass that is different than vegetation on the surrounding slopes. In coniferous forests, landslides typically are covered with deciduous trees for the first 100 years after the failure.

If one can see the actual sediments making up the landform, landslide debris and colluvium are unsorted, with all particles mixed together in a random fashion. Colluvium can be confused with glacial till, because both are unsorted.

see also Forest Hydrology; Groundwater; Volcanoes and Water.

Scott F. Burns

Bibliography

Ritter, D. F., Kochel, R. C., and Miller, J. R. Process Geomorphology, 3rd ed. Dubuque, IA: Wm. C. Brown Publishers, 1995.

Turner, A. K., and R. L. Schuster, eds. Landslides: Investigation and Mitigation. Washington, D.C.: Transportation Research Board, National Research Council, Special Report 247 (1996).

^* See "Volcanoes and Water" for a photograph of the Mount St. Helens eruption.

Landslide

Landslide is a general term used to describe a variety of geologic processes involving the movement of fine-grained earth, coarse-grained debris, or rock down a slope under the influence of gravity . This broad definition of downslope movements includes falling, toppling, sliding, spreading, and flowing. Landslides can also occur under water (submarine landslides) and trigger tsunamis.

Commonly used landslide classification systems rely on separate terms to describe the type of movement (falling, toppling, sliding, spreading, or flowing) and the type of material involved. Unlithified material is classified as debris if it is predominantly coarse-grained (sand , gravel, cobbles, or boulders) and earth if it is primarily fine-grained (silt or clay ). Thus, a debris flow would involve the down slope flow of predominantly coarse-grained material, whereas a debris slide would involve the sliding of the same kind of material along a well-defined slip surface. Additional detail can be included by specifying the rate of movement (which can range from several millimeters per year to tens of meters per second) and the water content of the moving mass (which can range from dry to very wet). Landslides moving at velocities faster than a few meters per minute, particularly when they are large, have the potential to cause catastrophic damage and loss of life. The volume of material involved in a landslide, however, is irrelevant to its classification and can range from a few cubic centimeters to several cubic kilometers. Landslides can also change modes as they move. For example, a debris slide may mobilize into a debris flow as the debris begins to move down slope.

The term mudslide, which is often used in news reports, does not exist within the classification systems used

by most geologists and engineers. It is an imprecise term that is best avoided.

Landslides occur when the forces tending to keep a soil or rock mass in place (resisting forces) are exceeded by those promoting movement (driving forces). Resisting forces most commonly arise due to the shear strength of the material acting over an area , such as the slip surface beneath a landslide, or as a consequence of engineered works such as retaining walls. The primary driving force—the component of the weight of the earth, debris, or rock mass acting parallel to the slope—and the force occurring when the potential landslide mass is accelerated during an earthquake can also trigger landsliding. Changing the geometry of a slope, for example by excavating some areas and placing fill in others during construction, can alter the balance of resisting and driving forces enough to trigger a landslide.

It is a widely held misconception that landslides occur because slopes are lubricated by water. Water does not act as a lubricant in landslides, but instead decreases the shear strength of the earth, debris, or rock by decreasing the normal force acting across a potential slip surface. It is well known from basic physics that the sliding of a block down an inclined plane is resisted by the product of the normal force acting on the plane and a coefficient of friction. Similarly, a decrease in the normal force acting across a potential slip surface will decrease the resistance to sliding. Sources of water leading to landsliding can include infiltrating rain and melted snow, leaking water pipes, and irrigation.

It is difficult to estimate the monetary costs of landslides because they can include both direct and indirect costs. Direct costs include damage to structures and roads, whereas indirect costs include items such as decreased property values, lost productivity, and the expense of driving longer distances when roads are blocked. Difficulties aside, in 1985 the National Research Council estimated that landslides cost between $1 billion and $2 billion per year in the United States alone. Estimates for other countries range from tens of millions to billions of dollars per year.

See also Catastrophic mass movements; Debris flow; Lahar; Mass movement; Mass wasting; Mud flow; Rockfall; Slump; Talus pile or talus slope

landslide rapid slipping of a mass of earth or rock from a higher elevation to a lower level under the influence of gravity and water lubrication. More specifically, rockslides are the rapid downhill movement of large masses of rock with little or no hydraulic flow, similar to an avalanche . Water-saturated soil or clay on a slope may slide downhill over a period of several hours. Earthflows of this type are usually not serious threats to life because of their slow movement, yet they can cause blockage of roads and do extensive damage to property. Mudflows are more spectacular streams of mud that pour down canyons in mountainous regions during major rainstorms where there is little vegetation to protect hillsides from erosion. The runoff from the storm and mud becomes a thin slurry that funnels down the canyons until it thickens and stops. Earthquakes also may cause landslides by shaking unconsolidated or weathered material from slopes. Rockslides triggered by an earthquake in Montana in 1959 caused an entire mountainside to slide into the Madison River gorge, killing 27 people in its path, damming the gorge, and forming a new lake. Humans have triggered a number of tragic landslides that have caused great damage and loss of life. In the Los Angeles area of California, extensive real estate development carried out on hillsides has resulted in widespread mudflows after winter rains have saturated the over-steepened embankments of soil. In some areas, slow-moving earthflows have been initiated by the lubrication of certain types of underlying clays by septic tank effluent. Submarine slides, or a sliding mix of seawater and sediment, are called turbidity currents. Undersea landslides can travel several hundred miles across very gradual slopes, riding on a thin film of water that reduces friction.

land·slide / ˈlan(d)ˌslīd/ • n. 1. the sliding down of a mass of earth or rock from a mountain or cliff. 2. an overwhelming majority of votes for one party in an election: winning the election by a landslide [as adj.] a landslide victory.

landslide See MASS-WASTING.

landslide See mass-wasting.

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