Engineering geology

What Causes Landslides?

There are two primary categories of causes of landslides: natural and human caused. Sometimes, landslides are caused, or made worse, by a combination of the two factors.

Natural Occurrences

This category has three major triggering mechanisms that can occur either singly or in combination —(1) water, (2) seismic activity, and (3) volcanic activity. Effects of all of these causes vary widely and depend on factors such as steepness of slope, morphology or shape of terrain, soil type, underlying geology, and whether there are people or structures on the affected areas.

Landslides and Water

Slope saturation by water is a primary cause of landslides. Saturation can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and surface- water level changes along coastlines, earth dams, and in the banks of lakes, reservoirs, canals, and rivers. Landslides and flooding are closely associated because both are related to precipitation, runoff, and the saturation of ground by water. Flooding may cause landslides by undercutting banks of streams and rivers and by saturation of slopes by surface water (overland flow). In addition, debris flows and mudflows
usually occur in small, steep stream channels and commonly are mistaken for floods; in fact, these two events often occur simultaneously in the same area. Conversely, landslides also can cause flooding when sliding rock and debris block stream channels and other waterways, allowing large volumes of water to back up behind such dams. This causes backwater flooding and, if the dam fails, subsequent downstream flooding. Moreover, solid landslide debris can “bulk” or add volume and density to otherwise normal streamflow or cause channel blockages and diversions, creating
flood conditions or localized erosion. Landslides also can cause tsunamis (seiches), overtopping of reservoirs, and (or) reduced capacity of reservoirs to store water. Steep wildfire-burned slopes often are landslide-prone due to a combination of the burning and resultant denudation of vegetation on slopes, a change in soil chemistry due to burning, and a subsequent saturation of slopes by water from various sources, such
as rainfall. Debris flows are the most common type of landslide on burned slopes. Wildfires, of course, may be the result of natural or human causes.
Figure 1 shows a devastating landslide caused by rainfall, and possibly made worse
by a leaking water pipe, which added even more water to the soil.

Figure 1. The Mameyes, Puerto Rico, landslide, 1985. This landslide destroyed 120 houses and killed at least 129 people. The
catastrophic slide was triggered by a tropical storm that produced extremely heavy rainfall. Contributing factors could also have
included sewage saturating the ground in the densely populated area, and a leaking water pipe at the top of the landslide. (Photograph
by Randall Jibson, U.S. Geological Survey.)

Landslides and Seismic Activity

Many mountainous areas that are vulnerable to landslides have also experienced at least moderate rates of earthquake activity in recorded times. Earthquakes in steep landslide-prone areas greatly increase the likelihood that landslides will occur, due to ground shaking alone, liquefaction of susceptible sediments, or shaking-caused dilation of soil materials, which allows rapid infiltration of water. For instance, the 1964 Great Alaska earthquake in the United States caused widespread landsliding and other ground failure, which led to most of the monetary loss attributed to the earthquake. Other areas in North America, such as the State of California, the Puget Sound region in Washington, and the St. Lawrence lowlands of eastern Canada, have experienced landslides, lateral spreading, and other types of ground failure classified as landslides, due to moderate to large earthquakes. Rockfalls and rock topples can also be caused by loosening of rocks or rocky formations as a result of earthquake
ground shaking. Figure 2 shows damage from a landslide that was triggered by an earthquake. There is also a great danger of landslide dams forming in streams and rivers below steep slopes, a result of rock and earth being shaken down by the earthquake. These landslide dams often completely or partially block the flow of water, causing water to back up behind the landslide dam, flooding areas upriver. As these dams are often unstable, they may erode either quickly or over a period of time and fail catastrophically, unleashing the backed up water as a rapid deluge below the dam. This deluge is capable of causing a great deal of damage downriver.

Figure 2. Earthquake-induced landslide damage to a house built on artificial fill, after
the 2004 Niigata Prefecture earthquake in Japan. (Photograph by Professor Kamai,
Kyoto University, Japan.)

Landslides and Volcanic Activity

Landslides due to volcanic activity represent some of the most devastating
types of failures. Volcanic lava may melt snow rapidly, which can form a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path. These volcanic debris flows (also known as lahars, an Indonesian term) can reach great distances after they leave the flanks of the volcano and can damage structures in flat areas surrounding the volcanoes. Volcanic edifices are young, unconsolidated, and geologically weak structures that in many cases can collapse and cause rockslides, landslides, and debris avalanches.
Many islands of volcanic origin experience periodic failure of their perimeter areas (due to the weak volcanic surface deposits), and masses of soil and rock slide into the ocean or other water bodies, such as inlets. Such collapses may create massive sub-marine landslides that may also rapidly displace water, subsequently creating deadly tsunamis that can travel and do damage at great distances, as well as locally.
Figure 3 shows a collapse of the side of a volcano and the resulting devastation.

Figure 3. The side of Casita Volcano in Nicaragua, Central America, collapsed on
October 30, 1998, the day of peak rainfall as Hurricane Mitch moved across Central
America. This lahar killed more than 2,000 people as it swept over the towns of
El Porvenir and Rolando Rodriguez. (Photograph by K.M. Smith, U.S. Geological Survey.)

Human Activities

Populations expanding onto new land and creating neighborhoods, towns, and cities is the primary means by which humans contribute to the occurrence of landslides. Disturbing or changing drainage patterns, destabilizing slopes, and removing vegetation are common human-induced factors that may initiate landslides. Other examples include oversteepening of slopes by undercutting the bottom and loading the top of a slope to exceed the bearing strength of the soil or other component material. However, landslides may also occur in once-stable areas due to other human activities such as irrigation, lawn watering, draining of reservoirs (or creating them), leaking pipes, and improper excavating or grading on slopes. New construction on landslide-prone land can be improved through proper engineering (for example, grading, excavating) by first identifying the site’s susceptibility to slope failures and by creating appropriate landslide zoning.

By Lynn M. Highland, and Peter Bobrowsky,

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