Landslides and slope stability

Landslides occur under specific geological conditions that are usually detectable. Site assessments done by qualified geologists are important to land-use planning and engineering design; much of the tragedy and expense of landslides is preventable.

Falls, Slides, and Flows

Slope failure, or mass wasting, is the gravity-induced downward and outward movement of Earth materials. Landslides involve the failure of Earth materials under shear stress and/or flowage. When slope failures are rapid, they become serious hazards. Areas of the United States that are particularly susceptible to landslides include the West Coast, the Rocky Mountains of Colorado and Wyoming, the Mississippi Valley bluffs, the Appalachian Mountains, and the shorelines and bluffs around the Great Lakes. The downslope movement of soil and rock is a natural result of conditions on the planet’s surface. The constant stress of gravity and the gradual weakening of Earth materials through long-term chemical and physical weathering processes ensure that, through geologic time, downslope movement is inevitable.

Slope failures involve the soil, the underlying bedrock, or both. Several types of movements (falling, sliding, or flowing) can take place during the failures. Simple rockfalls, or topples, may occur when rock overhangs a vertical road cut or cliff face. Other failures are massive and include flows and slides. Slides involve failure along a discrete plane. The failure planes in soils are usually curved, as in the illustration of a rotational slide or slump. The failure planes in bedrock can be curved or straight. Failures often follow planes of weakness, such as thin clay seams, joints, or alignment of fabric in the rock. Slides may be slow or rapid but usually involve coherent blocks of dry material. Flows, in contrast, behave more like a fluid and move downslope much like running water. Earthflows, mudflows, sand flows, debris flows, and avalanches occur when soils or other unconsolidated materials move rapidly downslope in a fluidlike manner. The movement destroys the vegetative cover and leaves a scar of hummocky deposits where the flow occurred. Although flows usually involve wet materials, rare exceptions, such as the destructive flows in Kansu, China, occur in certain types of dry materials that are finely grained and loosely consolidated.

Slides and flows are terms applied to failures that produce rapid movement. Rock slides are those slides that involve mostly fresh bedrock; debris slides include those movements that are mostly rock particles larger than sand grains but with significant amounts of finer materials; mudslides involve even finer material and water, but the failure plane is straight. Earthflows involve mostly the soil overburden and move over a slope or into a valley rather than failing along a rock bedding plane; mudflows involve more water than earthflows and have a downslope movement much like flowing water.

Creep

“Creep” is a term given to very slow movement of rock debris and soils. Creep in itself does not usually pose a life-threatening danger. When creep occurs beneath human-made structures, however, it leads to economic damage that requires repair or reconstruction in a new location. Examples include the gradual cracking and destruction of buildings, misalignment and breaking of power lines and fences, the filling of drains along highways, and the movement of soil into streams and reservoirs. Sometimes creep precedes a more rapid failure, and therefore new evidence of creep requires careful monitoring and an evaluation of the conditions that produce it.

Solifluction is a special type of creep that occurs in permafrost regions, in cold climates where the soil is frozen most of the year. In summer, the ice in the upper layer of soil melts, and the soil becomes waterlogged and susceptible to downslope movement. Solifluction is an important consideration for the design of structures in cold climates: For example, the Alaska pipeline, used for transporting petroleum, could not be buried but had to be set above ground on supports that were anchored at a depth below the seasonal depth of thaw to escape solifluction movement. Houses and other buildings in such areas must be set on supports and insulated in order to keep heat from the structures from melting the underlying soils.

Role of Water

Water is an important agent in promoting instability in slopes. Where soils are saturated, water in large pores will flow naturally in a downward direction. The resulting pressure of the water pushing against the soil grains is called pore pressure. As pore pressures increase, the grains are forced apart and cohesion decreases. The saturated soils become easier to move downslope. Water flowing along a bedding plane or joint can also exert pressure on either side of the joint and decrease cohesion, causing the block above the discontinuity to move downslope.

Loose volcanic materials often can absorb so much water that they flow quickly down even gentle slopes. A mudflow buried the Roman city of Herculaneum at the base of Mount Vesuvius in 79 CE. (although the city had already been hit by pyroclastic flows); and mudflows generated by the Mount St. Helens eruption of May 18, 1980, destroyed many properties. An especially destructive mudflow on the volcano Nevado Ruiz in Colombia in 1986 struck an unsuspecting town at night and killed 22,000 people.

Global warming and climate change have sigfinicantly increased the amount and intensity of rainfall and, as a result, the number of landslides. Sudden, heavy rainfall in southern India in July 2024 buried hundreds of people. Experts from World Weather Attribution estimated that climate change increased the amount of rain in the region by 10 percent. A landslide in Ethiopia during the same month killed 257 people. Heavy rain caused by climate change has caused landslides in the United States as well. Extreme rainfall has led to landslides and rising sea levels that have reshaped California's landscape, causing many buildings to be razed or fall into the ocean.

Role of Human Development

Designers may unwittingly assemble, in a human-made structure, conditions that produce slope failures. Dry materials such as mine wastes have sometimes been stacked into piles that are steeper than their angle of repose after saturation. Much later, a rainstorm or Earth tremor can send the piles into motion, destroying all structures around them, as happened at Aberfan, Wales, in 1964. When the supporting toe is removed from the base of a slope, as during such excavations that occur for a highway or building foundation, this action produces many landslides, evidence of which can be found on most highways constructed through hilly terrain. An unstable slope can be set into motion by loading the slope from above, which occurs when a structure such as a building, a storage tank, or a highway is built on materials that cannot remain stable under the load. Catastrophic failures have occurred in which mudflows were produced when dams built from mine tailings burst as a result of slope failure. The mine wastes used for these dams were susceptible to swelling, absorption of water, and weakening over time.

Human development alters the natural drainage of an area and increases runoff. Occasionally, water from storm drains, roof gutters, septic tanks, or leaking water mains reaches a sensitive slope and generates movement. This instability is particularly likely to occur where intensive housing development takes place in several levels on a long slope.

Regional Studies

The stability of slopes is evaluated over large regions from aerial photographs, satellite photographs, and images made by remote sensing techniques. Investigators look for telltale signs, such as hummocky topography and old scars left by slides, that may not be evident when viewed from the ground.

Regional studies involve an evaluation of the history of past landslides within the region. History often reveals particular geological formations that have an association with landslides. For example, the shales of the Pierre formation are well known by engineering geologists in the area of Denver, Colorado, as materials in which many slope failures occur. A geologic map that shows where this formation is exposed at the ground surface reveals potentially dangerous areas. Ignoring evidence of past landslides invites disaster.

The regional study also defines loose surficial materials that are likely to fail. Soils rich in clay minerals that swell and expand when wet are notorious for slumping and flowing. Usually, movement occurs in these soils in the spring, when the soil is very saturated from soil thaw and snowmelt. Other soils fail simply because they have low cohesion and large amounts of open space (pore space) between the tiny soil grains. Collapse of these soils requires no wetting; strong vibrations can trigger the movement. The most tragic example of this type took place in 1920, when thick deposits of fine loess (a type of soil deposited by the wind) settled rapidly during an earthquake in Kansu, China, and the resulting flows toppled and buried the many homes built upon them; more than 100,000 people died.

Regional studies also look at the earthquake history of an area, because a tremor, even a fairly mild one, can provide the coup de grâce to a slope that has been resting for decades in a state of marginal instability. Huge blocks of the shoreline slid beneath the ocean at Valdez, Alaska, during the 1964 earthquake when the rotational slumping of materials occurred below sea level. Landslides triggered by that same earthquake destroyed much of Anchorage, Alaska. Much of the failure was due to the sudden liquefaction of buried clay layers.

Finally, the regional study includes a history of weather events. When the right geological conditions exist, periods of intense rainfall can trigger the movement of unstable slopes. In the southern and central Appalachian Mountains, periods of increased slide frequency often coincide with severe local summer cloudbursts and thunderstorms. Intense rainfall events associated with hurricanes that have moved inland also trigger landslides over larger areas. Studies in the Canadian Rockies reveal a definite link between rainstorms and rockfalls, and landslides are particularly abundant during the rainy season along the West Coast of North America.

Onsite Investigation

Once knowledge is collected on the region, more specific questions are considered about the local site itself. The investigator first looks at the steepness of the slopes and the earth materials present. In the case of loose materials and soils, the angle of repose is very important. Dry sand poured carefully onto a table to form an unsupported conical pile cannot achieve a cone with sides steeper than approximately 30 degrees, because the cohesion between loose dry sand grains is not strong enough to allow the material to support a steeper face. The 30-degree angle is the maximum angle of repose for dry sand. The angle of repose changes with water content, mineral content, compaction, grain shape, and sorting. Soils that contain clay may be tough and cohesive when dry and have natural repose angles greater than 30 degrees. When wet, their angle of repose may be only 10 degrees. This is particularly true if the soils contain clay minerals such as montmorillonite that absorb large amounts of water. Those soils, sometimes called “quick clays,” can fail instantaneously and flow downslope almost as rapidly as pure water.

The orientation of discontinuities in rocks is as important in determining the stability of a slope as is the type of rock involved. Bedding planes that dip downslope serve as directions of weakness along which failure may occur. Other planar weaknesses may develop along joints and faults and parallel fabrics produced by the alignment of platy and rodshaped minerals that are oriented downslope.

The investigator will check to see if natural processes are removing the supporting material at the bases of slopes. Landslides are particularly common along stream banks, reservoir shorelines, and large lake and seacoasts. The removal of supporting material by currents and waves at the base of a slope produces countless small slides each year. Particularly good examples are found in the soft glacial sediments along the shores of the Great Lakes of the United States and Canada.

Finally, the investigator will look for evidence of actual creep at the site. Damage to structures already on the site—such as curved tree trunks (where tilting is compensated for by the tree’s tendency to resume vertical growth), the offset of fences and power lines, or the presence of hummocky topography on slopes—can demonstrate the presence of recent motion.

Costs and Remediation of Stability Problems

An annual economic loss of between $1 billion and $1.5 billion is a reasonable estimate for costs of landslides within the United States. Expenses include the loss of real estate around large lakes, rivers, and oceans; loss of productivity in agricultural and forest lands; depreciated real estate in areas of slide development; public aid for victims of large landslides; and the contribution of sediment to streams, which decreases water quality, injures aquatic life, and results in the loss of reservoir storage space. Small-scale damage from soil creep is not dramatic but very widespread all the same. In the United States, approximately twenty-five lives are lost each year from landslides. Elsewhere, in densely populated areas, single landslide events cause death tolls in the thousands.

The remediation of slope stability problems involves contributions from both geologists in the investigation of the site and civil engineers in the design of the project. Geologists employed by state geological surveys and the US Geological Survey provide a tremendous service in several ways—constructing geological and slope stability maps based on knowledge of the soils and rock formations, use of remote-sensing methods such as satellite and high-altitude photography, and field study of suspect areas. These maps are made readily available to engineers, contractors, developers, and homeowners; they show color-coded areas of active and potentially active landslides. Such maps have been produced for many areas with a high population density. Residents in the United States may contact their local state’s geological survey, which distributes these maps.

Principal Terms

angle of repose: the maximum angle of steepness that a pile of loose materials such as sand or rock can assume and remain stable; the angle varies with the size, shape, moisture, and angularity of the material

avalanche: any large mass of snow, ice, rock, soil, or mixture of these materials that falls, slides, or flows rapidly downslope

cohesion: the strength of a rock or soil imparted by the degree to which the particles or crystals of the material are bound to one another

creep: the slow and more or less continuous downslope movement of Earth material

earthflow: a term applied to both the process and the landform characterized by fluid downslope movement of soil and rock over a discrete plane of failure; the landform has a hummocky surface and usually terminates in discrete lobes

hummocky: a topography characterized by a slope composed of many irregular mounds (hummocks) that are produced during sliding or flowage movements of earth and rock

landslide: a general term that applies to any downslope movement of materials; includes avalanches, earthflows, mudflows, rockfalls, and slumps

mass wasting: collective term for all forms of downslope movement propelled mostly by gravity, including avalanche, creep, earthflow, landslide, mudflow, and slump

mudflow: both the process and the landform characterized by very fluid movement of fine-grained material with a high water content

slump: a term Lathat applies to the rotational slippage of material and the mass of material actually moved

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