Information about Landslide

A landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows. Although gravity's action on an over-steepened slope is the primary reason for a landslide, there are other contributing factors affecting the original slope stability:

What qualifies as a landslide? In order to clarify the meaning of the word "landslide", the main points of agreement among various authors in the terminological definitions of landslide events can be summarised as follows:

• Landslides represent one category of phenomena included under the general heading of mass movements. The term therefore describes a movement of a mass of rocks or soil from a higher point to a lower one.
• Gravity is the principal force involved. The movement of the masses is due to the action of the force of gravity, but other forces like those due to earthquake or due to water filtration can be involved.
• Movement must be moderately rapid. Creep that affects the regolith without definition of the sliding surface is therefore a slope movement but is not a landslide.
• Movements may include falling, sliding and flowing. The first is the movement of masses or blocks in free fall; the second is the movement along more or less well-defined surfaces, the third, is movements of masses in a fluid-plastic or viscous state.
• The plane or zone of movement is not identical with a fault. A fault can be a part of a sliding surface.
• Movements should be downwards and outwards with a free face, thus excluding subsidence. Subsidence is a mass movement, in which a mass goes downwards induced by gravity and/or specific water conditions.
• The displaced material has well-defined boundaries and usually involves only limited portions of the hillside. This shows that a landslide movement always involves a volume of material whose limits can be recognised or approximated and whose determination is at the basis of the identification of the mechanism movements.
• The displaced material may include parts of the regolithe and/or bedrock. The materials involved can therefore be rocks at various levels of alteration or the product of disgregation phenomena in the past.
• Frozen ground phenomena are usually excluded. Therefore solifluction due to frozen ground phenomena such as creep, can be classified as slope movement but is not a landslide.
• On the basis of what is stated above, a complete definition of a landslide event could be the following: "Movement of soil or rock controlled by gravity, superficial or deep, with movement from slow to rapid, but not very slow, which involves materials which make up a mass that is a portion of the slope or the slope itself".

Causes of landslides

Main article: Causes of landslides

Natural causes:

• erosion by rivers
• glaciers melting
• ocean waves create oversteepened slopes
• rock and soil slopes are weakened through saturation by snowmelt or heavy rains
• earthquakes create stresses that make weak slopes fail (see liquefaction, Hope Slide)
• volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
• thunder and lightning may trigger failure of weak slopes
• excess weight from accumulation of rain or snow
• groundwater pressure acting to destabilise the slope

Human causes:

• vibrations from machinery
• traffic and road construction
• blasting
• stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures
• in shallow soils, the removal of deep-rooted vegetation that binds the colluvium to bedrock
• Mining
• Logging
• Overgrazing

Debris flow

Slope material that becomes saturated with water may develop into a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Debris flow is often mistaken for flash flood, but they are entirely different processes.

Muddy-debris flows in alpine areas cause severe damage to structures and infrastructure and often claim human lives. Muddy-debris flows can start as a result of slope-related factors, and shallow landslides can dam stream beds, provoking temporary water blockage. As the impoundments fail, a "domino effect" may be created, with a remarkable growth in the volume of the flowing mass, which takes up the debris in the stream channel. The solid-liquid mixture can reach densities of up to 2 tons/m³ and velocities of up to 14 m/s (Chiarle and Luino, 1998; Arattano, 2003). These processes normally cause the first severe road interruptions, due not only to deposits accumulated on the road (from several cubic metres to hundreds of cubic metres), but in some cases to the complete removal of bridges or roadways or railways crossing the stream channel. Damage usually derive from a common underestimation of mud-debris flows: in the alpine valleys, for example, bridges are frequently destroyed by the impact force of the flow because their span is usually calculated only for a water discharge. For a small basin in the Italian Alps (area = 1.76 km²) affected by a debris flow, Chiarle and Luino (1998) estimated a peak discharge of 750 m3/s for a section located in the middle stretch of the main channel. At the same cross section, the maximum foreseeable water discharge (by HEC-1), was 19 m³/s, a value about 40 times lower than that calculated for the debris flow that occurred.

Earth flow

Earthflows are downslope, viscous flows of saturated, fine-grained materials, that move at any speed from slow to fast. Typically, they can move at speeds from .17 to 20 km/h. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that flows in that they are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be.

These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are usually derived from the slumping at the source.

Earthflows occur much more during periods of high precipitation, which saturates the ground and adds water to the slope content. Fissures develop during the movement of clay-like material creates the intrusion of water into the earthflows. Water then increases the pore-water pressure and reduces the shearing strength of the material.^[1]

Sturzstrom (sp)

A sturzstrom is a rare, poorly understood type of landslide. Often very large, these slides are unusually mobile, flowing very far over a low angle, flat, or even slightly uphill terrain. They are suspected of "riding" on a blanket of pressurized air, thus reducing friction with the underlying surface. Earth flow

A rock slide in Guerrero, Mexico.Earthflows are downslope, viscous flows of saturated, fine-grained materials, that move at any speed from slow to fast. Typically, they can move at speeds from .17 to 20 km/h. Though these are a lot like mudflows, overall they are slower moving and are covered with solid material carried along by flow from within. They are different from fluid flows in that flows in that they are more rapid. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to earthflows. The velocity of the earthflow is all dependent on how much water content is in the flow itself: if there is more water content in the flow, the higher the velocity will be.

These flows usually begin when the pore pressures in a fine-grained mass increase until enough of the weight of the material is supported by pore water to significantly decrease the internal shearing strength of the material. This thereby creates a bulging lobe which advances with a slow, rolling motion. As these lobes spread out, drainage of the mass increases and the margins dry out, thereby lowering the overall velocity of the flow. This process causes the flow to thicken. The bulbous variety of earthflows are not that spectacular, but they are much more common than their rapid counterparts. They develop a sag at their heads and are usually derived from the slumping at the source.

Shallow landslide

Landslide in which the sliding surface is located within the soil mantle or weathered bedrock (typically to a depth from few decimetres to some metres). They usually include debris slides, debris flow, and failures of road cut-slopes. Landslides occurring as single large blocks of rock moving slowly down slope are sometimes called block glides.

Deep-seated landslide

Landslides in which the sliding surface is mostly deeply located below the maximum rooting depth of trees (typically to depths greater than ten metres). Deep-seated landslides usually involve deep regolith, weathered rock, and/or bedrock and include large slope failure associated with translational, rotational, or complex movement.

Related phenomena

An avalanche, similar in mechanism to a landslide, involves a large amount of ice, snow and rock falling quickly down the side of a mountain. Usually the snow builds in cornices or forms over a weaker layer of snow which increases the danger of an avalanche.

A pyroclastic flow is caused by a collapsing cloud of hot ash, gas and rocks from a volcanic explosion that moves rapidly down an erupting volcano.

Historical landslides

• The Storegga Slide, Norway
• Cliff landslip of the Undercliff near Lyme Regis, Dorset, England, on 24 December 1839
• The Cap Diamant Québec rockslide on September 19, 1889
• Frank Slide, Turtle Mountain, Alberta, Canada, on 29 April 1903
• The Riñihuazo lanslide in Chile after the Great Chilean Earthquake, on 22 May 1960
• The 1966 Aberfan disaster
• St-Jean-Vianney, Québec, Canada. Small village near Saguenay river destroyed on May 1971. http://archives.cbc.ca/IDCC-0-13-51-945/desastres_tragedies/saint-jean-vianney/
• Landslides associated with the Mount St. Helens eruption on May 18, 1980.
• Thistle, Utah on 14 April 1983
• The Mameyes Disaster - Ponce, Puerto Rico on October 7, 1985
• Thredbo landslide, Australia on 30 July, 1997
• Payatas, Manila garbage slide on 11 July, 2000.
• Southern Leyte landslide in the Philippines on 17 February, 2006
• Devil's Slide, an ongoing landslide in San Mateo County, California
• 2007 Chittagong mudslide, in Chittagong, Bangladesh, on June 11, 2007.

See also

• Geotechnical engineering
• Landslide mitigation
• Mass wasting
• Slope stability
• Geotechnics

References

External links

• United States Geological Survey site
• British Columbia government landslide information
• Slide!, a program on B.C.'s Knowledge Network, with video clips
• Geoscience Australia Fact Sheet http://www.ga.gov.au/urban/factsheets/landslide.jsp/
• Pictures of Slope Failure

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