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The Mechanisms of Shallow Landslides and Their Causes

written by: ciel s cantoria•edited by: Sarah Malburg•updated: 7/13/2011

It can be said that human activities create simulations of the mechanisms that cause a slope to lose its soil stability. All these have contributed to the increased frequency by which shallow landslides occur. Understand that rainfalls merely trigger mudflows in a condition that is already unstable.

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    Understanding a Geological Phenomenon

    800px-FEMA - 27426 - Photograph by Marvin Nauman taken on 01-10-2007 in Washington 

    Landslides are looked upon by geologists as a geological phenomenon, to which changes in the composition of a slope area had built-up and created instability. The Earth’s gravitational energy is considered as the main driving force by which soil, rocks and other organic matters are being pulled-down as a natural course. Yet the gradual shift from a stable condition to an unstable state makes the slope susceptible to debris-flow that becomes uncontrollable. Once the trigger mechanisms all act together or even alone, a landslide actually occurs.

    The downward and outward flow of the rocks and the slope-fill is characterized by the sliding and spreading of mud, combined with toppling and falling rocks. A shallow landslide occurrence is one where the processes produce mud flow that is less than 2.0 meters deep and cover areas that could reach as far as 50 to 1,000 square meters. Mud flows are also considered to be dangerous because they cause damage to cultivated lands, to physical infrastructures and even result in loss of lives.

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    How and Why Shallow Landslides Happen?

    800px-FEMA - 43689 - Road damageand workers in California 

    A landslide that is categorized as shallow often occurs in slopes where the surface soil is highly porous while covering a substrate or layer of low-porous or slightly permeable bottom-soil. As the surface soil allows great volumes of water to seep through, the latter gets trapped and creates a pressure against the loose and highly porous top soil.

    Rainfall is the most common of the trigger mechanisms that induce these types of landslides. The precipitation can be heavy and intense within a short span or at low intensity for a prolonged period.

    As rainfall gets more intense or prolonged, soil that has become saturated with water, gives in to the downward-pull of gravity and flows rapidly over bedrocks. It becomes a messy pile of rock, mud and other organic matters that may have been uprooted or anything that gets dragged down while traveling in a downward movement.

    In the image to the left, of Angeles Crest Highway, Los Angeles County, the occurrence of a landslide was triggered by heavy rains and severe winter storms. The area was already unstable due to a wildfire that swept across the mountains a year before, whereby the slopes were observed to have eroded and sustained heavy soil losses. (Please click on the image to get a larger view.)

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    Factors That Affect or Alter a Slope’s Stability

    800px-Lushoto 800px-West Seattle shore 02 

    (1) The absence or loss of tree roots and other forms of vertical vegetations. Human activities, like logging or farming are largely responsible for the deforestation or denudation of the slope.

    (2) Wildfire incidents can lead to loss of soil nutrients and changes in the soil structure, which subsequently causes the roots to rot and weaken the structure of surface soil.

    (3) Rivers and coastal areas where the soil tends to erode as the land is continuously battered by sea waves or water movements.

    (4) Ground tremors of an earthquake that bring about stressful conditions to the soil, which can rob the soil of its stiffness and lead to the deterioration of its composition.

    (5) Snowmelts, thawing glaciers and heavy rains tend to saturate and add stress to the soil.

    (6) The hot, molten lava discharged during volcanic eruptions that destroys the root system, in addition to altering the soil nutrients and the soil structure.

    (7) Other human activities that create ground-shaking movements similar to an earthquake such as:

    • continuous vibrations emanating from vehicular traffic
    • vibrating machinery or equipment used in construction, mining or oil explorations
    • blasting devices used for road construction or mining sites
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    What are the Trigger Mechanisms?

    Streets in the family housing section of Naval Support Activity Mid-South are under water Saturday, May 1, 2010 during major flooding after heavy rains breeched nearby protective levees 

    Water- The weakened soil reaches a saturation point in the amount of accumulated water pressure carried by its particles. As water continuously seeps in even before the particles can unload some of the water pressure, both soil and water seek for a way out to release the stress. Due to the fact that the soil has already been pre-conditionally weakened; the mudflow will proceed with a rapid downward movement owing to the landform’s natural inclination.

    Seismic or Ground-Shaking Activities – Ground shaking allows water to rapidly infiltrate the soil and thus result in a type of water pressure that is overwhelming to the surface soil. An unstable slope will likewise give in to a landslide event, the moment it surpasses the threshold level for liquefied soil.

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    Forecasting a Mudflow Event

    Numerous studies and models have been used for the purpose of forecasting landslides, since the frequency by which they occur have considerably increased. To date however, researchers still find it difficult to determine the extent by which slope stability is affected by the causal factors. Forecasting initiatives, therefore, are still regarded as unreliable.

    The slope inclination is also considered as a contributing factor that enables rainfall to trigger mudflow occurrences. The gravitational pull of a 20-degree inclination is said to be low when compared to those with 45-degree steepness in its inclination. Some researchers contend that the soil strength and stability, even if the area is heavily forested, can be influenced by the weight being pulled down by the Earth’s gravity.

    Although there are several factors that contribute to the geological changes of a slope, increased human activities that altered not only the Earth’s landscape but also the soil's chemistry and atmospheric climate, have largely contributed to the increase in the frequency of shallow landslide occurrences.

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    Reference Materials and Image Credit Section

    References

    • NationalAtlas.gov - Land Slide Types and Processes. Retrieved from http://www.nationalatlas.gov/articles/geology/a_landslide.html#one

    Image Credits