Slope Failures – Causes and Mitigation

What are Slope Failures?

Slope failures are major natural hazards occurring both globally and locally. They are referred to as the downslope movement of rock debris and soil in response to gravitational stresses. Slope failures are generally classified according to the type of downslope movement namely falls, slides, and slows. Unfortunately, slope failure is a geohazard that impacts a wide range of landscapes and also many types of infrastructures.

Landslide at Pascal Village, Mahe.


So, what causes Slope Failures?

Rockfall at La Batie, Mahe.


Like all geohazards, the causes are myriad and complex. Generally speaking, slope stability is based on the interaction of two forces namely driving and resisting forces. Driving forces promote the downslope movement of slope material whilst resisting forces resist movement. Common causes of slope failure include:

  • Slope Steepness: Steeper slopes have greater risks for instability. The natural tendency of steep slopes is to move some of its materials downwards until the natural angle of repose is found. Any form of slope modification will eventually impact the stability of a slope.
  • Drainage and Stream Action: Excessive water in slopes is never good as it destabilizes the slope by adding weight, destroying cohesion between grains, and reducing friction. When water takes the place of air between the grains of soil, it will most likely increase the probability of downslope mass movement and lead to slope failures as the earth in slopes become a lot heavier. Streams can also erode away the bottom of the slope overtime resulting in decrease in slope stability.
  • Vegetation: The amount and type of vegetation on a slope is proportional to the strength of that slope. Generally, the roots of vegetation hold the soil in place and makes it more resistant to erosion. Therefore, the more vegetation present, the more stable the slope is likely to be.
  • Human Modifications: Humans modify stability of slopes in many ways which may trigger the sudden mass movement of the soil in slopes. Such includes the excavation and removal of the slope’s base to build roads, the passage of heavy trucks, blasting, loading of the slope or crest, surface or groundwater manipulation, irrigation and mining.

Slope Failures Mitigation Measures?

Landslide at Montagne Posee, Mahe.


Slope Failure Mitigation or Repair, is not a one-size-fits-all task.  The materials to be used as well as the reinforcement design are influenced by a myriad of factors. Some of the common methods used for slope failure mitigation and repair includes:

  • Proper Drainage: Plans to repair a slope must be accompanied by drainage rehabilitation plans. The planned drainage system must be able to efficiently channel water away from the slope without affecting slope stability or causing erosion. Weep holes in retaining walls and French drains are just two of the more popular drainage options.
  • Terracing & Benching: The nearer a slope is to its natural angle of repose, the more stable it is. It is for this reason that terracing or benching is a popular way of dealing with steep slopes.  This involves making the slope more manageable by dividing it into several smaller and less steep slopes reinforced by retaining walls and friction piles, to name a few.
  • Retaining Walls: Retaining walls are used to stabilize the slopes surrounding a property. Concrete retaining walls are walls that are designed to “retain” or hold in place a substantial amount of soil. They are built on the lower part of a slope to directly suppress a collapse of that part and also to check coming-down collapsed soil and stop it before houses.
  • Soldier piles and Lagging works: This slope stabilization technique often used in projects involving soft soils. Sheet piles can be made of wood planks, vinyl or steel. Piles driven 2/3 of their length into the slope towards load bearing strata (layer that can bear stress) to restrain the collapse of the surface soil layer and installed in such a way that the sheets overlap. Sometimes, lagging or panels are installed between piles to form a retaining wall that will prevent the downward movement of soils.
  • Rock Bolts: Much like piles, rock bolts are used to stabilize slopes that are composed mainly of fractured rocks. The bolts connect the fractured and weak surface to the stronger rock layers underneath, giving the slope stability.
  • Grating Crib: Concrete frames are laid on a slope, within which plants grow to protect the slope from weathering and erosion. It is also possible to directly suppress slope collapse by using the frames in combination with ground anchors, or to allow trees remaining on the slope to be retained by adjusting the arrangement of the frames.
  • Biotechnical Slope Stabilization: In layman’s terms, biotechnical slope stabilization is simply letting plants and vegetation stabilize a slope. Vegetation is good for slope stability and planting different layers of vegetation on the slope is an effective way of stabilizing this slope.

Based on the above discussion, you should realize that repairing a slope and preventing a landslide is not a simple task. If you’re going to do it, then you should do it right. Band-Aid solutions will only be a waste of money as they could also give you a false sense of security, which could ultimately cost you your house and out your family at risk. If your house is on a slope or located near one, get the slope inspected to understand if you are at risk and, if you are, learn about the best slope failure solutions that you can apply to mitigate such risk.

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