Wednesday, December 21, 2011

Slopes Stability

In a sloping soil mass, forces acting on it try to cause the soil mass to move from higher elevation to lower elevation. The forces which try to cause instability are force due to gravity, imposed loads and seepage forces. These forces produce shearing stress in the soil, and unless the shearing resistance of the soil is sufficient to withstand them the slope fails in shear along a well defined failure surface. The safety of a slope against failure is its stability. The failure of a mass of soil located beneath a slope is called a slide. To avoid failure, a thorough analysis is needed else the failure may lead to loss of human life as well as a loss of national economy.

The stability can be determined with a reasonable accuracy, if the geological cross section of the slope and the shear strength parameters of the soil are known. The accuracy of the result depends on the accuracy with which the shear strength is predicted.

Causes of Slope Instability
The causes of slope instability may be due to:
a. Increase in shear strength.
b. Reduction in shear strength.

These may be due to :
  1. Gravitational force
  2. Force due to seepage of water
  3. Erosion of surface due to flowing water
  4. The sudden lowering of water adjacent to a slope
  5. Forces due to earthquakes
Many slope failures are associated with the exceeding presence of water during heavy rainfall and flood. The introduction of the exceeding water content to the soil contributes to both increases in the shear stresses as well as the reduction in the shear strength due to the increase in the pore water pressure.

Remedial measures for Slope Stability
  1. Flattening the slope reduces the weight of the potential sliding mass and consequently the driving force, resulting in the increase of the factor of safety.
  2. The presence of a berm adjacent to the toe of the slope increases the resisting forces and consequently the factor of safety will be increased. This is especially useful when there is a possibility of base failure.
  3. Proper drainage of water is one of the most effective methods to increase the stability of earth slope. Surface drainage and sub drainage are provided to increase the stability of the slope.
  4. Densification of the ground increases the shearing resistance of the soil, thus increases the stability of the slope. a) Addition of chemical additives (lime or cement) for clay soils & b) Vibro-floatation (deep densification of cohesion-less soils)
  5. Construction of earth retaining structures (retaining walls, reinforced earth, etc.) at the toe of the slope increases the resistance of the potential sliding mass.
  6. Planting the surface of the slope is beneficial to protect the slope against shallow slides. The plants enhance the stability of the slope in two different ways:
  • Consolidation of the soil by a network of roots and therefore increasing the resistance to shear.
  • Drying out the surface layers by water suction by the roots, increasing the shearing resistance of the soil.

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