Question

Explain the modes of failure with different factors influencing the decrease in shear strength of slope. Draw neat sketches wherever necessary.

Answer 1

Ans:- Slope failure occurs when the downward movements of material due to gravity and shear
stresses exceeds the shear strength.

Therefore, factors that tend to increase the shear stresses or decrease the shear strength increase the chances of failure of a slope.

Different processes can lead to reduction in the shear strengths of rock mass.

Increased pore pressure, cracking, swelling, decomposition of clayey rock fills, creep under sustained loads, leaching, strain softening, weathering and cyclic loading are common factors that decrease the shear strength of
rock mass.

In contract to this the shear stress in rock mass may increase due to additional loads
at the top of the slope and increase in water pressure in cracks at the top of the slope, increase in soil weight due to increased water content, excavation at the bottom of the slope and seismic effects.

In addition to these reasons factor contributing in failure of slope are properties of rock mass, (slope geometry), state of stress, temperature and erosion.

DIFFERENT FACTORS INFLUENCING THE DECREASE IN SHEAR STRENGTH

1. GEOMETRY SLOPE:-

Important parameters of slope geometry affecting its stability include height and angle of slope.
The critical height of slope depends on shear strength, density and bearing capacity of the slope foundation.

Slope stability generally decreases with increase in height of slope.

As the slope height increases, the shear stress within toe of slope increases due to added weight.

Shear stress is also related to the mass of the material and the slope angle.

With increasing slope angle, the tangential stress increases which result in increase in shear stress thus reducing its stability.

2. TEMPERATURE

The effects of temperature also influence the performance of a rock slope.

Large temperature changes can cause rock to spall due to the accompanying contraction and expansion.

Freezing of water in discontinuities causes more significant damage by loosening the rock mass.
Repeated freeze cycles may result in gradual loss of strength.

Except for periodic maintenance requirements, such effects are a surface phenomenon and are most likely of little concern for permanent slopes.

However, in a few cases, surface deterioration could trigger slope instability on a larger scale.

3.EROSION

Two aspects of erosion need to be considered from slope stability point of view.

The first is alarge scale erosion, such as a river erosion occurring at the base of a slope.

The second is a relatively localized erosion caused by groundwater or surface runoff.

In the first type, erosion changes the geometry of the potentially unstable rock mass. The removal of material at the toe of a potential slide reduces the confining stress that may be stabilizing the slope.

Localized erosion of joint filling material, or zones of weathered rock, can effectively decrease interlocking between adjacent rock blocks.

Loss of such interlocking significantly reduces the rock mass shear strength. The resulting decrease in shear strength may allow a previously stable rock mass to move causing slope failure.

In addition, localized erosion may also result in increased permeability and ground-water flow thus affecting the stability of rock slope.

4.SEISMIC EFFECT

Seismic waves passing through rock adds stress which could causes fracturing in the rock
mass. As a result, friction is reduced in unconsolidated masses as they are tarred apart which may induce liquefaction.

Landslide is one of the major hazards resulting due to earthquakes. Blasting and earthquakes events affect rock slopes in two distinct ways with different time scales.

The first effect is in the form of immediate co-seismic detachment of rock from a slope
face.

:- The important geotechnical properties affecting stability of a slope are shear strength of material, particle size distribution, density, permeability, moisture content, plasticity and angle of repose. The strength of rockmass is a very important factor that affects the stability  of slopes.