Slope failures occur as a result of slope instability which is the movement of a mass of soil, rock, debris, or snow in response to gravity when the support for such mass of body has been removed or disturbed. Slope instability is one of the major ecological challenges in the world presently. Slope instability can be induced by natural processes such as earthquake-induced soil liquefaction, heavy and sustained rainfall that triggers gully erosion, etc., however, most cases of slope instability are caused by the actions of man such as construction activities that lead to the disturbance of natural soil structure and alteration of natural soil/rock slope. The cases of slope failures are aggravated by climate change, hence, it is imperative that continuous efforts are made at finding sustainable measures for preventing and remedying slope failures.
Causes of Slope Failure
Slope failure is caused by natural forces such as gravitational forces, seepage forces, and earthquake forces. It can also be caused by actions of man such as excavation near the base of the slope. Slope failures occur when the forces causing failure are greater than the shearing resistance (shear strength) developed along the critical surface of failure. Factors leading to the failure of slope can be categorized into two:
Factors that lead to an increase in the shear stresses of the soil such as the weight of water causing saturation of soils, surcharge loads, seepage pressure, and steepening of a slope by excavation or natural erosion and
Factors that lead to a decrease in the shear strength of the soil such as an increase in water content, an increase in pore water pressure, shock or cyclic loads, weathering processes, etc.
When a slope fails, it creates environmental nuisance and leads to enormous economic losses such as loss of soil mass, loss of infrastructures, loss of agricultural products, and in some cases, loss of lives.
Most slope failures occur during the rainy seasons as a result of the presence of water which causes both increase in stress and loss of shear strength.
Types of Slope Failure
Natural slopes, slopes of excavation, or slopes of an embankment can fail by four different means viz: rotational failure, translational failure, compound failure, and wedge failure.
1. Rotational Failure
In rotational failure, rotational slips, the shape of the failure surface in a section may be a circular arc or a non-circular curve. Circular slips are usually associated with homogenous, isotropic soil conditions while non-circular slips are usually associated with non-homogenous conditions (see Figure). Failure here occurs by the rotation along a strip surface by downward and outward movement of the soil mass.
Rotational failures are further divided into:
i. Toe failure
ii. Slope failure and
iii. Base failure
2. Translational Failure
Translational failures occur in an infinite slope along a long failure surface parallel to the slope or along slopes of layered materials. The shape of the failure surface is influenced by the presence of any hard stratum at a shallow depth below the slope surface.
3. Compound Failure
A compound failure is usually a combination of the rotational slips and the translational slip and occurs when a hard stratum exists at a considerable depth below the toe. This failure surface is unique by being curved at the two ends and plane in the middle portion.
4. Wedge Failure
This type of failure is also known as plane failure or block failure. It occurs when distinct blocks and wedges of the soil mass become separated. It is similar to translational failure but unlike the translational failure that usually occurs in an infinite slope, a plane failure may occur even in a finite slope consisting of two different materials or in a homogenous slope having cracks, fissures, joints or any other specific plane of weakness.
Measures of Slope Stability
To fix a failed slope often take years and gulps large sums of money, hence, it is of utmost importance to prevent slope failure.
Slope stability entails measures to stabilize slopes and prevent them from failure. The first approach to slope stability are slope stability analysis. The basis of slope stability analysis is the determination of the factor of safety against failure. In engineering, a factor of safety relates the strength of a system to an intended load. Thus, there should be a minimum acceptable strength for an intended load if the system would remain stable under the load.
In slope stability analysis, three different definitions of factor of safety are usually encountered:
Factor of safety with respect to shear strength
Factor of safety with respect to cohesion
Factor of safety with respect to friction
Factor of safety with respect to shear strength: this is defined as the ratio of the shear strength of the soil to the shear stress along the surface of failure.
Factor of safety with respect to cohesion: this is the ratio of the available cohesion intercept to the mobilized cohesion intercept.
Factor of safety with respect to friction: this is the ratio of the available frictional strength to the mobilized frictional strength.
Methods of Slope Stability Analysis
Slope stability analysis can be carried out by a number of methods that include:
- Culmann Method
- Logarithmic Spiral Method
- Method that consider slope failure under undrained conditions (ϕ = 0 analysis method)
- Friction-circle Method
- Taylor Stability Number Method
- Swedish Circle Method
- Bishop Method
- Bishop and Morgenstern Method
- Spencer Method
Methods ‘a’ and ‘b’ are limiting equilibrium methods that assume that Coulomb’s failure criterion is satisfied along the assumed failure surface. Methods ‘c’, ‘d’, and ‘e’ consider only the whole free body of the slope while methods ‘f’, ‘g’, ‘h’, and ‘i’ divide the free body into many slices that consider the equilibrium of each slice.
Innovative Measures to Mitigate and Prevent Slope Failures
Researchers have continued to seek sustainable measures to stabilize slopes and prevent slope failures. Attached are videos of lectures on alternative measures to stabilize slopes. The videos were embedded from the website of Advances in Geotechnical Engineering – from Research to Practice (AGE-RP).
Video 1: Applications of Unsaturated Soil Mechanics to Slope Stability:
Video 2: Rainfall-Induced Slope Processes and Failures:
Video 3: Soil Erosion and Design Applications:
Video 4: Erosion Control Techniques for Drainage Channels: from traditional methods to latest sustainable techniques:
Disclaimer: The videos are shared here to advance knowledge. It does not imply that the author has any agreement or partnership with AGE-RP.
Arora, K.R. (2014). Soil Mechanics and Foundation Engineering (7th edition). Standard Publishers Distributors, New Delhi, India.
Murthy, V.N.S. Geotechnical Engineering: Principles and Practices of Soil Mechanics and Foundation Engineering. Marcel Dekker Inc. 270 Madison Avenue, New York, USA.
Venkatramaiah, C. (2006). Geotechnical Engineering (3rd edition). New Age International Limited, New Delhi, India.