Soil instability is the inclination of soil for upward, lateral, or downward movement due to natural factors like pore water pressure, cracking, earthquakes, etc.
It is crucial to understand the factors that cause slope instability for two reasons. First, for the construction and design of new slopes, and second, for repairing the old slopes that fail before their intended lifespan.
When designing a new slope, it is important to anticipate the modifications in the chemical and mechanical properties of the soil within the slope. These modifications usually develop over time under different loading and seepage conditions and may affect the slope stability in the long run.
For repairing the slopes that fail before their intended lifespan, it is crucial to recognize the situations and components that led to the failure, so the stability of such slopes can be maintained, and failures can be avoided in the future.
This article talks about the various reasons that lead to slope failures.
- Reasons for Slope Failure
- 1. Decline in Shear Strength of soil
- 2. Increase in Shear Stress of Soil
- Frequently Asked Questions
Reasons for Slope Failure
The basic criterion to achieve a stable slope is to ensure that the shear strength of the soil is higher than the shear stress that may cause the failure. If this basic requirement is not fulfilled, the slope may get unstable and fail.
Following are the ways that can affect the stability of slopes :
- The decrease in the shear strength of the soil.
- The increase in the shear stress that ultimately causes the failure of soil.
1. Decline in Shear Strength of soil
Several factors can result in a decreased shear strength of the soil. The following factors are of particular significance with respect to slope stability.
1.1 Increase in Pore Water Pressure
The frequent increase in the groundwater table and upward seepage, as an outcome of uncommonly heavy rains, are the most common reasons for increased pore water pressure. As a result, the associated effective stresses decrease with increased pore water pressure.
More importantly, the permeability of soil determines the time required for effective discharge of pore water pressure, thus, for highly permeable soils, modifications in groundwater conditions can happen quickly, whereas, for low permeability soil, modifications are slow.
Mostly, the clayey soils have a very low permeability index. Thus, the change in shear strength of clayey soil determines the long term stability of the slope while in case of sandy soils, the short term stability needs to be evaluated.
Slope failures are often preceded by the advancement of fractures through the soil near the crest of the slope. These fractures appear as an outcome of tension in the soil at the ground surface that goes beyond the tensile strength of the soil. Therefore, as the tensile strength of soil reduces, the shear strength on crack-plane also reduces.
Highly plastic and over-consolidated clay easily swells when it comes in contact with water. A classic example to delineate failure due to swelling can be taken from a case in Houston, Texas, where highway embankments constructed with extremely plastic compacted clays failed after ten years as a result of swelling and the loss of shear strength.
1.4 Decomposition of Clayey Rock Fills
Claystone and shales as a filling material in rock joints can be used by breaking them into pieces to form a sound rock that can be relatively stable after compaction. However, with time, as the compacted fill gets in contact with groundwater or seepage water, the disintegration of compacted fill can result in chunks of clay particles.
These chunks of clayey particles then swell into the open spaces within the fill, causing a reduction in the shear strength of soil, and making the fill unstable.
Under sustained loading, the highly plastic clays undergo constant deformation. Therefore, after a certain period, clays might ultimately fail, even at low shear stresses.
The impact of creep is worsened under cyclic loads such as freezing, thawing, wetting, and drying conditions. When such cyclically differing conditions are at their unfavorable extremes, movement of soil within the slope takes place in the downhill direction. Therefore, in the long term, a downslope motion may develop that ultimately results in the failure of slope along the critical plane.
As the water seeps through the voids of the soil, the chemical and mechanical properties of soil start undergoing modifications. This process is known as leaching.
In the case of marine clays, leaching plays an important role as it contributes to the development of quick clay conditions, and such clays have significantly no strength when disturbed.
1.7 Strain Softening
Strain softening phenomenon is associated with brittle soils. In a stress-strain curve of brittle soils, when the critical stress reaches the peak, the shear strength of brittle soils reduces with more constant strain. This type of stress-strain behavior makes a progressive failure, thus creating a path for slope failure.
The process in which rocks and soils lose their strength due to the modifications in physical, chemical, and mechanical properties by external agents such as water, wind, temperature change, etc. is known as weathering.
Weathering significantly reduces the shear strength of the soil. In the worst case, weathering can change the whole structure of rocks and soil, transforming a strong and stable slope into an unstable one.
1.9 Cyclic loading
Under the impact of cyclic loads, the bond between the soil particles may break, and the pore water pressure might increase, resulting in the loss of strength. The loose soil may get saturated due to the increase in pore water pressure and lose all its strength under cyclic loading because of liquefaction.
2. Increase in Shear Stress of Soil
Even if the shear strength of the soil stays intact, the change in the shear stress due to an increase in loading may destabilize the slopes.
The factors through which shear stresses can be increased are discussed below:
2.1 Loads at the Top of the Slope
If the ground at the top of a slope is loaded, the shear stress needed for equilibrium of the slope will be more. If the loads are kept away from the slope’s crest, it would be possible to prevent the increase in shear stresses.
2.2 Water Pressure in Fractures
A slope can become unstable if fractures at the top of a slope are filled with water. The pressure created due to water in the fractures loads the soil and increases the shear stresses. If the cracks stay filled with water for seepage towards the slope face to establish, then the pore water pressure in the soil will increase, leading to an even worse condition.
2.3 Due to an Increase in Soil Weight
Seepage into the soil within a slope can increase the water content of the soil, thereby increasing its weight. If this increase in weight is considerable, specifically with the combination of other forces, it can lead to slope failure.
Excavation that makes a slope steeper will increase the shear stress in the soil within the slope and reduce stability. The disintegration of soil by a stream at the base of a slope has the same result.
2.5 Drop in Water Level at the Base of a Slope
External water pressure acting on the face of the slope provides a stabilizing result. Rather, the slope will become unstable if the water content reduces by increasing shear stress.
When this level drops quickly, and the pore pressures within the slope are not reduced in accordance with the drop in the water level outside, the slope will be less stable. This phenomenon is known as the rapid drawdown condition and is important for the design of partially submerged slopes.
In the event of an earthquake, slopes are subjected to vertical and horizontal velocities that result in cyclic variations in stresses within the slope. This increases them above their static values for brief durations, lasting for seconds or fractions of a second. Even if the shaking doesn’t cause any modification in the strength of the soil, the stability of the slope will reduce during those quick instants when the dynamic forces act in adverse directions.
Frequently Asked Questions
Slickensided surface usually areas appear in clays, especially in extremely plastic clays, due to slip on critical shear planes. Due to shear displacement on unique planes, realignment of clay particles occurs parallel to the critical plane of slip. This leads to the development of a smooth surface with a dull luster known as a slickenside.
The surface areas of the slickenside surface are weaker than the surrounding clay and the friction angle is very low. Therefore, the shear strength of slopes reduces significantly in that region, leading to an unstable slope.