What are the Causes of Slope Failure?

Slope failure, which is known as landslides or slope instability, is one of the primary geotechnical concerns that are triggered by a large number of causes, whether natural or man-induced. 

In all engineering projects, mining operations, or natural terrain, knowledge of the triggers for slope failure can avoid or at least reduce the disastrous events associated with it. Thus, major triggers of slope failure include gravity forces, water-related effects, seismic activities, construction, and others.

1. Gravitational Force

The primary cause for slope failure is gravity itself, as every natural or artificial slope is affected by this downward pull of gravity, which tends to move soil, rocks, and debris downslope. 

It leads to slope failure when the shear stress induced because of gravity exceeds that of the shear strength of the material within the slope. 

Other circumstances occur less often; although gravitational force is always present, failure happens most of the time when other factors weaken the resisting forces of the slope or increase the driving forces.

Example: Rockfalls along steep mountain slopes occur because of the constant action of gravity on fractured or weakened rock.

2. Force Due to Seepage Water

Seepage refers to the flow of water through the soil, which can impart additional forces to the slope. 

According to this seepage, the increment of pore water pressure builds up during the seepage process, thus reducing the effective stress that would hold the soil particles together. 

This, in most cases, leads to a decreased shear strength and hence to slope instability.

Example: Water seepages along riverbanks or dams can also weaken the soil and cause the slope to fail or trigger landslide.

3. Flowing Water Erosion of Slope Surface

Erosion occurs by transporting away material via flowing water so that, with time, the structure of the slope becomes weakened. 

Surface erosion may eventually destabilize a slope, especially as a result of heavy rains or storms. 

As the slope deteriorates further, the remaining soil might not be strong enough to carry its weight and the whole slopes may suddenly collapse.

Example: Coastal cliffs continue to be eroded by wave action, resulting in some landslides along shorelines.

4. Sudden Drainage of Water Close to a Slope

When water levels close to a slope, such as within a lake, river, or reservoir, drain away quickly, the resting pressure on the slope is weakened. 

Without this external support, the slope may collapse, as the removal of water takes away part of the friction that is holding the soil or rock in place.

Example: Prompt lowering of water in a reservoir after a flood can lead to the occurrence of landslides or slumping along the banks.

5. Earthquake-Induced Forces

Earthquakes generate seismic forces that can trigger slope failure by shaking the ground and weakening soil and rock materials in the process. 

The internal arrangement of the soil is ruptured by shaking, which, in some cases, leads to liquefaction (for saturated soils) or fast slips along fault lines. Earthquakes are among the main landslide-inducing sources in mountainous areas.

Example: On May 12, 2008, the Sichuan earthquake occurred in China. The major jolt in the mountainous regions triggered many landslides on these hills; the landslides covered entire villages and blocked rivers.

6. Rainfall

Heavy rainfall is one of the most widely experienced causes of slope failure. Water entering a slope can saturate the soil around it, thereby increasing pore water pressure and reducing the shear strength of soil. 

Large-scale landslides can easily result in weak or unconsolidated soils if rain falls for a long period. Intensified rainfall storms or extended periods during rainy seasons can be particularly hazardous for slopes.

Example: Heavy monsoon rainfalls, of which the tropical regions are very susceptible to, bring about landslides that displace large volumes of soil debris in mountainous terrain.

7. Construction Activities at the Toe of the Slope

Human activities, for example, construction or excavation on the toe of a slope, disturb the natural balance. 

Removing material from the toe reduces support to the upper slope and allows for collapse. Cut steep slopes during construction could also destabilize the area.

Example: Construction of highways in hilly areas normally involves cutting on the slopes, which may trigger off landslides when proper stabilization measures are not made.

8. External Loading

External loads such as heavy equipment, buildings, or infrastructure built on or near a slope have an effect on stress. 

When the super-imposed load exceeds the strength of the material or the natural support provided by the slope itself, failure occurs. 

Obviously, external loading also refers to heavy snow or debris accumulation on the slope due to a natural event.

Example: Large dams often impose large external loads on slopes, and careful geotechnical analysis is often the difference between slope stability and failure.

 Conclusion

In reality, slope failure is a complex natural and human-induced interaction phenomenon. Even though gravity remains the main core, other forces like water leakages, seepage, erosion, or seismic movements hasten the slope failure process. 

Human intervention, particularly construction and external loading, further affects the instability of the slope. Thus, knowledge of these causes is of paramount importance for effective management of slopes, hazard mitigation, and preservation of safety of infrastructure and communities at large.

Preventive measures would bring down the risk by slope reinforcement and related means, proper land use planning, and early warning systems. 

We could protect better from such catastrophic failures if the root causes were being handled right.

References:

Abramson, L. W., Lee, T. S., Sharma, S., & Boyce, G. M. (2002). Slope Stability and Stabilization Methods. Wiley.

Duncan, J. M., Wright, S. G., & Brandon, T. L. (2014). Soil Strength and Slope Stability. Wiley.

National Research Council. (1996). Landslides: Investigation and Mitigation. Transportation Research Board.