Landslides are a form of mass wasting, a geological process defined by the downslope movement of rock, debris, or earth. This movement occurs when the forces pulling the material down the slope exceed the internal strength of the slope material itself. A landslide is the end result of a complex interaction between static geological conditions and dynamic triggering factors. The immediate cause of a slide is the imbalance created when the gravitational force overcomes the slope’s resistance.
Fundamental Conditions for Slope Failure
The potential for a landslide is determined by the balance between two opposing forces: shear stress and shear strength. Shear stress is the component of gravity that acts parallel to the slope, attempting to pull the material downward. Shear strength is the material’s internal resistance to this sliding force, acting as the slope’s natural defense against failure.
This inherent strength comes primarily from cohesion (molecular bonding) and internal friction (resistance created as particles rub against and interlock). A slope’s stability is largely controlled by its angle and the type of material it is composed of. Steeper slopes naturally experience higher shear stress, making them more susceptible to failure, especially if the underlying soil or rock is inherently weak or fractured.
The Critical Role of Water and Saturation
Water is the most frequent and significant catalyst for slope failure, often tipping the balance from stability to movement. It affects a slope through three primary mechanisms, all of which reduce the overall shear strength of the material.
Water saturating the soil adds substantial weight to the slope mass, which directly increases the downward-pulling shear stress. This added load can be enough to surpass the existing strength of the slope material.
Furthermore, water acts as a lubricant, reducing the friction between individual soil or rock particles, thereby decreasing the material’s internal resistance to sliding. The most complex effect is the generation of pore water pressure, which is the pressure exerted by water trapped in the voids between soil grains. As water fills these spaces, it pushes the grains apart, reducing the effective stress—the force with which the particles press against each other.
This reduction in effective stress causes a loss of shear strength, which can lead to rapid failure even on relatively gentle slopes. Prolonged or intense rainfall drives this saturation, leading to a rise in the groundwater table and increased pore water pressure throughout the slope mass.
Sudden Natural Triggers
While water saturation typically causes failure over a period of time, certain high-energy natural events can trigger immediate, rapid landslides. Seismic activity, such as earthquakes, is a major trigger, as the ground shaking temporarily reduces the effective stress and strength of the slope materials. The vibrations generate cyclic stress, which can cause saturated, loose, granular soils to undergo liquefaction.
Liquefaction occurs when the shaking increases the water pressure in the soil voids so severely that the soil temporarily behaves like a dense liquid, losing nearly all its shear strength. This process can cause massive landslides, even on slopes that are not particularly steep. Volcanic activity can also trigger landslides; eruptions can melt snow and ice rapidly, creating destructive mudflows called lahars, or destabilize the flanks of the volcano itself through ground deformation.
More localized triggers include rapid weather fluctuations, such as sudden snowmelt or cycles of freezing and thawing. The expansion of freezing water in rock fractures and soil pores slowly weakens the material over time, a process known as weathering. When this weakened material is subjected to a subsequent trigger, it is much more likely to fail.
How Human Activity Destabilizes Slopes
Human actions frequently alter the natural conditions of a slope, inadvertently increasing its susceptibility to failure.
A common cause is the improper excavation or grading of slopes during construction, often involving cutting into the hillside to create level platforms for roads or buildings. This process can steepen a slope beyond its natural angle of repose or remove material from the base, eliminating the support—known as toe support—that was holding the slope in place.
Deforestation is another major contributing factor, as it removes the root systems that naturally bind soil particles together, significantly reducing the soil’s internal shear strength. Without the soil-anchoring effect of vegetation, the land becomes more prone to erosion and the surface runoff of water increases. Furthermore, tree removal means more water is available to infiltrate the ground, accelerating saturation and the buildup of pore water pressure.
Finally, human changes to natural drainage patterns often concentrate water flow into vulnerable areas. Activities like poorly planned irrigation, leaky utility lines, or the construction of impermeable surfaces can channel large volumes of water. This concentrated water increases local saturation, sometimes dramatically raising the groundwater level and triggering landslides.