Landslides involve the rapid downhill movement of rock, debris, or earth. This geological process occurs when the forces pulling material down a slope overcome the forces resisting its movement. Landslides are significantly more common on steep slopes due to fundamental principles of physics, the influence of water, geological material characteristics, and triggering events.
Gravity’s Influence on Slope Stability
Gravity is the fundamental force driving all landslides, pulling material downward. On a sloped surface, this gravitational force resolves into two components: normal stress and shear stress. Normal stress acts perpendicular to the slope, pressing the material into the incline and contributing to its stability. Shear stress, conversely, acts parallel to the slope, exerting a force that pulls the material downhill.
As the steepness of a slope increases, the component of gravity acting as shear stress (the downhill-pulling force) also increases. Simultaneously, the normal stress (the force holding the material against the slope) decreases. This shift means that on steeper slopes, material has a greater propensity to slide because the downhill force becomes stronger, while resistance weakens. A slope will remain stable as long as the material’s shear strength, which includes friction and cohesion, exceeds the shear stress exerted by gravity. When the shear stress surpasses the material’s shear strength, a landslide occurs.
The Critical Role of Water
Water plays a significant role in reducing slope stability. First, it adds substantial weight to the soil or rock mass. As water infiltrates the ground, filling pore spaces and fractures, it increases the overall mass of the slope material. A water-saturated body of sediment can weigh approximately 13% more than when dry, directly increasing the shear force on the slope.
Second, water reduces the cohesion between soil particles and the friction within the material. Water can act as a lubricant, making it easier for particles to slide past each other. In granular sediments like sand, water can push grains apart, diminishing the friction that typically holds them together.
Third, water increases pore water pressure within the slope. As water fills the empty spaces (pores) between soil and rock particles, it exerts pressure that effectively pushes these particles apart. This increased pore water pressure lowers the material’s shear strength. When pore water pressure rises significantly, it can reduce shear strength to zero, leading to slope failure.
How Soil and Rock Properties Matter
The inherent characteristics of soil and rock materials significantly influence a slope’s stability. Different soil types possess varying strengths, cohesiveness, and permeability. For instance, clay-rich soils tend to absorb moisture and soften, making them susceptible to movement when saturated. Shales, composed of clay- and silt-sized grains, are often associated with landslides, especially when they weather into clayey soil where water content fluctuates.
Conversely, sand and silt-textured soils can be more prone to landslides than clay textures, particularly because sand is loose and grainy, with less bonding between particles. When water is introduced, sand can flow quickly. The presence of loose, unconsolidated materials or highly fractured rocks inherently reduces stability on steep inclines. Geological discontinuities such as faults, joints, and bedding planes also act as zones of weakness, along which material can easily slide.
Factors That Initiate Landslides
While inherent slope conditions and material properties create susceptibility, specific events often trigger landslides. Prolonged or intense rainfall is a common trigger, as it saturates the ground, leading to the effects of increased weight, reduced cohesion, and elevated pore water pressure.
Earthquakes are another significant trigger. Seismic shaking can destabilize slopes by imposing inertial loading or by causing a loss of strength in the slope materials. Ground vibrations can lead to soil liquefaction, where saturated granular soils temporarily lose their strength and behave like a liquid. Large earthquakes can trigger tens of thousands of landslides across vast regions, significantly altering the landscape.
Human activities also frequently initiate landslides by altering natural slope stability. Excavation for roads or construction can oversteepen slopes or remove supportive material from their base. Deforestation removes root systems that help bind soil together, making slopes vulnerable. Other human actions like irrigation, overloading slopes with fill material, or vibrations from machinery or traffic can contribute to pushing an unstable slope past its breaking point.