Mass movement is the bulk movement of soil, rock, and debris down a slope under the direct influence of gravity. This process is a fundamental part of Earth’s surface sculpturing, linking weathering and the transport of material by streams or glaciers. Gravity constantly drives downhill movement, but failure only occurs when the gravitational force pulling the material down overcomes the slope’s internal resistance. This requires either a reduction in the material’s strength or an increase in the load. Potential failure is determined by pre-existing weaknesses in the slope combined with specific events that act as the final trigger.
Static Predisposing Factors
The inherent characteristics of a slope determine its baseline vulnerability to movement long before any external trigger is applied. This vulnerability is defined by a dynamic balance between the driving force of gravity and the material’s ability to resist that force.
The geometry of the slope dictates how the force of gravity is resolved into components that either drive or resist motion. A steeper slope increases the shear stress, the component of gravity acting parallel to the slope surface and attempting to pull the material downslope. Conversely, the normal stress, acting perpendicular to the slope, is reduced on steeper angles, lessening the friction that holds the material in place. When the steepness exceeds the material’s angle of repose—the steepest angle at which loose material remains stable—the slope is highly predisposed to failure.
The geological structure and composition of the slope material also represent long-term weaknesses. Different rock types possess varying strengths, with crystalline rocks like granite being stronger than sedimentary rocks such as shale or unconsolidated sediments. Discontinuities, including fractures, faults, and bedding planes, act as pre-determined surfaces of weakness where movement can initiate. If these planes align closely with the slope angle, they provide a smooth, low-friction surface for a slide.
The overall resistance of the material to being pulled apart or sliding is termed shear strength. This strength is derived from two primary factors: the internal friction between individual soil or rock particles and the cohesion that holds the particles together. A slope remains stable only as long as its shear strength is greater than the shear stress imposed by gravity. When conditions weaken this internal strength, the material becomes primed for failure even without a change in slope angle.
The Critical Role of Water and Pore Pressure
Water is the most common factor contributing to mass movement, primarily reducing the slope’s resistance to failure. When water infiltrates a slope, it directly adds mass to the soil or rock, increasing the overall gravitational driving force. A saturated body of sediment can weigh over 10% more than when dry, often sufficient to push a marginally stable slope past its breaking point.
The most significant effect of water is its influence on the internal mechanics of the soil through pore water pressure. Soil consists of solid grains with void spaces, or pores, between them. When these pores fill with water, the water exerts an outward pressure on the grain boundaries.
This outward pressure pushes the soil grains apart, reducing the force with which they press against each other, known as the effective normal stress. Since shear strength is directly proportional to the effective normal stress, increased pore pressure causes a substantial drop in the material’s ability to resist shearing. This mechanism can reduce shear strength by up to 60%, making the slope material behave more like a fluid.
Water also contributes to instability by exerting hydrostatic pressure within cracks and fissures in rock masses. As water fills a fracture, the pressure acts as a wedge, forcing rock structures further apart and widening the discontinuity. This action reduces the overall coherence and strength of the rock mass, preparing it for collapse. A rising water table is a major concern, as it drastically increases pore water pressure at depth, often preceding a deep-seated landslide.
Dynamic Triggers and Human Modification
While static factors create the potential for movement, dynamic triggers provide the final energy input needed to overcome the slope’s weakened resistance. These triggers are rapid, external events that momentarily increase shear stress or dramatically decrease shear strength.
Seismic activity, such as an earthquake, is a powerful and sudden trigger. Ground shaking generates inertial forces that momentarily increase the gravitational load on the slope material. The cyclical loading from seismic waves can instantaneously exceed the material’s shear strength, resulting in widespread failure. In saturated, loose granular soils, intense shaking can also lead to liquefaction, where increased pore water pressure causes the soil to lose all effective stress and flow like a liquid.
Volcanic activity can also act as a trigger, often through the rapid introduction of heat and water. The sudden melting of snow and ice, combined with ash and loose debris, creates fast-moving, destructive mudflows called lahars. Additionally, the rapid accumulation of heavy ash increases the load, while volcanic earthquakes and hydrothermal weakening contribute to edifice collapse.
Human modification of the landscape frequently acts as a trigger by exacerbating existing weaknesses. Undercutting the toe of a slope, such as during road construction, removes natural support and instantly increases shear stress on the remaining mass. Conversely, placing excessive load on the top of a slope, through construction or dumping waste, increases the overall gravitational driving force.
Deforestation removes the binding strength provided by root systems, which anchor the soil and sediment. Plant roots also remove moisture from the soil through evapotranspiration, helping to maintain lower pore water pressures. The removal of this natural reinforcement and drainage mechanism significantly lowers the shear strength of the near-surface material, making the slope highly susceptible to failure during heavy rainfall.