critical slip surface
A landslide is the rapid downslope movement of rock, soil, or debris under the influence of gravity. Landslides occur when gravitational driving force exceeds the shear strength of the slope material, which depends on cohesion, friction angle, and the effective stress reduced by pore water pressure. They range from slow soil creep (millimetres per year) through rotational slumps to catastrophic rock avalanches travelling hundreds of kilometres per hour.
The stability of a slope is quantified by the factor of safety (FoS): FoS = resisting forces / driving forces. FoS > 1 means stable; FoS = 1 means on the verge of failure; FoS < 1 means unstable. Key factors that reduce FoS include slope steepening (by erosion or undercutting), increased water content (raising pore pressure and reducing effective stress), removal of supporting material (natural or by construction), and earthquake shaking (providing dynamic driving force and briefly exceeding static shear strength).
Landslide hazard assessment combines geological mapping, historical inventory analysis, slope stability modelling (infinite slope analysis, limit equilibrium methods, finite element simulation), and remote sensing. Early warning systems using GPS, extensometers, piezometers, and rainfall gauges monitor precursor movements and trigger evacuations. Climate change is increasing landslide frequency in many regions by intensifying rainfall events and accelerating permafrost thaw, which destabilises mountain slopes previously held together by ice.
Common triggers include intense or prolonged rainfall (raising pore water pressure in soils), earthquake shaking (providing additional driving force and liquefying saturated soils), snowmelt, volcanic eruptions, undercutting by rivers or waves, removal of vegetation (roots that stabilise soil), and human activities such as road construction, excavation, or poorly managed irrigation.
The factor of safety (FoS) is the ratio of the forces resisting slope failure (friction and cohesion along a potential failure surface) to the driving forces (gravity component along that surface). FoS = 1.0 means the slope is on the verge of failure; FoS < 1 means failure is imminent. Engineers typically design slopes with FoS of 1.3–1.5, depending on the consequences of failure.
Water saturating soil raises the pore water pressure between soil particles. By Terzaghi's principle of effective stress, the effective stress (which drives friction and cohesion) equals total stress minus pore pressure. Rising pore pressure reduces effective stress and thus the frictional resistance to sliding, potentially bringing FoS below 1. Clay-rich soils are particularly vulnerable because absorbed water reduces cohesion and makes them plastic.
A debris flow is a fast-moving mixture of water, soil, and fragmented rock that behaves as a viscous fluid. Unlike a solid landslide that moves as a coherent mass along a failure surface, a debris flow mobilises and flows like a dense slurry, often channelled into valleys and travelling long distances at speeds of 10–50 km/h. Debris flows are particularly destructive because they are hard to predict and can inundate large areas.
Monitoring uses a combination of: GPS and InSAR (satellite radar) to measure surface deformation, extensometers to measure crack opening, inclinometers to track subsurface movement, piezometers to measure pore water pressure, and rain gauges to monitor triggering rainfall. Machine learning models correlate historical landslide inventory data with slope angle, geology, land cover, and rainfall to produce regional susceptibility maps used in land-use planning and early warning systems.