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What causes a landslide to begin on a slope?
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landslide
This 1995 landslide at La Conchita, a coastal town in California, swept away a hillside road and destroyed a number of houses.
A landslide begins when the gravitational and other shear stresses acting upon a slope exceed the shear strengthāthe material's internal resistance to shearing.12
This imbalance is typically triggered by processes that either increase stress or weaken the slope's material:
- Increased Stress: Factors such as the oversteepening of a slopeās base (via erosion or excavation) and the loading of the slopeācaused by water inflow, rising groundwater levels, or the accumulation of debrisābuild up internal stress.2
- Weakened Strength: The shear strength of slope material can be reduced by the removal of vegetation, which decreases stability. Additionally, water saturation can weaken materials by reducing interparticle friction or dissolving cements that bond particles together.32
- Short-term Triggers: Sudden events, including earthquakes and intense rainstorms, often act as the immediate catalyst for movement. In some cases, soil liquefaction caused by seismic activity can cause a sudden loss of strength, leading to a landslide.124