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Why can rising groundwater levels make a slope unstable?
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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.
Rising groundwater levels can make a slope unstable primarily by reducing the forces that hold soil particles together. Slopes remain stable because the downward pull of gravity is countered by the internal friction and cohesion between soil particles.1
When the amount of water within a slope increases, it negatively impacts these stabilizing forces in two key ways:
- Reduced Friction and Cohesion: An increase in water content drastically reduces the internal friction and cohesion that resist movement, making the soil more prone to shearing or sliding.1
- Increased Pressure: Flowing groundwater exerts pressure on soil particles, which impairs the overall stability of the slope.2
Additionally, saturation from rainfall can increase the total weight of the debris on a slope, further contributing to the likelihood of downslope movement. Because of these factors, maintaining slope stability often requires effective drainage to prevent excess water from accumulating.21