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How does water weaken sandy slope materials?

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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.

Water weakens sandy slope materials primarily by reducing the friction between the individual particles that make up the slope.1

In geology, the shear strength of a material—its resistance to movement—is largely dependent on the frictional strength between its constituent particles. Because sand grains rely heavily on this interparticle friction for stability, an increase in water content disrupts these interactions. Additionally, some sandy materials possess a loose, open sediment fabric that becomes inherently unstable when flooded with water or mechanically disturbed.1

When the resulting reduction in shear strength causes the material's resistance to fall below the gravitational and other shear stresses acting upon the slope, a landslide may occur.12