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How can excavation increase the risk of a landslide?

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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.

Excavation increases the risk of a landslide primarily by causing the oversteepening of the base of a slope.1

In geological terms, slopes remain stable because the downward pull of gravity is balanced by the internal shear strength of the materials, which includes frictional strength and cohesive strength. When excavation occurs at the base of a slope, it removes supporting material, which increases the shear stresses acting within the slope. If these stresses exceed the shear strength of the slope-forming materials, the balance is upset, potentially triggering a collapse.312

To maintain safety in construction projects such as highway cuts or railway cuts, engineers must design slopes to specific standards of stability, often employing methods like gradient leveling, drainage, or the installation of retaining structures to counteract the destabilizing effects of excavation.31