A2 Rapid Landslides and Mass Flows

Topic

Slope failures occur when the shear stress mobilizing the ground exceeds the shear strength that the soil or rock itself can provide; this interplay between the two forces defines the slope's factor of safety, and when that factor drops below one, the slope fails.

Heavy rainfall is the most common trigger for this type of failure: as water infiltrates the ground, it raises the soil's pore-water pressure. This increase in pressure reduces the soil's effective strength—as described by the Mohr-Coulomb criterion—bringing the slope closer to the failure threshold mentioned earlier.

In slopes already saturated with water, this pore-water pressure can completely eliminate the soil's apparent cohesion; this total loss of cohesion triggers planar or translational landslides, where an entire mass of ground slides as a single unit along a well-defined failure surface.

When the material mobilized in these landslides mixes with water, the concentration of solids within the mixture determines the resulting type of movement: a mixture with a higher solid content behaves like a debris flow, whereas a more dilute mixture behaves like a mudflow.

Liquefaction, meanwhile, occurs when pore-water pressure completely exceeds the total stress acting on the soil; at this point, the soil loses all shear strength and begins to behave like a fluid rather than a granular solid.

Physical models describing this entire process range from infinite slope analysis—which evaluates how rainfall destabilizes a slope—to the Bingham and Herschel-Bulkley rheological models, which describe how the material flows once the landslide has transformed into a mass flow. Thus, ranging from the interplay between shear stress and resistance—with which this document begins—to the rheological models that conclude it, and encompassing factors such as rainfall that raises pore pressure, saturation that eliminates cohesion, and liquefaction that transforms soil into a fluid, slope failures and rapid mass flows can cause a seemingly stable slope to collapse and turn, in a matter of seconds, into a moving mass of soil, rock, and water.

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