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How do shear stress and shear strength determine whether a slope fails?
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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 slope fails when the shear stress acting upon it exceeds the shear strength of the materials forming that slope.12
- Shear Stress: This is the force that promotes downslope movement by causing deformation through slippage along a plane. It is increased by processes such as the oversteepening of a slopeās base (via erosion or excavation), the loading of the slope (such as by water accumulation or debris), and short-term events like earthquakes or heavy rainstorms.32
- Shear Strength: This represents the material's resistance to shearing. It is primarily determined by two factors:1
- Frictional strength: The resistance to movement between interacting particles.2
- Cohesive strength: The bonding between particles.2
Slope failure occurs when these resisting forces are overcome, often because the shear strength has been weakened. For example, increased water content can reduce interparticle friction in sandy materials or dissolve cements and hydrate minerals in clays, thereby lowering the overall shear strength of the slope.2