Shear Strength

Definition

Shear strength is the ability of a material to resist sliding or shearing forces when subjected to stress. It plays a critical role in understanding how soils behave under loading conditions, particularly during seismic events, where sudden ground movements can lead to instability and failure. The shear strength of soil is influenced by factors such as cohesion, friction, and the effective stress within the material, which are essential for evaluating risks related to liquefaction and lateral spreading during earthquakes.

5 Must Know Facts For Your Next Test

1. Shear strength is essential for predicting potential soil failure during seismic activity, particularly in saturated soils that may undergo liquefaction.
2. The two main components of shear strength in soils are cohesion and internal friction angle, both of which are influenced by the soil's physical properties.
3. During an earthquake, a sudden increase in pore water pressure can reduce the effective stress, leading to a decrease in shear strength and potential failure.
4. Liquefaction occurs when saturated soils lose their shear strength due to increased pore water pressure, causing them to behave like a liquid.
5. Lateral spreading can result from reduced shear strength in soil layers, causing ground movement away from a point of rupture during seismic events.

Review Questions

• How does shear strength influence the behavior of soils during an earthquake?
  • Shear strength is crucial in determining how soils will respond to seismic forces. When an earthquake occurs, the ground shaking can increase pore water pressure in saturated soils, which reduces their effective stress and consequently their shear strength. If this reduction leads to shear strength falling below the mobilized stresses, it can result in soil failure, causing phenomena like liquefaction where solid ground behaves like a fluid.
• Discuss the role of cohesion and friction in defining the shear strength of soil under lateral spreading conditions.
  • Cohesion and friction are key factors that define shear strength and are particularly important during lateral spreading. Cohesion provides resistance against sliding due to internal bonding forces among soil particles, while friction relates to how well particles interlock and resist motion under applied forces. In conditions where lateral spreading occurs, a loss of effective stress can reduce both cohesion and frictional resistance, making soils more susceptible to movement and instability.
• Evaluate how effective stress changes during an earthquake impact the shear strength of saturated soils and their potential for liquefaction.
  • Effective stress changes significantly during an earthquake due to rapid increases in pore water pressure caused by shaking. When pore water pressure rises, it effectively reduces the normal stress acting on soil particles, leading to a decrease in shear strength. If this reduction causes shear strength to drop below what is required to maintain stability, it can trigger liquefaction, where saturated soils lose their rigidity and behave as a viscous fluid. This transformation can lead to severe structural damage and ground deformation.

Related terms

Cohesion: The attractive force between particles in a soil that helps resist external forces, contributing to the overall shear strength.

Effective Stress:

The stress that contributes to the strength of the soil, calculated as the total stress minus pore water pressure.

Mohr-Coulomb Failure Criterion: A mathematical model that describes the relationship between shear strength, normal stress, and internal friction in soil mechanics.