Cohesion

Definition

Cohesion refers to the internal resistance of a soil or material to shear stress, which helps maintain stability in structures like slopes and earth retaining walls. It plays a crucial role in determining the overall stability of slopes and retaining structures by influencing how well the soil particles stick together, thereby affecting their ability to withstand external forces such as water pressure and gravity.

5 Must Know Facts For Your Next Test

1. Cohesion is particularly significant in clayey soils, where the particle attraction due to moisture plays a major role in stability.
2. In cohesionless soils, like sandy soils, cohesion is minimal, and stability largely depends on friction between particles.
3. Cohesive strength can be affected by factors such as moisture content, temperature, and the presence of organic materials.
4. When analyzing slope stability, cohesion is often included in calculations using methods like the Mohr-Coulomb failure criterion.
5. A decrease in cohesion can lead to increased risk of landslides or failure of earth retaining structures during heavy rain or flooding.

Review Questions

• How does cohesion influence the stability of slopes and retaining structures?
  • Cohesion is a key factor that influences how well soil particles stick together, which directly affects the stability of slopes and retaining structures. Higher cohesion means greater internal resistance to shear stress, making it less likely for a slope to fail or for a retaining wall to collapse under pressure. Engineers must assess cohesion when designing structures to ensure they can withstand environmental forces.
• Discuss the role of effective stress in relation to cohesion and its impact on slope stability.
  • Effective stress is crucial in understanding how cohesion contributes to slope stability. It is calculated by subtracting pore water pressure from total stress and reflects the actual stress acting on soil particles. As pore water pressure increases, effective stress decreases, reducing cohesion and potentially leading to slope failure. Thus, managing water levels is essential for maintaining cohesive strength and ensuring structural integrity.
• Evaluate how changes in soil composition might affect cohesion and subsequently influence the design of earth retaining structures.
  • Changes in soil composition can significantly alter cohesion levels, impacting the design parameters for earth retaining structures. For example, if a construction site has a high percentage of clay, it may exhibit strong cohesive properties that require different design considerations compared to sandy soils with low cohesion. Engineers must assess these changes carefully, as insufficient consideration can lead to structural failures under load or adverse weather conditions. This evaluation is critical for creating safe and effective designs.

Related terms

Shear Strength: The maximum resistance of a material to shear deformation, influenced by both cohesion and internal friction.

Friction Angle: The angle at which soil particles resist sliding over each other, contributing to the overall stability of a slope.

Effective Stress: The stress that contributes to the strength of soil, calculated by subtracting pore water pressure from total stress.