Cohesive Soils

Definition

Cohesive soils are types of soils that exhibit significant cohesion due to the presence of fine particles, such as clay, which have a strong tendency to stick together. This property makes cohesive soils critical in understanding engineering principles, as they affect the behavior of structures built on or within them.

5 Must Know Facts For Your Next Test

1. Cohesive soils typically have high plasticity, which means they can change shape without breaking when wet.
2. They are characterized by their ability to retain water and can become very soft when saturated, affecting their load-bearing capacity.
3. Cohesive soils often require specific testing methods, like unconfined compression tests, to determine their mechanical properties.
4. In slope stabilization efforts, cohesive soils may need reinforcement due to their susceptibility to erosion and instability under load.
5. The effective stress principle is vital for understanding how changes in pore water pressure affect the shear strength of cohesive soils.

Review Questions

• How do cohesive soils influence the selection of field density tests in engineering projects?
  • Cohesive soils significantly impact field density tests because their unique properties require different approaches than granular soils. In cohesive soils, tests like nuclear density gauges are preferred as they provide accurate measurements of moisture content and density without disturbing the soil structure. Understanding these properties is essential for engineers to assess stability and settlement potential for foundations in areas with cohesive soil.
• Discuss how soil classification systems help in the identification and understanding of cohesive soils' behavior.
  • Soil classification systems like USCS and AASHTO categorize cohesive soils based on particle size and plasticity. By identifying these soils correctly, engineers can predict their behavior under loading conditions and environmental changes. Cohesive soils are often classified as clays, which indicates their high plasticity and shear strength characteristics. This classification informs design decisions related to construction methods and foundation stability.
• Evaluate the implications of drained versus undrained shear strength in cohesive soils during foundation design.
  • Understanding drained and undrained shear strength is crucial for foundation design in cohesive soils. Drained shear strength applies when pore water pressures dissipate over time, allowing the soil to consolidate and regain strength. Conversely, undrained shear strength is significant during rapid loading conditions where pore pressures cannot dissipate, often leading to temporary failures. Engineers must consider these conditions when designing foundations to ensure they can withstand both immediate and long-term loads.

Related terms

Plasticity Index: A measure of the range of moisture content over which a cohesive soil remains plastic or workable, indicating its ability to deform without cracking.

Atterberg Limits: The moisture content thresholds that define the transitions between the liquid, plastic, and solid states of cohesive soils, crucial for understanding soil behavior.

Shear Strength: The maximum resistance of soil to shear stress, significantly influenced by the cohesion and internal friction angle of cohesive soils.