Cohesionless soils

Definition

Cohesionless soils are types of soils that do not exhibit any significant cohesion between their particles, primarily relying on friction to resist shear stress. These soils, such as sand and gravel, are typically granular in nature and behave differently under varying conditions like drainage, load stress, and historical stress paths. Understanding cohesionless soils is essential when analyzing factors affecting shear strength and evaluating the bearing capacity of foundations.

5 Must Know Facts For Your Next Test

1. Cohesionless soils have a low internal friction angle, which is crucial in determining their shear strength under various conditions.
2. These soils are often susceptible to liquefaction during seismic events due to their loose structure and high pore water pressure.
3. Cohesionless soils can experience significant changes in effective stress with variations in loading conditions, impacting their stability.
4. The bearing capacity of structures built on cohesionless soils is highly influenced by the soil's density and drainage conditions.
5. When analyzing cohesionless soils, the use of effective stress principles is essential for predicting behavior under loading.

Review Questions

• How do drainage conditions affect the shear strength of cohesionless soils?
  • Drainage conditions play a vital role in the shear strength of cohesionless soils because they determine the pore water pressure within the soil. In fully drained conditions, effective stress increases, enhancing the frictional resistance between particles and thus increasing shear strength. Conversely, in undrained conditions, high pore water pressures can lead to reduced effective stress, significantly lowering the shear strength and potentially causing failure.
• Discuss the differences between the shear strength mechanisms of cohesionless soils compared to cohesive soils.
  • Cohesionless soils primarily rely on inter-particle friction for their shear strength, with minimal contribution from cohesion. This means that their strength is influenced significantly by factors such as particle size, density, and moisture content. In contrast, cohesive soils derive much of their strength from cohesive forces between particles, resulting in different behavior under loading. Cohesive soils often exhibit plasticity and can retain shape under stress due to these cohesive forces, while cohesionless soils are more prone to instability when subjected to high loads.
• Evaluate the implications of using bearing capacity theories for foundation design on cohesionless soils, considering factors such as stress history and soil type.
  • The application of bearing capacity theories like Terzaghi's, Meyerhof's, and Vesic's for foundation design on cohesionless soils requires careful consideration of stress history and soil type. For instance, prior loading can influence the current effective stresses within the soil and affect its bearing capacity. The chosen theory must consider these factors to accurately estimate how much load the foundation can support without failure. Ignoring these aspects could lead to overestimating or underestimating the soil's capacity, ultimately affecting the safety and performance of structures built on these soils.

Related terms

Granular Soils: Soils composed predominantly of sand and gravel particles, characterized by high permeability and low plasticity.

Effective Stress: The stress carried by the soil skeleton, which influences the soil's strength and behavior under loading conditions.

Shear Strength: The maximum stress a soil can withstand without failing in shear; influenced by factors like particle interlocking and friction angle.