Purely cohesive soils

Definition

Purely cohesive soils are types of soils that primarily consist of fine-grained particles, such as clay, which exhibit significant cohesion due to the electrochemical forces between particles. These soils are characterized by their ability to retain water and their low permeability, resulting in unique engineering properties that influence their behavior under loading conditions. Understanding purely cohesive soils is critical for evaluating the bearing capacity and stability of foundations and structures.

5 Must Know Facts For Your Next Test

1. Purely cohesive soils have a high plasticity index, which means they can change shape significantly when subjected to mechanical stress without cracking or breaking.
2. The bearing capacity of purely cohesive soils can be determined using various theories, including Terzaghi's bearing capacity equation that factors in cohesion.
3. These soils often exhibit a failure mechanism called undrained shear strength, where the soil strength is defined without considering drainage conditions.
4. In purely cohesive soils, the cohesion value plays a vital role in the overall shear strength, often dominating the soil's response under short-term loading conditions.
5. Moisture content has a significant impact on purely cohesive soils, as increased water can reduce cohesion and lead to instability in slopes and foundations.

Review Questions

• How do purely cohesive soils differ from granular soils in terms of mechanical properties and behavior under load?
  • Purely cohesive soils differ significantly from granular soils due to their fine-grained nature and cohesive properties. While granular soils rely mainly on intergranular friction for stability, purely cohesive soils depend on molecular attraction between particles for strength. This results in purely cohesive soils exhibiting different behaviors under load, such as higher plasticity and unique failure mechanisms like undrained shear strength, making them crucial for foundation design.
• Discuss how Terzaghi's bearing capacity theory applies specifically to purely cohesive soils and its implications for foundation design.
  • Terzaghi's bearing capacity theory provides a framework for assessing the load-bearing capacity of purely cohesive soils by incorporating cohesion into the equation. The formula allows engineers to calculate the ultimate bearing capacity based on factors like soil cohesion and depth of the foundation. Understanding this relationship is vital for foundation design because it helps ensure structures are built on stable ground, reducing the risk of settlement or failure.
• Evaluate the implications of pore water pressure on the effective stress principle in purely cohesive soils during loading scenarios.
  • The effective stress principle plays a critical role in understanding how pore water pressure affects purely cohesive soils under loading scenarios. As loads are applied, any increase in pore water pressure reduces the effective stress acting on soil particles, potentially leading to decreased shear strength. This situation is particularly important during undrained conditions where excess pore pressure cannot dissipate quickly, highlighting the need for careful consideration in geotechnical design and analysis to prevent structural failures or landslides.

Related terms

Cohesion: The internal molecular attraction between particles in soil that contributes to its strength and stability.

Shear Strength: The resistance of soil to shear stress, which is an essential factor in determining the stability of slopes and foundations.

Effective Stress Principle: A fundamental concept in soil mechanics that relates the stress carried by soil skeleton to the pore water pressure, influencing the behavior of cohesive soils.