Cohesive soil

Definition

Cohesive soil is a type of fine-grained soil that exhibits strong inter-particle attraction, primarily due to its clay content, which allows it to retain shape and resist deformation when subjected to external forces. This soil is significant in various engineering applications due to its unique properties, such as high plasticity and compressibility, influencing factors like drainage, load-bearing capacity, and stability in construction projects.

5 Must Know Facts For Your Next Test

1. Cohesive soils are typically classified based on their plasticity, with low plasticity clays being more stable for construction purposes compared to high plasticity clays.
2. The water content significantly affects the behavior of cohesive soils; as water content increases, cohesion decreases, making the soil more susceptible to failure.
3. Cohesive soils can undergo consolidation over time when subjected to load, which can affect the settlement and stability of structures built on them.
4. Seepage analysis in cohesive soils is critical because water movement can influence pore pressure, which directly impacts the effective stress and strength of the soil.
5. Foundation design in cohesive soils often requires consideration of both bearing capacity and potential for excessive settlement due to the compressible nature of these materials.

Review Questions

• How do the properties of cohesive soil influence the design of shallow foundations?
  • The properties of cohesive soil, such as its high plasticity and compressibility, directly influence shallow foundation design by dictating the bearing capacity and settlement characteristics. Designers must account for the soil's ability to support loads without excessive deformation or failure. This involves using appropriate foundation types and sizes to distribute loads effectively and mitigate potential issues such as differential settlement, which can lead to structural damage.
• Discuss how cohesive soils affect seepage analysis and flow nets in geotechnical engineering.
  • Cohesive soils play a significant role in seepage analysis because their low permeability can lead to higher pore water pressures during drainage conditions. Flow nets help visualize how water moves through these soils, allowing engineers to predict potential changes in effective stress and shear strength. Understanding seepage through cohesive soils is essential for ensuring stability in structures like embankments or retaining walls, as improper drainage can lead to failure.
• Evaluate the impact of cohesive soil characteristics on deep foundation capacity and construction methods.
  • The characteristics of cohesive soil significantly impact deep foundation capacity as these soils can provide substantial resistance due to their cohesion but may also pose challenges related to compressibility and consolidation. When using methods like pile installation in cohesive soils, considerations around skin friction, end bearing capacity, and potential heave must be evaluated. Engineers often utilize static and dynamic load testing methods on piles to determine their actual capacity in cohesive environments, ensuring they meet safety standards while addressing unique site conditions.

Related terms

Clay: A type of fine-grained natural soil material that is plastic when wet and hardens when dried; clay is a major component of cohesive soils.

Plasticity Index: A measure of the range of moisture content over which a soil behaves plastically; it indicates the workability of cohesive soils.

Shear Strength: The resistance of a material to shear stress, crucial for understanding how cohesive soils will behave under loading conditions.