Cohesionless soil

Definition

Cohesionless soil is a type of granular soil that lacks the ability to stick together, resulting in low shear strength and high permeability. This soil primarily consists of particles such as sand and gravel, which do not exhibit significant cohesion due to the absence of water or other binding agents. When dealing with structures like piles, understanding how cohesionless soils behave is crucial for assessing pile capacity and the overall stability of foundations.

5 Must Know Facts For Your Next Test

1. Cohesionless soils typically have a friction angle that governs their shear strength, which can be influenced by factors like particle size and density.
2. These soils are prone to liquefaction during seismic events, where they can lose strength and behave like a liquid.
3. In pile load tests, the interaction between cohesionless soils and pile design is essential to determine the actual load-bearing capacity.
4. The design of foundations in cohesionless soils often relies on static or dynamic methods to evaluate how piles will perform under various loads.
5. Drainage conditions significantly affect the behavior of cohesionless soils, impacting their stability and load-bearing capacity.

Review Questions

• How does the lack of cohesion in cohesionless soils affect the selection of pile types and their installation methods?
  • The absence of cohesion in cohesionless soils necessitates careful selection of pile types, such as driven piles or drilled shafts, which can better transfer loads into the soil. The installation methods must also account for potential soil displacement and settlement issues. Because these soils rely heavily on friction for load support, ensuring proper embedment depth and side friction is crucial for achieving desired pile capacities.
• Discuss the significance of conducting pile load tests in cohesionless soils and how these tests inform engineering decisions.
  • Pile load tests are critical in determining how piles perform in cohesionless soils because they provide real-world data on load capacity and settlement behavior. These tests help engineers understand the interactions between the pile and the surrounding soil, enabling them to make informed decisions about design parameters and safety factors. The results guide adjustments to pile dimensions or arrangements to optimize performance under expected loads.
• Evaluate the impact of effective stress principles on the performance of piles in cohesionless soils during dynamic loading conditions.
  • In dynamic loading scenarios, such as earthquakes or heavy machinery vibrations, effective stress principles play a vital role in understanding how cohesionless soils respond. As pore water pressure increases during these events, the effective stress within the soil decreases, potentially leading to reduced shear strength. This condition can cause instability or even liquefaction, severely affecting pile performance. Engineers must consider these effects when designing foundations in such environments to ensure safety and structural integrity.

Related terms

Granular Soil: Soil composed of larger particles, such as sand and gravel, that have low cohesion and high drainage capacity.

Effective Stress Principle: A principle that states the strength of soil is determined by the effective stress, which is the difference between total stress and pore water pressure.

Settlement: The downward movement of a structure or its foundation due to changes in the soil beneath it, often influenced by soil type and load conditions.