Cohesive Soils

Definition

Cohesive soils are fine-grained soils characterized by their ability to stick together due to their particle size and the presence of clay minerals. These soils exhibit plasticity and can undergo significant deformation without cracking, which is vital for understanding how they behave under dynamic loading conditions, especially during seismic events.

5 Must Know Facts For Your Next Test

1. Cohesive soils, primarily composed of clay particles, have a high plasticity index, making them suitable for construction but challenging during seismic activities.
2. During dynamic loading, such as earthquakes, cohesive soils may exhibit liquefaction if saturated, leading to a loss of strength and increased risk of failure.
3. The Atterberg limits (liquid limit and plastic limit) are crucial for identifying the behavior of cohesive soils under varying moisture conditions.
4. Cohesive soils often experience increased pore water pressure during seismic events, affecting their shear strength and overall stability.
5. Effective stress principles are essential in understanding the behavior of cohesive soils during dynamic loading, as they determine how stresses are transmitted through the soil structure.

Review Questions

• How does the plasticity index affect the behavior of cohesive soils during seismic events?
  • The plasticity index indicates how much moisture a cohesive soil can hold while remaining plastic. Soils with a higher plasticity index can deform significantly before failing, which can help absorb seismic energy. However, if the moisture content becomes too high, these soils may lose strength and become susceptible to liquefaction during earthquakes, leading to severe structural failures.
• Discuss how pore water pressure influences the shear strength of cohesive soils during an earthquake.
  • Pore water pressure increases in cohesive soils during an earthquake due to rapid loading conditions. This rise in pressure reduces the effective stress within the soil matrix, subsequently decreasing its shear strength. As a result, the soil may behave less like a solid structure and more like a fluid, which is critical to understand for predicting potential failures and ground instability in areas prone to seismic activity.
• Evaluate the implications of consolidation on the long-term stability of structures built on cohesive soils in seismically active regions.
  • Consolidation affects how cohesive soils settle over time under applied loads. In seismically active areas, structures built on these soils must consider both immediate dynamic responses and long-term consolidation effects. If consolidation leads to significant settlement after an earthquake, it can compromise structural integrity. Engineers must assess both immediate liquefaction risks and long-term settlements to design foundations that ensure stability and safety in these challenging conditions.

Related terms

Plasticity Index: A measure of the range of moisture content over which a soil remains plastic, indicating its ability to change shape without breaking.

Shear Strength: The maximum resistance of soil to shear stress, essential for understanding how cohesive soils will perform under loads.

Consolidation: The process by which soils decrease in volume under pressure over time, important for predicting long-term settlement in cohesive soils.