Secondary compression

Definition

Secondary compression refers to the gradual and long-term deformation of soil that occurs after primary consolidation has taken place. This process is particularly significant in fine-grained soils, such as clays, where additional settlement can continue to occur due to factors like particle rearrangement, changes in pore water pressure, and the dissipation of excess pore pressure over time. Understanding secondary compression is essential for accurate predictions of settlement in various geotechnical applications.

5 Must Know Facts For Your Next Test

1. Secondary compression can occur over months or years after primary consolidation is complete, depending on soil type and environmental conditions.
2. In clays, secondary compression is often caused by the gradual rearrangement of soil particles and the slow dissipation of pore water pressure.
3. Unlike primary consolidation, which is rapid and measurable, secondary compression is more difficult to quantify and predict.
4. The degree of secondary compression can be influenced by factors like drainage conditions, load history, and the physical properties of the soil.
5. It is crucial for engineers to account for secondary compression in settlement calculations for structures built on clayey soils to ensure long-term stability.

Review Questions

• How does secondary compression differ from primary consolidation in terms of its causes and effects on soil settlement?
  • Secondary compression differs from primary consolidation primarily in its timing and causes. Primary consolidation occurs quickly as excess pore water is expelled under load, leading to immediate settlement. In contrast, secondary compression takes place much later and is caused by factors like particle rearrangement and slow pore pressure dissipation. This results in ongoing settlement that can last for years after initial loading, making it essential to consider in long-term geotechnical assessments.
• What role does preconsolidation pressure play in predicting secondary compression behavior in saturated clays?
  • Preconsolidation pressure serves as a critical benchmark for understanding a saturated clay's response to new loads. It indicates the maximum stress level that the clay has previously endured without undergoing significant deformation. When new loads exceed this threshold, both primary consolidation and secondary compression can occur. Engineers use this knowledge to assess potential future settlements, as soils with a high preconsolidation pressure may exhibit less secondary compression compared to those with a lower value.
• Evaluate the implications of ignoring secondary compression in geotechnical design and construction projects involving clayey soils.
  • Ignoring secondary compression in geotechnical design can lead to serious consequences such as unexpected and excessive settlement of structures over time. If engineers do not account for this long-term deformation, they risk underestimating the potential movement that could affect structural integrity and serviceability. Such oversight may result in costly repairs or even catastrophic failures. Consequently, incorporating secondary compression into settlement predictions ensures that structures are designed to withstand not only initial loading but also ongoing soil behavior over their lifespan.

Related terms

primary consolidation: The initial reduction in soil volume due to the expulsion of water from the pores under applied load, leading to immediate and short-term settlement.

settlement: The downward movement of the ground surface resulting from soil compression under loading, which can be immediate or delayed over time.

preconsolidation pressure: The maximum past pressure that a soil has experienced, influencing its future consolidation behavior and potential for secondary compression.