Chapter 11 Compressibility of Soil - Lecture 2A: Empirical Correlations

Summary

This lecture delves into the concept of pre-consolidation pressure in clays, crucial for understanding soil behavior under stress. It differentiates between normally consolidated (NC) and over-consolidated (OC) clays, highlighting how their stress history affects loading paths. The lecture also explores reasons for over-consolidation, such as previous structures, glaciers, water table changes, desiccation, and weathering. Emphasizing the impact of soil disturbance on void ratio and effective stress plots, it provides methods to correct these effects. Lastly, it discusses determining compression and recompression indices, and pre-consolidation pressure using lab tests or empirical correlations, essential for settlement calculations.

Takeaways

• 📚 Pre-consolidation pressure is the maximum effective past pressure a clay has experienced, influencing its stress history and behavior under different loading paths.
• 📈 The relationship between current effective stress and pre-consolidation pressure determines the loading paths for clays, with different void ratio changes for the same stress increment.
• 🔍 NC (Normally Consolidated) clays have a current effective stress equal to the maximum experienced, while OC (Over Consolidated) clays have a current effective stress less than the past maximum.
• 🔄 Over consolidation in clays can occur due to previous structures, glacial loading, water table changes, desiccation, or weathering processes that remove loads from the clay.
• 📊 The over consolidation ratio is defined as the ratio of pre-consolidation pressure to the current effective stress, which is crucial for settlement calculations.
• 🧪 Laboratory tests, such as the 1D consolidation test, are essential for obtaining parameters like compression index (CC), recompression index (Cs), and pre-consolidation pressure (σ'c).
• 📉 Disturbance during soil sampling can obscure the pre-consolidation point and decrease the compression index, leading to an underestimation of settlements.
• 🔗 Empirical correlations can be used to estimate CC and CS when lab tests are not available, with common methods including relations to liquid limit (LL) and plasticity index (PI).
• 📝 The plasticity index can be used to estimate both CC and CS, with a rule of thumb suggesting that CS is about one-fifth to one-tenth of CC.
• 🌐 Settlement calculations are a key focus in geotechnical engineering, where understanding the stress history and current state of clays is vital for accurate predictions.

Q & A

What is pre-consolidation pressure in the context of clay?

Pre-consolidation pressure refers to the maximum effective past pressure that the clay has experienced. It is a key parameter in understanding the stress history of clay, which influences its behavior under different loading conditions.

How does the pre-consolidation pressure affect the loading paths of clay?

The pre-consolidation pressure affects the loading paths of clay by determining how the clay will respond to stress increments. Depending on the relationship between the current effective stress and the pre-consolidation pressure, clay may follow different paths, such as initial loading with a steeper slope or unloading and reloading with a flatter slope.

What are the two types of clays mentioned in the script, and how are they defined?

The two types of clays mentioned are normally consolidated (NC) clay and overconsolidated (OC) clay. NC clays are where the current effective stress is equal to the maximum stress the clay has ever experienced. OC clays are where the current effective stress is less than the past maximum effective stress.

What is the overconsolidation ratio, and how is it calculated?

The overconsolidation ratio is the ratio of the pre-consolidation pressure to the current effective stress (sigma_c' / sigma') and is used to characterize the degree of overconsolidation in clay.

What are some reasons for clay becoming overconsolidated?

Clay can become overconsolidated due to several reasons such as the presence of previous structures, glaciers, water table changes, desiccation, and weathering processes that involve the removal of top soil layers.

How does disturbance affect void ratio versus effective stress plots obtained from lab tests?

Disturbance, even when minimal, can obscure the pre-consolidation point and decrease the compression index (cc) value on the e-log sigma' plot. This can lead to an underestimation of the expected settlement.

What is the significance of the compression index (cc) and the recompression index (cs) in soil mechanics?

The compression index (cc) and the recompression index (cs) are important parameters in soil mechanics. The cc value indicates the compressibility of the soil, while the cs value represents the soil's behavior under reloading conditions. These indices help in predicting the settlement of soil under different loading scenarios.

How can the compression index (cc) and the recompression index (cs) be estimated if lab tests are not available?

If lab tests are not available, the compression index (cc) and the recompression index (cs) can be estimated using empirical correlations based on properties such as the liquid limit, plastic limit, or plasticity index of the clay.

What is the relationship between the plastic index and the compression index (cc) as per the empirical correlation mentioned?

According to one of the empirical correlations mentioned, the compression index (cc) can be estimated using the plastic index (PI) with the formula cc = PI / 370.

Why is the 1D consolidation test considered the best method to obtain cc, cs, and sigma_c'?

The 1D consolidation test is considered the best method to obtain cc, cs, and sigma_c' because it provides a direct measurement of the soil's behavior under controlled loading conditions, allowing for the determination of these parameters from the e-log sigma' plot.