Effective Stress in Soil Mechanics

Effective stress is a fundamental concept in soil mechanics, which is defined as the stress that is transmitted through soil particles. It is an important parameter that helps to understand the behavior of soil and its interaction with external loads.

Definition of Effective Stress

Effective stress is defined as the stress that is transmitted through the solid skeleton of the soil, excluding the pore water pressure. In other words, it is the stress that is carried by the soil particles themselves and not by the interstitial water.

Mathematically, effective stress is given by the following equation:

σ' = σ - u

where σ is the total stress, u is the pore water pressure, and σ' is the effective stress.

Importance of Effective Stress

Effective stress is an important concept in soil mechanics for the following reasons:

1. It is a function of engineering properties of soil: Effective stress is related to the strength and stiffness of soil. It determines the shear strength of soil, which is a critical parameter for many geotechnical engineering applications.

2. It affects the permeability of soil: The effective stress influences the flow of water through soil. Higher effective stress leads to lower permeability, which is an important consideration in designing drainage systems.

3. It governs the settlement of soil: The effective stress plays a significant role in the settlement of soil. It determines the degree of consolidation that occurs when a load is applied to soil.

Measurement of Effective Stress

The effective stress can be measured using various laboratory and field tests. Some of the commonly used methods are:

1. Triaxial compression test: This test is used to determine the shear strength of soil and the effective stress at failure.

2. Consolidation test: This test is used to determine the rate and magnitude of settlement of soil under a given load.

3. Piezometer: This is an instrument used to measure the pore water pressure in soil, which can be used to calculate the effective stress.

Conclusion

Effective stress is a fundamental concept in soil mechanics, which is essential for understanding the behavior of soil under external loads. It is a function of engineering properties of soil and affects many geotechnical engineering applications. The effective stress can be measured using various laboratory and field tests.