Skempton's pore pressure coefficients, denoted as A and B, are fundamental concepts in soil mechanics, particularly in understanding the behavior of soils under changes in stress. These coefficients help quantify how pore water pressure changes in response to changes in total stress. The question specifically asks about Skempton's pore pressure coefficient B for saturated soil.

Skempton's Pore Pressure Coefficient Explained

The pore pressure coefficient B quantifies the change in pore water pressure (\(\Delta u\)) due to a change in isotropic confining pressure (\(\Delta \sigma_3\)) under undrained conditions. It is defined by the following relationship:

$$B = \frac{\Delta u}{\Delta \sigma_3}$$

Here:

• \(\Delta u\) represents the change in pore water pressure.
• \(\Delta \sigma_3\) represents the change in the all-round or isotropic confining stress (minor principal stress).

This coefficient helps in understanding how much of the applied total stress is transferred to the pore water pressure within the soil.

Saturated Soil Conditions and B Value

When soil is fully saturated, its pore spaces are completely filled with water. Water is considered nearly incompressible compared to the soil skeleton. Therefore, under undrained conditions, any change in total stress applied isotropically to a saturated soil mass will cause a significant change in the pore water pressure.

• Incompressible Water: Since water is almost incompressible, it resists changes in volume. In a saturated soil, if an isotropic stress is applied without allowing drainage (undrained condition), the water in the pores has nowhere to go.
• Stress Transfer: Consequently, the applied total stress is almost entirely transferred to the pore water. This means that if you increase the confining pressure by \(\Delta \sigma_3\), the pore water pressure will also increase by approximately the same amount, \(\Delta u \approx \Delta \sigma_3\).
• Resulting B Value: Because \(\Delta u\) is approximately equal to \(\Delta \sigma_3\) for a saturated soil under isotropic stress change, the ratio \(B = \frac{\Delta u}{\Delta \sigma_3}\) becomes approximately 1.

Therefore, for saturated soil, Skempton's pore pressure coefficient B is considered to be 1.

Pore Pressure Response in Saturated Soil

The value of Skempton's pore pressure coefficient B provides crucial insights into the behavior of soil:

• For a completely saturated soil, B = 1. This indicates that the entire change in isotropic total stress is transferred to the pore water pressure. This is a critical assumption in many geotechnical analyses, especially concerning rapid loading or undrained conditions where water cannot escape immediately.
• For a partially saturated soil, the value of B lies between 0 and 1 (0 < B < 1). In this case, some of the air voids can compress, meaning not all the applied total stress is transferred to the pore water.
• For a dry soil, B = 0. There is no water in the pores, so no pore pressure change occurs due to changes in isotropic total stress.

Understanding the value of B is essential for predicting pore pressure changes during construction activities, such as embankment loading or excavation, and for analyzing the stability of slopes and foundations in various soil conditions.