Question

A soil sample was consolidated at a cell pressure of 20 kPa and a back pressure of 10 kPa for 24 hours during a consolidated undrained (CU) triaxial test. The cell pressure was increased to 30 kPa on the next day and it resulted in the development of pore water pressure of 1 kPa. The soil sample failed when the axial stress was gradually increased to 50 kPa. The pore water pressure at failure was recorded as 21 kPa. The value of Skempton’s pore pressure parameter \( B \) for the soil sample is ______ (rounded off to 2 decimal places).

Hint:

Skempton’s pore pressure parameter \( B \) is critical for understanding the behavior of soil under stress conditions in geotechnical engineering, indicating how changes in stress are translated into changes in pore water pressure.

Answer 1

Step 1: Calculate the initial increase in pore water pressure when the cell pressure was increased. Initially, the soil was under a cell pressure of 20 kPa and a back pressure of 10 kPa. When the cell pressure was increased to 30 kPa, the increase in pore water pressure was 1 kPa. This provides: \[ \Delta u = 1 \text{ kPa} \] \[ \Delta \sigma = 30 \text{ kPa} - 20 \text{ kPa} = 10 \text{ kPa} \] The Skempton's pore pressure parameter \( B \) at this stage is calculated by: \[ B = \frac{\Delta u}{\Delta \sigma} = \frac{1 \text{ kPa}}{10 \text{ kPa}} = 0.10 \]