A soil layer is partially saturated due to capillary action and the degree of saturation is 50%. The height of point ‘x’ under consideration measured from groundwater table is 0·25 m. Assume unit weight of water is 10 kN/m³. What is the approximate pore water pressure at point ‘x’ ?
- A– 2·5 kN/m²
- B– 1·5 kN/m²
- C– 1·25 kN/m²Correct
- D– 2·75 kN/m²
Solution
Concept
Pore water pressure in the capillary zone (partial saturation)
Formula
u = − S γw h
Step-by-step solution
1. In a partially saturated capillary zone, the pore water pressure is negative. 2. Using u = − S γw h = − 0.50 × 10 × 0.25 = − 1.25 kN/m².
Explanation
For fully saturated capillary soil u = −γw h = −2.5 kN/m². When the degree of saturation is only 50 percent, the pore pressure acting on the soil skeleton is reduced in proportion (u = −S γw h).
Why the other options are wrong
−2.5 kN/m² would be for 100% saturation. The other values do not match S γw h.
Shortcut
Partial saturation: u = −S γw h.
Common mistake
Using the fully saturated value −γw h.
Correct answer: C) – 1·25 kN/m²