The pore water pressure at a point on the phreatic surface is
- A0 kPa (absolute)
- B0 kPa (gauge)Correct
- C100 kPa (gauge)
- D–10 kPa (gauge)
Solution
Concept
Phreatic Surface Pore Pressure
Formula
No formula needed; by definition, pore water pressure at the phreatic surface is zero gauge pressure (u = 0).
Step-by-step solution
1. Identify that the phreatic surface corresponds to the water table level. 2. Recognize that water pressure at the water table is equal to atmospheric pressure. 3. Express atmospheric pressure in gauge terms, which is equal to 0 kPa (gauge). 4. Select option b.
Explanation
The phreatic surface, commonly known as the water table, is the boundary line where the water in the soil is at atmospheric pressure. Since gauge pressure measures pressure relative to atmospheric pressure, the pressure at this surface is exactly 0 kPa (gauge).
Shortcut
Remember: Phreatic = Free water table = Atmospheric pressure = 0 Gauge pressure. Above it is negative (suction), below it is positive.
Common mistake
Confusing absolute pressure with gauge pressure and selecting option A instead of option B, forgetting that atmospheric pressure corresponds to 0 kPa in gauge pressure terms.
Correct answer: B) 0 kPa (gauge)