Question

26. Rise of water table above the ground surface causes

• A. Equal increase in pore water pressure and total stress.
• B. Equal decrease in pore water pressure and total stress.
• C. Increase in pore water pressure but decrease in total stress.
• D. Decrease in pore water pressure but increase in total stress.

Hint:

Any change in water level above the ground surface increases total stress and pore pressure by the exact same amount (\(\gamma_w \Delta h\)).
Because both values increase equally, the effective stress of the soil remains completely unaffected by standing water above the ground.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
To analyze changes in soil stress, we must use Terzaghi's effective stress principle, which states:
\[ \sigma' = \sigma - u \]
where \(\sigma'\) is the effective stress, \(\sigma\) is the total stress, and \(u\) is the pore water pressure.
Total stress (\(\sigma\)) is the total downward force per unit area acting on a horizontal plane, which includes the weight of the soil solids and water above that plane.
Pore water pressure (\(u\)) is the hydrostatic pressure of water filled within the soil void spaces.

Step 2: Detailed Explanation:
Consider a soil layer and let us evaluate the stress changes at a depth \(z\) below the ground surface when the water table rises by a height \(h\) above the ground surface.
When the water level rises above the ground, it creates a layer of standing water of depth \(h\) over the entire area.
This standing water layer acts as a uniform surcharge pressure on the ground surface.
The increase in total stress (\(\Delta\sigma\)) at the depth \(z\) is equal to the pressure exerted by the weight of this additional water column:
\[ \Delta\sigma = \gamma_w \cdot h \]
Similarly, because the hydraulic head at any point in the soil increases by the same height \(h\) of water, the hydrostatic pore water pressure (\(\Delta u\)) at depth \(z\) also increases by:
\[ \Delta u = \gamma_w \cdot h \]
Since both total stress and pore water pressure increase by the exact same amount (\(\gamma_w \cdot h\)), the effective stress remains unchanged:
\[ \sigma'_{\text{new}} = (\sigma + \Delta\sigma) - (u + \Delta u) = \sigma - u = \sigma' \]
Thus, the rise of the water table above the ground surface results in an equal increase in both total stress and pore water pressure.

Step 3: Final Answer:
The rise of the water table above the ground surface causes an equal increase in pore water pressure and total stress.