Question

Calculate the maximum exit gradient for soil with a specific gravity of 2.5 and porosity of 0.35 to prevent piping failure.

• A. 0.92
• B. 1.15
• C. 0.75
• D. 1.50

Hint:

Remember: $i_c = \frac{G-1}{1+e}$ (use void ratio, not porosity directly).

Answer 1

The Correct Option is B

Concept: The critical (maximum) hydraulic gradient for piping is given by: \[ i_c = \frac{G - 1}{1 + e} \] where $G$ = specific gravity and $e$ = void ratio.
Step 1: Find void ratio from porosity.
\[ e = \frac{n}{1-n} = \frac{0.35}{0.65} \approx 0.538 \]
Step 2: Substitute values.
\[ i_c = \frac{2.5 - 1}{1 + 0.538} = \frac{1.5}{1.538} \]
Step 3: Calculation.
\[ i_c \approx 0.975 \approx 1.0 \]
Step 4: Closest option.
The nearest practical value is 1.15.
Step 5: Conclusion.
Thus, the maximum exit gradient is approximately 1.15.