Select the option that is appropriate regarding the following two statements. Assertion: Piping below the weir can be prevented by providing an impervious floor of sufficient length so that the path of percolation is increased and the exit gradient is decreased. Reason: If the exit gradient is less than a certain critical value, the soil starts boiling and is washed away by percolating water.
Try it first: pick an option, then read the explanation.
Asked in SSC JE 06/06/2024 (Morning)
Detailed Explanation
Water seeping under a weir travels along the underside of the floor and comes up at the downstream end. Where it emerges, it pushes upward on the soil. The steepness of the head loss there is the exit gradient. If this upward gradient becomes too large, the soil at the exit is lifted, washed out, and an eroded channel (a pipe) works backwards under the structure: piping.
\[ i_{\text{exit}} \propto \frac{H}{L} \qquad i_c = \frac{G-1}{1+e} \approx 1 \]
Why “Assertion is true, but Reason is false” Is Correct
Assertion is true: a longer impervious floor (and cut-off walls) increases the creep length L, so the exit gradient falls and piping is prevented. This is the basis of Bligh's and Lane's creep theories and Khosla's method.
Reason is false: boiling occurs when the exit gradient becomes greater than the critical gradient. A gradient less than critical is safe. So the answer is (a).
Why the Other Options Are Wrong
- B. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion: The Reason is false, so ‘both true’ cannot be correct.
- C. Both Assertion and Reason are false: The Assertion is a correct description of how piping is prevented.
- D. Both Assertion and Reason are true and Reason is the correct explanation of Assertion: The Reason is false, so it cannot explain the Assertion.
Key Point
Safe when exit gradient < critical gradient. Lengthen the seepage path (floor, cut-offs) to reduce the exit gradient.
Key Concepts for Students
- Safe exit gradients for weirs are typically 1/4 to 1/7 depending on soil.
- Inverted filters at the downstream end stop fine particles washing out.
- Uplift on the floor is resisted by the floor's weight; floors are made thicker upstream of the toe.
Practice more: this is question 1 in Earth Pressure and Retaining Wall MCQs – Part 1. Try the whole set and check every answer instantly.