The sliding factor of a gravity dam is defined as :
- AThe stabilizing moment / The overturning moment
- BThe sum of the vertical forces / The sum of the horizontal forces
- CThe overturning moment / The stabilizing moment
- DThe sum of the horizontal forces / The sum of the vertical forcesCorrect
Solution
Concept
Sliding factor versus factor of safety against sliding
Formula
Sliding factor = ΣH / ΣV; FOS against sliding = μ ΣV / ΣH
Step-by-step solution
1. The sliding factor is the ratio of the total horizontal force to the total vertical force: ΣH / ΣV. 2. It is compared with the coefficient of friction μ: the dam is safe against sliding if ΣH/ΣV < μ.
Explanation
The sliding factor expresses the tendency to slide, and it must be less than the friction coefficient between dam and foundation. The factor of safety against sliding is its inverse multiplied by μ.
Why the other options are wrong
(a) is the factor of safety against overturning (stabilising moment ÷ overturning moment). (c) is its inverse. (b) is the inverse of the sliding factor.
Shortcut
Sliding factor = ΣH/ΣV < μ.
Common mistake
Inverting the ratio.
Correct answer: D) The sum of the horizontal forces / The sum of the vertical forces