Question

For a rectangular masonry dam, the weight of dam, uplift pressure and horizontal force causing the sliding are 300 kN, 60 kN and 80 kN respectively. If the coefficient of friction between the dam base and soil is 0.5, then the factor of safety against sliding is

• A. 1.07
• B. 1.5
• C. 1.88
• D. 2.0

Hint:

The formula for FOS against sliding is a critical check in dam design.
FOS = $\frac{\mu (\Sigma V)}{\Sigma H}$.
Remember that uplift force reduces the net vertical force, which in turn reduces the frictional resistance.
Always use the net vertical force ($Weight - Uplift$) in the calculation.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Weight=300kN, uplift=60kN, horizontal=80kN, μ=0.5. Find FOS against sliding.

Step 2: Key Formula (Alternate):
FOS = Resisting/Driving = μ(W-U)/H.

Step 3: Detailed Explanation:
Net vertical = 300-60 = 240kN. Friction resistance = 0.5×240 = 120kN. FOS = 120/80 = 1.5. Should be >1.0 (typically >1.5 for safety). Also check overturning and bearing pressure.

Step 4: Final Answer:
1.5.