Retaining Walls MCQ with answer and solution

Concept

The wall of an underground tank is checked for the worst case. This is the tank empty, with full earth pressure (and any ground water) acting from outside and no water inside to balance it.

Formula

P_a = K_a × γ × H² / 2, with K_a = (1 - sin φ)/(1 + sin φ)

Step-by-step solution

1. Outside the wall, soil (and maybe ground water) always pushes inward. 2. Inside the wall, stored water pushes outward. 3. When the tank is full, the inside water pressure partly cancels the outside earth pressure. 4. When the tank is empty, nothing cancels the earth pressure, so the wall gets the largest net load. 5. Uplift on the base is also largest when the tank is empty and the water table is high. 6. So the tank-empty case is the governing case. Answer: C.

Explanation

In an underground tank, earth pressure acts from outside all the time. When the tank is empty, there is no water inside to resist it, so the wall bending is highest. Codes also ask to check the tank full before backfilling, but the main case for an underground tank is tank empty with soil outside.

Why the other options are wrong

A: water inside partly balances the earth pressure, so it is not critical. B: the full tank gives the most balance once soil is placed. It is checked only for the testing stage before backfilling. D: half full still gives some balancing water pressure, so it is less severe than empty.

Shortcut

Underground tank: design for empty (earth outside). Elevated or ground tank: design for full.

Common mistake

Choosing 'tank full' because water load is largest. For an underground tank, the outside earth pressure is the governing load.