The bearing capacity of a rectangular footing of plan dimensions 1.5 m × 3 m resting on the surface of a sand deposit was estimated as 600 kN/m² when the water table is far below the base of the footing. The bearing capacities in kN/m² when the water level rises to the depths of 3 m, 1.5 m and 0.5 m below the base of the footing are
- A600, 450, 350
- B600, 600, 400Correct
- C600, 500, 250
- D600, 400, 250
Solution
Concept
Water-table correction
Formula
Cw = 0.5 + 0.5 Dw/B (0 ≤ Dw ≤ B)
Step-by-step solution
1. If Dw ≥ B (3 m and 1.5 m, B = 1.5 m) there is no reduction: 600 kN/m². 2. For Dw = 0.5 m: Cw = 0.5 + 0.5 × 0.5/1.5 = 0.667. 3. q = 0.667 × 600 = 400 kN/m².
Explanation
The water table affects the capacity only when it is within the depth B below the base.
Why the other options are wrong
Other options reduce the capacity for Dw ≥ B.
Shortcut
Dw ≥ B → no reduction.
Correct answer: B) 600, 600, 400