Shallow Foundations MCQ with answer and solution

Concept

To avoid adverse interference of stress bulbs and undue settlement, adjacent footings must be spaced apart by at least the width of the wider footing.

Formula

Minimum horizontal spacing ≥ Width of the wider footing (B_wider)

Step-by-step solution

1. Consider an existing footing and a proposed new footing located nearby at approximately the same level. 2. The pressure distribution below each footing extends outward and downward at an angle of roughly 2:1 (or 30° to 45° with the vertical), forming a stress bulb. 3. To ensure that the stress bulb of the new footing does not induce significant additional stress or settlement on the existing footing (and vice-versa), standard codes specify that the minimum horizontal clear spacing between the footings must be at least equal to the width of the wider footing.

Explanation

According to IS 1904 (Clause 7.2) and standard foundation design principles, to prevent the overlapping of stress bulbs and the exertion of additional lateral or vertical pressure on an existing footing, the minimum clear horizontal distance between adjacent footings should at least be equal to the width of the wider (larger) footing. If the footings are placed closer than this, stress zones overlap significantly, potentially causing excessive settlement or shear failure.

Why the other options are wrong

A: Twice the width of the smaller footing is not standard and may still cause stress overlap if the other footing is much larger. B: Spacing equal to the width of the smaller footing is insufficient to prevent interference from the larger footing's stress bulb. D: Twice the width of the wider footing is an overly conservative design rule not specified as the minimum requirement.

Shortcut

Remember: The spacing is governed by the larger zone of influence, hence width of wider footing.

Common mistake

Candidates often confuse the width of the smaller footing with the width of the wider footing.