Pile Foundations MCQ with answer and solution

Concept

Under-reamed piles are bored cast-in-situ concrete piles having one or more bulbs (under-reams) in expansive soils like black cotton soil. Adding a second bulb increases the safe load capacity in both compression and uplift by approximately 50%.

Formula

Capacity of double under-reamed pile ≈ 1.5 × Capacity of single under-reamed pile

Step-by-step solution

1. Identify the pile type: single under-reamed pile versus multi-under-reamed pile as per IS 2911 (Part 3). 2. Note the standard codal provision: The safe load carrying capacity of a single under-reamed pile in uplift (tension) or compression increases by 50% by the provision of an additional bulb. 3. Therefore, the percentage increase is 50%.

Explanation

According to IS 2911 (Part 3), the load carrying capacity of an under-reamed pile in compression or uplift can be increased by providing an additional bulb. Providing a second bulb (making it a multi-under-reamed pile) increases the capacity by about 50% over a single under-reamed pile.

Why the other options are wrong

A: 5% is negligibly small and does not represent the effect of an additional bearing bulb. B: 10% is incorrect according to IS 2911 (Part 3). C: 20% is incorrect; the bulb significantly enhances both bearing and shearing area, giving a 50% increase.

Shortcut

Additional bulb adds 50% capacity over a single bulb pile (Capacity ratio = 1 : 1.5).

Common mistake

Confusing the ratio with total percentage (150% total vs 50% increase), or confusing with depth/spacing requirements.