Question

Pile foundation is mostly preferred in

• A. Soft rocks
• B. Soft and loose soils
• C. Hard rocks
• D. Compact granular soils

Hint:

\textbf{Pile Foundations (Deep Foundations):} Used to transfer structural loads to deeper, more competent soil or rock strata.
\textbf{Primary situations for using piles:}
Weak, compressible, or loose upper soil layers with low bearing capacity.
Heavy structural loads.
To resist uplift or large lateral loads.
Problematic soils (expansive, collapsible).
If hard rock or very competent soil is near the surface, shallow foundations are usually preferred.

Answer 1

The Correct Option is B

Pile foundations are a type of deep foundation. They consist of long, slender columns (piles) made of materials like concrete, steel, or timber, which are driven, drilled, or jacked into the ground to transfer structural loads to deeper, stronger soil or rock strata. Pile foundations are preferred or necessary in several situations:
When the upper soil layers are weak, soft, or loose: If the soil near the surface has low bearing capacity (e.g., soft clay, loose sand, filled ground), it cannot support the loads from shallow foundations (like strip, pad, or raft foundations) without excessive settlement or failure. Piles are used to transfer the loads through these weak layers down to a firmer stratum (end-bearing piles) or to develop support through friction along their length in moderately competent soil (friction piles). (This matches option b).
For heavy structural loads: When loads from the superstructure are very high (e.g., tall buildings, heavy industrial structures, bridges), shallow foundations might not be adequate even if the upper soil is reasonably good. Piles can distribute these heavy loads to deeper, stronger ground.
To resist uplift forces or lateral loads: Piles can be designed to resist uplift (tension) forces (e.g., in structures subject to buoyancy or wind overturn) or large horizontal loads (e.g., from earthquakes, wind, bridge abutments).
In expansive or collapsible soils: To bypass problematic soils near the surface that are prone to large volume changes.
For structures over water or in areas with high water table. Let's evaluate the options:
(a) Soft rocks: If "soft rock" refers to weathered or weak rock that still has decent bearing capacity, shallow foundations might still be feasible if the rock is near the surface. If it's very soft/decomposed rock overlying stronger rock, piles could be used to reach the stronger layer. However, "soft and loose soils" is a more classic scenario for piles.
(b) Soft and loose soils: This is a primary reason for using pile foundations. These soils have low bearing capacity and are prone to excessive settlement. Piles bypass these weak upper layers.
(c) Hard rocks: If hard rock is at or near the surface, shallow foundations (like footings directly on rock) are usually the most economical and effective solution, as hard rock has very high bearing capacity. Piles would generally not be needed unless loads are exceptionally high or there are other specific reasons (like resisting uplift in rock anchors).
(d) Compact granular soils (e.g., dense sand, gravel): These soils generally have good bearing capacity. Shallow foundations are often suitable. Piles might be used if loads are very heavy or if liquefaction is a concern in seismic areas for loose granular soils (but "compact" implies good). Therefore, pile foundations are most commonly preferred and necessary when dealing with soft and loose soils near the surface that cannot adequately support the structure using shallow foundations. \[ \boxed{\text{Soft and loose soils}} \]