What is a structural foundation type suitable for weak soils and low bearing capacity?

• A. Shallow spread footings on compacted soil.
• B. Floating raft on water with no soil contact.
• C. Deep foundations such as piles or a mat (raft) foundation.
• D. Timber footings and posts.

Answer: (C)

When soils are weak and have low bearing capacity, the structure needs a foundation that either reaches deeper into stronger material or spreads the load over a much larger area to keep stresses within what the soil can safely carry. Deep foundations do this by transferring the load down to deeper, competent layers—piles driven or drilled to reach those layers provide reliable support where surface soils are inadequate. A mat (raft) foundation achieves a similar goal by forming a thick concrete slab that distributes the entire building load over a large footprint, reducing contact pressure and controlling settlements on very soft soils. Both approaches are chosen specifically for weak soils because they either bypass weak layers or dilute the load across a larger area to stay within soil capacity.

Shallow spread footings on compacted soil assume a solid, stronger surface layer and can experience excessive settlement or failure if the surface soils are weak or variable. A raft floating on water with no soil contact isn’t a practical foundation in real construction. Timber footings and posts typically offer insufficient bearing capacity and are vulnerable to moisture and decay, making them unsuitable for transferring modern structural loads into weak soils.