Choosing foundations for heavy structures on sandy soil For heavy structures founded on sandy deposits, which type of shallow foundation is generally preferred to distribute loads effectively?

Introduction / Context:
Sandy soils can possess adequate bearing capacity but are sensitive to stress distribution and settlement under heavy loads. Foundation choice must control contact pressure and mitigate differential settlement risk. Raft (mat) foundations are commonly employed for large, heavy structures where multiple columns must be supported uniformly.

Given Data / Assumptions:

• Structure is heavy with multiple columns or large load footprint.
• Soil is predominantly sand with reasonably shallow bearing stratum.
• Shallow foundation solutions are under consideration.

Concept / Approach:
Raft foundations spread the total load over a large area, reducing net contact pressure and evening out differential settlements. In sands, immediate settlement dominates; distributing load through a raft helps maintain serviceability. Combined and strap footings are useful for limited column groups or property line constraints but do not provide the same uniformity as a continuous mat for very heavy loading. Pier footings are more suited to isolated column loads on competent strata.

Step-by-Step Solution:

Verification / Alternative check:
Settlement analyses using elasticity theory or Schmertmann’s method often show reduced peak settlement and differential settlement with mats compared to isolated footings under the same total load.

Why Other Options Are Wrong:

• combined footing: Serves two closely spaced columns but not ideal for very heavy, widespread loading.
• pier footing: Isolated and may induce higher contact pressures.
• strap footing: Used to balance eccentricity near property lines, not for uniformly supporting heavy structures.

Common Pitfalls:
Overlooking groundwater effects and potential punching in thin mats; failing to check settlement compatibility among columns.

Final Answer:
raft footing