Foundation choice for layered clays — firm clay over deep soft clay During site investigation, a thick layer of fairly firm clay is encountered overlying a deep, soft clay stratum. For such a soil profile supporting a building, which type of foundation is generally most useful?

Introduction / Context:
Layered soil profiles often dictate the most economical and safe foundation type. When a reasonably firm clay layer of adequate thickness lies at the surface, while a softer clay extends deeper below, designers aim to distribute loads within the firm layer and avoid excessive stress transmission to the weaker stratum.

Given Data / Assumptions:

• Thick, fairly firm clay near the surface; deep soft clay underneath.
• Building loads are spread over an area (no extreme point loads beyond raft design capability).
• Conventional settlement control and bearing checks are required.

Concept / Approach:

A raft (mat) foundation spreads loads over the entire footprint, reducing contact pressure and making better use of the firm upper clay layer. It mitigates differential settlement by plate action. Pile foundations would transfer loads to deeper strata; if those strata are soft for great depth, piles become long and uneconomical or may settle. Grillage foundations are usually for isolated heavy columns on relatively shallow competent layers and are less effective for uniform distribution over soft underlying clay.

Step-by-Step Solution:

Verification / Alternative check:

Plate-on-elastic-foundation analyses and settlement calculations typically show reduced maximum contact pressures and improved uniformity with rafts on firm-over-soft profiles.

Why Other Options Are Wrong:

• Piles would require deep penetration into weak strata or reaching very deep firm layers, often uneconomical.
• Grillage focuses load under discrete columns and does not address broad-area settlement control.
• “None” is incorrect because raft is a standard solution here.

Common Pitfalls:

• Ignoring long-term consolidation of the deeper soft clay; preloading or staged construction may still be needed.
• Overlooking groundwater effects and basal heave in excavations.

Final Answer:

Raft foundation.