Raft (mat) foundation bridging soft spots: for safe bridging of loose pockets in the soil mass, the soft area should be smaller than which fraction of the column spacing?

Introduction / Context:
Raft (mat) foundations spread loads over a large area and can bridge over localized soft pockets. However, there are empirical limits on the permissible size of such weak zones relative to the structural grid to avoid excessive differential settlement and punching.

Given Data / Assumptions:

• Columns rest on a continuous raft slab.
• A localized soft patch exists within the plan.
• Bridging is considered without special ground improvement.

Concept / Approach:
Design heuristics state that if a soft area is sufficiently small compared to the column spacing, the raft's stiffness can redistribute stresses around it, “bridging” the gap. A commonly referenced guideline is that the soft pocket should be smaller than about one-third of the column spacing for safe bridging without undue settlements.

Step-by-Step Solution:
Identify the governing rule-of-thumb: soft area size < (1/3) × spacing.Evaluate options: one-third meets the conventional guideline.Select “One-third the column spacing.”

Verification / Alternative check:
Structural-geotechnical texts suggest similar limits; beyond this, ground improvement or raft thickening/ribs may be necessary.

Why Other Options Are Wrong:

• Half or three-fourths spacing: too large; risk of differential settlement.
• Full spacing/none: unconservative or undefined.

Common Pitfalls:
Assuming any size can be bridged by simply thickening the raft; ignoring settlements and shear checks around soft inclusions.

Final Answer:
One-third the column spacing