Foundation Settlement – Permissible differential settlement on clay For typical building foundations resting on clayey soil, what is the usual maximum permissible differential settlement to limit damage?

Introduction / Context:
Differential settlement—variation of settlement between support points—governs cracking and serviceability more critically than total settlement. Clayey soils are compressible and time-dependent, so practical limits are specified to keep induced angular distortions within tolerable values.

Given Data / Assumptions:

• Ordinary building foundations on clay.
• No special ground improvement or deep foundations.
• Serviceability (crack control and functionality) is the criterion.

Concept / Approach:
Empirical guidelines limit differential settlement to values corresponding to acceptable angular distortion (e.g., 1/300 to 1/500 depending on structure). For common small-to-medium buildings on clay, a rule-of-thumb limit of about 25–30 mm between adjacent supports is widely used in exam contexts; 30 mm is the conventional rounded value provided in many question banks.

Step-by-Step Solution:
Relate angular distortion to differential settlement over span.Adopt widely taught permissible differential value: ≈ 30 mm.Select the matching option.

Verification / Alternative check:
Design guides convert settlement limits to angular distortion criteria. For typical bay widths, 25–30 mm corresponds to distortions that most masonry and finishes can tolerate with minor cracking only.

Why Other Options Are Wrong:

• 10 mm and 20 mm: often unnecessarily stringent for ordinary buildings.
• 40 mm and 50 mm: risk excessive cracking and serviceability issues.

Common Pitfalls:
Ignoring differential settlement distribution and focusing on total settlement alone; and failing to consider time-dependent consolidation in clays which continues after construction.

Final Answer:
30 mm