Foundations on sandy soils — typical limit for differential settlement For building foundations resting on sandy soil, what is the usual maximum permissible differential settlement to be adopted in design and serviceability checks?

Introduction / Context:
Differential settlement can damage structural and non-structural elements even when total settlement is moderate. In sandy soils, immediate settlements dominate and are often limited by serviceability criteria. Designers use standard limits to prevent cracks, misalignment, and service issues.

Given Data / Assumptions:

• Shallow foundations on sands with typical building loads.
• Serviceability limit approach, not ultimate capacity.
• Uniform construction quality and drainage assumed.

Concept / Approach:
Typical guidance limits differential settlement to a small value so that angular distortion remains within acceptable ranges for masonry and frame structures. For many conventional buildings on sand, a common rule-of-thumb limit is about 25 mm over the relevant span between supports, aligned with avoiding visible cracking and door/window jam issues.

Step-by-Step Solution:

Verification / Alternative check:
Serviceability checks often also specify angular distortion limits (e.g., 1/500 to 1/300); with typical spans, 25 mm aligns with these ratios.

Why Other Options Are Wrong:

• 15 mm: Too conservative for many practical buildings.
• 35 mm, 45 mm, 55 mm: Often exceed acceptable angular distortion; risk cracking and misalignment.

Common Pitfalls:
Confusing total settlement with differential settlement; ignoring span length and stiffness when applying a millimetre limit.

Final Answer:
25 mm