Foundation detailing for heavy wall loads on soil with low safe bearing capacity: which bed should be provided?

Introduction / Context:
When a masonry or concrete wall imposes heavy line loads on a soil of low safe bearing capacity, the foundation must distribute pressure effectively and control settlement. The choice between lean concrete, mass concrete, and reinforced concrete beds depends on load intensity and soil behavior.

Given Data / Assumptions:

• Wall carries heavy load.
• Subsoil shows low bearing capacity.
• Objective: spread load safely and limit differential settlement.

Concept / Approach:
Lean concrete is primarily a leveling course (blinding) and does not appreciably spread heavy loads. A thick mass concrete bed can spread loads but is inefficient on weak soils because it relies on its own mass rather than structural action. A reinforced concrete (R.C.) bed acts as a slab distributing and bridging over soft spots—its reinforcement provides tensile capacity, achieving better load spread with controlled thickness and crack resistance.

Step-by-Step Solution:
Assess soil: low SBC → need structural action to spread load.Evaluate lean concrete: leveling only → not adequate alone.Evaluate thick mass concrete: heavy and uneconomical; limited tensile capacity.Select R.C. bed: provides bending resistance and effective load distribution.

Verification / Alternative check:
Design guides recommend R.C. strip footings or combined footings on weak soils; reinforcement permits thinner sections and better serviceability than mass concrete alone.

Why Other Options Are Wrong:
Lean concrete only: Insufficient structural capacity.Thick mass only: Inefficient and may still crack without reinforcement.Combinations listed add cost without necessity if a properly designed R.C. bed is used.

Common Pitfalls:
Confusing blinding with structural foundation; overusing mass concrete where reinforcement is more effective.

Final Answer:
Reinforced concrete (R.C.) bed.