Foundations on Low Bearing Capacity Soils – Which foundation type is generally adopted? When the soil at shallow depth has low allowable bearing capacity and differential settlement must be minimized, which foundation type is commonly provided for buildings?

Introduction / Context:
Foundation selection balances bearing capacity, settlement control, constructability, and cost. On weak, relatively uniform soils, spreading loads over a larger area reduces contact pressure and helps limit differential settlement. This is the basis for choosing raft (mat) foundations in many low-capacity scenarios.

Given Data / Assumptions:

• Soil near the surface has low bearing capacity at the required founding level.
• Building has multiple columns/walls requiring support.
• Objective is to reduce bearing pressure and differential settlement.

Concept / Approach:

A raft (mat) footing is a large, continuous slab supporting many or all columns and walls, effectively distributing loads across the entire building footprint. By increasing foundation area, the average contact pressure is reduced, and the raft’s rigidity helps bridge local soil variability. Grillage footings serve primarily to spread a concentrated load from a column (often steel) and are less suited for whole-building distribution on weak soils compared with a full mat. Isolated column footings may be inadequate or lead to large, overlapping pads in weak soils.

Step-by-Step Solution:

Verification / Alternative check:

Rules of thumb: when the sum of isolated footing areas exceeds ~50% of the footprint, a raft is economical and performs better. Analytical settlement checks confirm improved performance on soft soils.

Why Other Options Are Wrong:

Isolated footings demand large sizes and may settle unevenly; grillage is localized; answering “All” would include inferior choices for the stated condition.

Common Pitfalls:

Ignoring groundwater effects and punching shear; not considering basement integration; overlooking need for soil improvement under the raft when necessary.

Final Answer:

Raft footing