Do Deeper Foundations Increase Load-Bearing Capacity?

This is a persistent misconception on building sites: if the ground cannot take the load, make the foundation deeper. In most cases, that is not the right answer.

What Controls Bearing Capacity?

The soil’s bearing capacity is a property of the ground at a given depth — it is expressed in kN/m² and represents the pressure the ground can safely sustain. When you calculate whether a foundation is adequate, you compare the contact pressure (load divided by base area) against the allowable bearing capacity of the soil.

The contact pressure is determined by the plan dimensions of the foundation, not its depth. A 600 × 600 mm pad carrying 50 kN exerts the same contact pressure whether the base is 300 mm or 600 mm thick. The soil does not feel the depth — it feels the footprint.

What Depth Does Help With

Increasing foundation depth is not without benefit, but it addresses different problems:

– Punching shear — a deeper pad provides more mass and a longer shear perimeter, reducing the risk of the column punching straight through the concrete
– Embedment — a deeper pad is less likely to be destabilised by frost, ground movement or edge erosion
– Reaching better soil — in some cases, soil quality genuinely improves with depth, so going deeper reaches strata with higher allowable bearing capacity

If the ground at 300 mm depth is inadequate, going to 600 mm might reach better soil — and that improves bearing capacity. But it is the quality of the soil at that deeper level that helps, not the extra concrete.

Why Reinforcement Still Matters

A393 mesh is specified in pad foundations for crack control (top layer) and to provide tensile resistance under flexure (bottom layer). Doubling the depth of a pad does not remove that requirement — a deep but unreinforced pad can still crack. The mesh specification remains regardless of how much deeper you go.

When to Ask for a Revised Design

If ground conditions on site are weaker than assumed (soft clay, made ground, voids), the right solution is not simply to fill the pit with more concrete — it is to contact your structural engineer. A revised design may require a larger pad footprint, a raft, or in some cases piling. Get in touch with PorthouseDean if your site conditions differ from the assumptions in your current drawings.