Soil Bearing Capacity Explained Clearly

When a foundation presses down on the ground, the soil has to push back. There is a limit to how hard it can push before it gives way, and that limit is what engineers call bearing capacity. It is one of the most fundamental quantities in foundation design, because it determines how much load a given patch of ground can safely carry and therefore how large a foundation needs to be.

This article explains what bearing capacity means, how it differs from the related question of settlement, what affects it, and how engineers apply it safely. The discussion describes principles and the factors that matter rather than presenting design formulas, which must be applied by a qualified engineer using site-specific ground data.

What bearing capacity actually means

Bearing capacity is the soil’s ability to support load without failing in shear. If a foundation applies more pressure than the ground can resist, the soil shears along a surface beneath and around the foundation, and the foundation punches into the ground or tilts as the soil is pushed aside. This is a sudden, dramatic failure, and avoiding it is a non-negotiable requirement of any foundation.

It helps to distinguish two related terms. The ultimate bearing capacity is the maximum pressure the soil can withstand just before it fails in shear. The allowable bearing capacity is a much lower value, the ultimate capacity divided by a factor of safety, which is the pressure an engineer permits in design. Working well below the ultimate value provides a margin against the uncertainties inherent in soil and loading.

Two distinct requirements, strength and settlement

A common misunderstanding is that bearing capacity alone decides whether a foundation is adequate. In reality a foundation must satisfy two separate requirements, and either one can govern the design.

Bearing capacity guards against shear failure, the catastrophic collapse of the ground. This is a strength, or ultimate, requirement.
Settlement guards against excessive movement. Even when the ground is nowhere near shearing, it may compress enough to damage the building. This is a serviceability requirement.

On many soils, particularly soft or compressible clays, settlement governs the design well before bearing capacity does. The foundation ends up sized not to prevent collapse, which a smaller foundation would achieve, but to keep movement within tolerable limits. The allowable pressure an engineer actually uses is therefore the lower of the two values that satisfy strength and settlement, and our explainer on soil mechanics describes the compression behaviour that drives settlement.

What affects bearing capacity

The bearing capacity of a site is not a fixed property of the soil alone; it depends on the foundation and the conditions as well. Several factors influence it.

Soil type and strength

The shear strength of the soil is the dominant factor. Dense sands and gravels, which resist through friction, generally offer high bearing capacity. Soft clays, which rely on cohesion and drain slowly, offer much less. Rock, where present at a reasonable depth, offers the highest capacity of all. The classification and strength of the soil therefore set the broad scale of what is achievable.

Foundation width and depth

A wider foundation spreads load over more soil and engages a larger volume of ground, generally increasing the load it can carry. Founding deeper also helps, because the weight of soil surrounding the foundation provides confinement that resists the shearing mechanism. These geometric effects are why the same soil supports a small footing and a large raft very differently.

Groundwater

Water reduces bearing capacity. A high water table lowers the effective stress in the soil, and since strength depends on effective stress, the capacity falls. A site assessed in a dry season may behave quite differently when the water table rises, which is why groundwater conditions are always part of the assessment.

The role of the factor of safety

Soil is variable and the loads a building experiences are not perfectly known, so engineers never design a foundation to sit at the ultimate bearing capacity. A factor of safety reduces the design pressure well below the failure value, providing a margin that absorbs uncertainty in the ground investigation, in the soil properties, and in the magnitude and combination of loads.

This margin is deliberate and conservative. It accounts for the simple fact that we cannot test every cubic metre of ground, that soil properties scatter, and that the consequences of a bearing failure are severe and sudden. The factor used reflects the reliability of the available data and the importance of the structure, and it is one reason why a thorough ground investigation pays for itself by allowing a more confident, less wasteful design.

How bearing capacity shapes foundation choice

Bearing capacity is the bridge between the ground and the foundation. Once the engineer knows the allowable pressure the soil can sustain and the loads coming down from the structure, the required foundation area follows directly: enough area to keep the applied pressure within the allowable value.

Where the ground is strong and the allowable pressure is high, modest shallow foundations such as pads or strips suffice. Where the allowable pressure is low, foundations grow larger, and at some point a raft spreading the entire building load becomes more sensible. If even a raft cannot satisfy the requirements, or if settlement would be unacceptable, the loads must be taken deeper through piles to firmer strata. The progression from pad to strip to raft to piles is driven largely by the available bearing capacity and the settlement the structure can tolerate. Deciding among these options is part of the work covered in our overview of what a structural engineer does.

Common problems linked to bearing capacity

When bearing capacity or settlement is misjudged, the symptoms show up in the building above. Excessive or uneven settlement distorts the structure, and one of the most visible signs is cracking, which is why ground movement is a frequent culprit behind the patterns described in our guide to types of concrete cracks. Doors and windows that stick, sloping floors and separating finishes can all point to a foundation that is moving more than intended.

These problems are far cheaper to prevent than to cure. Underpinning or strengthening a foundation after a building is occupied is disruptive and costly, which is the strongest argument for proper investigation and competent design at the outset. If you notice signs of movement in an existing building, our note on whether you need a structural engineer can help you decide when to seek advice.

Bearing capacity in the Singapore context

Singapore’s varied ground makes bearing capacity a site-specific question. Stiff residual soils and shallow rock in some areas offer generous capacity and allow economical shallow foundations, while the soft marine clays found in coastal and reclaimed areas offer low capacity and large, slow settlements, frequently pushing designs towards piling. Two sites a short distance apart can demand very different foundations for the same building.

Because the ground governs so much, foundation designs in Singapore rely on site investigation and must be endorsed by a Professional Engineer in accordance with Building and Construction Authority (BCA) requirements. Whatever the bearing capacity of your site turns out to be, it is far better understood early, before the foundation is committed. If you would like an assessment of what your ground can support, you can talk to our consultancy for guidance.

Frequently asked questions

What is bearing capacity?

Bearing capacity is the ability of soil to support load without failing in shear, and the allowable bearing capacity used in design is the failure value reduced by a factor of safety, representing the pressure a foundation may safely apply to the ground.

What is the difference between bearing capacity and settlement?

Bearing capacity is a strength limit guarding against sudden shear failure of the ground, while settlement is a serviceability limit guarding against excessive movement, and a foundation must satisfy both because on soft soils settlement often governs the design before bearing capacity does.

What reduces a soil’s bearing capacity?

Soft or weak soil, a narrow or shallow foundation, and a high groundwater table all reduce bearing capacity, with rising water being especially significant because it lowers the effective stress in the soil and therefore the shear strength on which capacity depends.

Why do engineers use a factor of safety?

A factor of safety lowers the design pressure well below the soil’s failure value to provide a margin against the variability of natural ground, the uncertainty in soil properties and applied loads, and the severe, sudden nature of a bearing failure should it occur.

Does higher bearing capacity always mean a cheaper foundation?

Generally yes, because higher allowable bearing pressure lets the same load be carried on a smaller, shallower foundation, but settlement must still be checked, and on compressible soils the need to limit movement can govern the foundation size regardless of bearing capacity.

Want to know what your site can safely support before committing to a foundation? Talk to our consultancy or get in touch.