A footing can pass the bearing-capacity check and still be a poor foundation design.
That is the central issue behind allowable bearing capacity vs settlement. Bearing capacity deals primarily with whether the soil has enough strength to support the applied load without a shear failure. Settlement deals with whether the foundation will move too much for the structure above it to perform properly.
Good foundation design must satisfy both.
For structural and foundation designers, the mistake is usually not forgetting one calculation. It is assuming that a satisfactory allowable bearing pressure automatically means settlement will also be acceptable.
It may not.
Allowable Bearing Capacity vs Settlement: Two Different Limits
Think of the two checks this way:
- Bearing capacity is primarily a strength limit.
- Settlement is primarily a serviceability limit.
A soil can have adequate shear strength and still compress enough to cause structural or architectural problems.
Conversely, a soil profile may be relatively compressible but still have a high calculated ultimate bearing capacity.
The allowable foundation pressure should therefore reflect the more restrictive condition.
Bearing Capacity: Can the Soil Carry the Load?
Ultimate bearing capacity represents the pressure at which a foundation could develop a shear failure mechanism in the supporting soil.
Depending on the soil and foundation conditions, the analysis considers factors such as:
- soil cohesion;
- effective friction angle;
- soil unit weight;
- footing width and shape;
- foundation embedment;
- groundwater conditions; and
- surcharge at foundation level.
A typical allowable bearing pressure may be obtained by applying an appropriate factor of safety to the calculated ultimate resistance.
Conceptually:
Allowable bearing pressure = Ultimate bearing capacity / Factor of safety
However, that is only the strength-based side of the problem.
A footing may be safely below the pressure required for shear failure and still experience unacceptable settlement.
That is why an experienced geotechnical engineer does not simply calculate an ultimate capacity, divide it by three, and stop.
Settlement Can Control Before Shear Failure Does
Settlement occurs because the soil beneath a footing changes volume or deforms as foundation loads are transferred into the ground.
The amount and rate of movement depend heavily on the soil profile.
Immediate Settlement
Immediate settlement develops relatively quickly as the soil deforms under load.
It can be important in:
- granular soils;
- unsaturated soils;
- stiff cohesive soils; and
- heavily loaded foundations.
Factors affecting immediate settlement include applied pressure, footing dimensions, soil stiffness, Poisson’s ratio, and the thickness of the stressed soil zone.
Consolidation Settlement
Consolidation settlement is particularly important in saturated cohesive soils.
When a load is applied, some of the initial stress increase is carried by pore water. As water gradually drains from the soil, the effective stress within the soil skeleton increases and the soil compresses.
Depending on permeability and drainage conditions, this process can continue for months or years.
A building may therefore perform normally shortly after construction but develop settlement progressively as consolidation occurs.
Differential Settlement
For most structures, differential settlement is often more damaging than total settlement.
A building that settles relatively uniformly may remain functional. Uneven movement between columns, walls, or adjacent foundations can cause:
- cracking in masonry and partitions;
- distortion of frames;
- sloping floors;
- sticking doors and windows;
- cladding distress;
- utility connection problems; and
- additional structural forces that were not anticipated in the original model.
Foundation recommendations should therefore consider not only how much movement may occur, but how uniformly that movement is expected to develop.
The Footing-Width Paradox
One of the most important concepts in allowable bearing capacity vs settlement is that increasing footing size does not always improve foundation performance in the way designers expect.
Suppose a footing appears too highly loaded.
The intuitive solution is often simple:
Make the footing larger.
Increasing the footing area reduces average contact pressure for a given column load. In some soil conditions, increased width can also increase calculated ultimate bearing capacity.
From a strength perspective, the larger footing may look better.
But there is another effect.
Larger Footings Influence Deeper Soil
Foundation stresses do not stop at the underside of the footing. They spread downward and outward through the soil.
As footing width increases:
- the loaded soil volume increases;
- the stress influence extends deeper;
- deeper compressible layers may become involved; and
- settlement behaviour can change significantly.
This becomes particularly important on layered sites.
Imagine the following profile:
- dense sand immediately below foundation level;
- several metres of competent soil beneath it; and
- a softer compressible clay layer at greater depth.
A relatively small footing may transfer most of its significant stress increase within the stronger upper soils.
A much larger footing may extend its stress influence deep enough to significantly load the underlying clay.
The designer may have improved the bearing-capacity calculation while unintentionally increasing the risk of deep-seated settlement.
This is why “bigger footing” and “better foundation” are not always synonymous.
Why Bearing Pressure Alone Can Be Misleading
Consider a structural designer who receives an allowable bearing pressure of 200 kPa [about 4.2 ksf].
The natural design approach is:
- Determine the service column load.
- Divide the load by the allowable pressure.
- Select the required footing area.
- Design the reinforced concrete footing.
That approach is reasonable only when the geotechnical recommendation clearly states the assumptions behind the 200 kPa value.
The designer should confirm:
- Is the value gross or net?
- Is it based on service loads?
- Is settlement already incorporated?
- What footing sizes were assumed?
- Does it apply to isolated and continuous footings?
- What founding stratum is required?
- Is there a minimum embedment?
- Does the recommendation change for larger footings?
An allowable pressure without these conditions can easily be misapplied.
Bearing Capacity May Change With Footing Geometry
Foundation geometry matters.
Bearing-capacity equations may include corrections for:
- footing shape;
- depth;
- load inclination;
- ground slope;
- eccentricity; and
- footing width.
For example, the soil contribution associated with the unit-weight term in common bearing-capacity formulations can increase with footing width.
However, the same increase in width changes the settlement problem.
At a similar applied contact pressure, settlement generally becomes more sensitive to footing width because a larger volume of soil participates in the response.
This is the engineering paradox:
A wider footing may improve the calculated bearing resistance while increasing the depth and volume of soil contributing to settlement.
Designers therefore need a foundation-specific view rather than treating the allowable pressure as a universal property of the site.
The Soil Profile Matters More Than a Single Number
One of the most useful things a foundation designer can do is review the actual borehole logs.
Do not jump directly to the foundation-recommendations table.
Look for:
- uncontrolled or variable fill;
- soft cohesive layers;
- loose granular zones;
- organic soils;
- changes in groundwater;
- weak layers beneath stronger near-surface material; and
- differences between boreholes across the building footprint.
A footing founded on competent soil can still experience settlement if a compressible layer lies within its stress influence zone.
This is particularly relevant for:
- heavily loaded columns;
- large combined footings;
- mat foundations;
- tanks and silos;
- industrial equipment foundations; and
- structures with significantly different loading across the footprint.
Groundwater Changes Both Strength and Settlement Behaviour
Groundwater should never be treated as a minor note in the borehole log.
A high groundwater table can influence:
- effective soil stress;
- bearing resistance;
- settlement;
- excavation stability;
- subgrade conditions; and
- construction dewatering requirements.
In granular soils, groundwater can reduce the effective unit weight used in portions of the bearing-capacity analysis.
In cohesive soils, groundwater and drainage conditions can strongly influence consolidation behaviour and the time required for settlement to develop.
Groundwater observations should also be interpreted carefully. A water level recorded immediately after drilling may not represent the stabilized or seasonal groundwater condition.
What Foundation Designers Should Ask the Geotechnical Engineer
Before finalizing foundation sizes, confirm a few critical items.
For Shallow Foundations
Ask:
- What soil layer should support the footing?
- What minimum founding depth is required?
- Is the bearing pressure gross or net?
- Is the recommendation based on allowable or factored resistance?
- Is settlement already considered in the recommended pressure?
- What total and differential settlement should be expected?
- Does the bearing recommendation change with footing width?
For Large Footings and Mats
Confirm:
- whether deeper compressible layers have been considered;
- whether settlement should be evaluated for actual foundation dimensions;
- whether soil-structure interaction warrants additional analysis; and
- whether a modulus of subgrade reaction or other stiffness parameter is appropriate for the structural model.
When the Structural Design Changes
Geotechnical recommendations should be revisited when:
- column loads increase substantially;
- isolated footings become combined footings;
- a mat foundation is introduced;
- the building footprint changes;
- foundation depths change; or
- significantly larger footing dimensions are required.
These are not minor structural revisions. They can change how the soil responds to loading.
Where project conditions evolve beyond the original assumptions, additional geotechnical engineering services may be needed to confirm that the original design parameters remain appropriate.
Do Not Design to the Highest Number in the Report
A geotechnical report may provide several allowable bearing pressures for different foundation widths, depths, soil conditions, or settlement criteria.
The largest number is not automatically the best design value.
The correct value is the one that corresponds to:
- the actual foundation geometry;
- the correct founding soil;
- the appropriate loading condition; and
- acceptable settlement performance.
This distinction is particularly important during value engineering.
Increasing bearing pressure to reduce footing dimensions may save concrete and reinforcement, but only if settlement remains acceptable.
Likewise, increasing footing size to reduce contact pressure may not solve the problem if the larger stress bulb begins loading a deeper compressible layer.
Bearing Capacity and Settlement Must Be Checked Together
The practical lesson in allowable bearing capacity vs settlement is that foundation design cannot be reduced to one soil-pressure number.
Bearing capacity asks:
Will the supporting soil fail in shear?
Settlement asks:
Will the foundation move enough to affect the structure?
A successful design must answer no to both questions.
For routine foundations on relatively uniform competent soils, this coordination may be straightforward. On sites with fill, soft layers, high groundwater, variable stratigraphy, or large structural loads, the interaction becomes much more important.
The most efficient foundation is therefore not necessarily the largest footing or the footing designed to the highest allowable pressure.
It is the foundation that satisfies strength and serviceability requirements while reflecting the actual soil profile, groundwater conditions, structural loads, and expected movement.
That requires one thing above all else: structural and geotechnical engineers working from the same assumptions before the concrete is placed.
Prepared by ZALIG Consulting Ltd.
ZALIG Consulting Ltd. provides geotechnical engineering, environmental consulting, and construction materials testing services for commercial, industrial, infrastructure, and private-sector projects.
