Bearing Capacity Versus Settlement

There are two separate questions about any footing, and they get confused constantly.

Will the ground fail under it, and how much will it move. The first is bearing capacity. The second is settlement. A footing has to satisfy both, and on most real sites the one that decides the size is settlement.

Bearing capacity is a strength question

Whether the soil shears and the footing punches into the ground.

It depends on the soil's strength, the footing's width and depth, and the groundwater. It is a collapse mechanism, so it is checked with a healthy factor of safety against it.

In practice it governs on genuinely weak ground: soft clay, loose fill, or a footing on a slope where the failure surface can escape sideways.

Settlement is a serviceability question

How much the footing moves downward as the soil beneath it compresses.

Which has nothing to do with whether the soil is close to failing. Ground can be a long way from a bearing failure and still compress enough to crack the building above it.

And it is almost always the governing criterion on competent ground, because the strength is high but the stiffness is finite. Which is why footings on good ground are frequently larger than a strength calculation alone would require.

Total settlement matters less than differential

A building that settles uniformly by twenty millimetres is largely unaffected. The same building where one footing settles twenty millimetres and the next settles five is cracked.

Which means the number that matters is the difference between adjacent supports, and the rate of change across the structure. The causes and the symptoms are in [differential settlement and what causes it](/blog/differential-settlement-and-what-causes-it).

Differential settlement arises from four things.

Variable ground, where one footing sits on sandstone and the next on fill. The most common cause in Sydney and the reason the investigation has to cover the whole footprint rather than one borehole.

Variable load, where a heavily loaded column settles more than a lightly loaded one on the same ground.

Variable footing size, where a wider footing stresses the soil to a greater depth and settles more than a narrow one even at the same bearing pressure. This is the result people find least intuitive and it is why footings on a project are proportioned for equal settlement rather than equal pressure.

Mixed footing types, where part of a building is on piles founded on rock and part is on pad footings in soil. Those settle by completely different amounts, and the junction between them is where the cracking appears.

Time, which depends on the soil

Sands settle almost immediately, as the load is applied. By the time the building is finished, the settlement has happened.

Clays settle over time, as water is squeezed out of the soil. Which can continue for years after construction, and it is why a crack can appear in a building that was fine for three summers.

That delay is why clay sites are assessed on long term settlement as well as immediate, and why the clay movement question in [reactive clay sites and footing design in NSW](/blog/reactive-clay-sites-footing-design-nsw) and [trees and footings on Sydney clay](/blog/trees-and-footings-on-sydney-clay) sits alongside it rather than replacing it.

The allowable bearing pressure on a drawing

A geotechnical report gives an allowable bearing pressure, and it usually already has a settlement limit built into it.

Which is worth understanding, because the number is not purely a strength value. It is the pressure at which both the factor against bearing failure and the expected settlement are acceptable for a particular footing size and a particular structure.

So the number is not transferable without thought. The allowable pressure for a two metre pad is not the allowable pressure for a six metre raft on the same ground, because the deeper stress bulb under the larger footing compresses more soil. Reading a report for that distinction is part of [understanding geotechnical investigation reports](/blog/understanding-geotechnical-investigation-reports).

What this changes about design

Proportion footings for equal settlement, not for equal pressure, so the structure settles as a unit.

Avoid mixing foundation systems under one structure, or where it is unavoidable, provide a movement joint at the change.

Found at a consistent level and in a consistent stratum where the ground allows it.

Design the structure for the settlement that will occur, which for a sensitive or long building can mean articulation joints rather than a stiffer footing. The brick version of that is in [articulation joints in brick veneer houses](/blog/articulation-joints-in-brick-veneer-houses).

Stiffen the footing system where the ground is variable and a joint is not acceptable, which is what a raft does and why it appears on difficult sites. The comparison is in [foundation design, raft versus piled versus strip](/blog/foundation-design-raft-vs-piled-vs-strip).

Existing buildings

When a building cracks, the useful question is which of the two mechanisms is at work, and it is nearly always settlement rather than bearing failure.

Which means the investigation looks for what changed: a tree removed or planted, a leak saturating the ground under a footing, an excavation nearby, a load added, or dewatering on an adjacent site. The last of those is the mechanism in [dewatering and groundwater control in excavations](/blog/dewatering-and-groundwater-control-in-excavations), and it damages buildings that nobody was working on.

Monitoring tells you whether it has stopped, which decides whether the repair is a repair or a holding action.

ACSES provides geotechnical engineering and footing design across Sydney. Talk to us about a project.