Every load in a structure ends up in the ground, and the foundation is where the handover happens. Foundations rarely fail by collapsing. They fail by settling unevenly: cracked masonry, jammed doors, a tilt that gets surveyed and argued over. Most foundation problems on real projects are settlement problems, not strength problems.
Design splits into two questions. A geotechnical one: can the ground carry the pressure without shearing or compressing too much. A structural one: can the concrete element spread the column load into that pressure without failing itself. This guide covers how the foundation type is chosen, why bearing capacity and settlement are checked separately, how the footing itself is designed, and when shallow foundations stop being the answer.
Choosing the foundation type
An isolated pad under each column is the default where the soil is competent and columns are far enough apart that their pads do not meet. It is the most economical option and the most direct to check.
Combined footings carry two or more columns on one base. They appear when columns sit too close for separate pads, or when a column stands hard against a boundary and its pad cannot spread symmetrically. The base is proportioned so the resultant of the column loads passes near the centroid, keeping the pressure distribution close to uniform.
Strip footings run continuously under load-bearing walls or closely spaced rows of columns. Rafts spread the whole structure over the full footprint: the move when the soil is weak, the loads are heavy or uneven, or when isolated footings would grow to cover around half the plan area, at which point joining them into one slab usually costs less anyway. Piled foundations stop asking the surface soil to work at all and carry the load down to something better.
| Type | Typical use | Governing concern |
|---|---|---|
| Isolated pad | Single columns on competent soil | Punching shear, base pressure |
| Combined | Columns close together, or at a boundary | Eccentricity and load resultant position |
| Strip | Walls, closely spaced column rows | Bending and differential settlement along the run |
| Raft | Weak soil, heavy or uneven loads, basements | Overall settlement, dishing, punching under heavy columns |
| Piled | Weak upper strata over a firmer layer | Pile capacity, group settlement, cap design |
The selection is rarely subtle. Read the borehole logs, note where the competent stratum sits and where the water table is, and most of the table above eliminates itself.
Bearing capacity and settlement are separate checks
Bearing capacity is a strength check on the soil. Push hard enough and the ground fails in shear along a slip surface beneath the footing. The ultimate capacity divided by a factor of safety, classically around 3, gives the safe pressure against that mode.
Settlement is a stiffness check. Under working pressure the ground compresses: almost immediately in sands, over months and years in clays as pore water is squeezed out. What damages buildings is not the total movement but the differential — the difference in settlement between adjacent supports. An angular distortion of about 1/500 is a common threshold for cracking in walls and finishes; structural distress arrives an order lower, near 1/150.
The two checks answer different questions and either can govern. On dense sands and gravels, capacity is generous and settlement decides the pressure. On soft clays the reverse can hold. Sizing a footing on capacity alone, without asking what it will settle, is the shortcut that surfaces two years later as diagonal cracks above door frames.
Designing the footing itself
Once the ground checks fix the plan area, the footing becomes a concrete design problem. The soil pushes back with a near-linear pressure, and the base behaves as a cantilever fanning out from the column.
- Flexure. The critical section sits at the column face. Upward soil pressure bends the base, and bottom reinforcement in both directions carries the moment. Bars must be anchored beyond the point of peak moment to develop their full strength, which on compact footings can govern bar size.
- Punching shear. The column tries to punch a truncated cone through the base on a perimeter close to its face; codes place the control perimeter between 0.5d and 2d out. This check usually sets the footing depth, because in a pad it is far more practical to add depth than to fix shear reinforcement.
- One-way shear. Checked as for a beam at a section a distance d from the column face. It seldom governs once punching has already set the depth.
- Detailing. Cover of 75 mm where concrete is cast against the ground, starter bars lapped for the column above, and on rectangular footings the short-direction steel concentrated in a central band under the column rather than spread evenly.
When shallow stops working
Shallow foundations lose the argument when no pad or raft achieves acceptable settlement at a buildable size. The usual triggers: soft or compressible clays near the surface, loose fill, organic soils, a high water table that complicates excavation, heavy concentrated loads, or uplift and lateral demands a pad cannot anchor.
Piles carry load through skin friction along the shaft, end bearing on a firmer stratum, or both. Two behaviours deserve respect at design time. First, a group settles more than a single test pile predicts, because the stressed zone below a group reaches far deeper than below one shaft. Second, the pile cap is a structural element in its own right, with its own punching and bending checks around both the column above and the piles below.
Foundation design rewards doing ordinary things in the right order: read the soil investigation before sizing anything, treat bearing and settlement as separate questions, let punching shear set the depth, and detail the reinforcement so the anchorage actually exists where the moment peaks. Modern analysis software carries a foundation from base pressure through to bar detailing in one model, which keeps the geotechnical assumptions and the structural output consistent with each other.
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FAQs
What is the difference between bearing capacity and allowable bearing pressure?
Ultimate bearing capacity is the pressure at which the soil fails in shear. The allowable pressure divides that by a factor of safety, commonly around 3, and is then reduced further if predicted settlement exceeds the project limit. On many soils the settlement condition governs, so the allowable figure in the report is effectively a settlement limit in disguise.
When should a raft foundation be used instead of isolated footings?
When isolated footings grow so large that they would cover roughly half the plan area or more, when the soil is weak or variable and differential settlement needs to be spread across the whole footprint, or when a basement slab is required in any case. A raft trades more concrete for lower and more uniform pressure on the ground.
Why does punching shear usually govern footing depth?
Because the column delivers its full load over a small contact area, and the footing must resist that load on a shear perimeter close to the column face. Shear reinforcement is awkward to fix in a shallow pad, so the practical remedy is depth: increasing the effective depth raises both the shear resistance and the length of the control perimeter at once.
When are piled foundations required?
When no shallow option achieves acceptable capacity or settlement: soft or compressible upper strata, loose fill, organic soils, heavy concentrated loads, or uplift and lateral demands that a pad cannot resist. Piles transfer load through skin friction along the shaft, end bearing on a firmer stratum, or a combination of the two, and the group settlement check matters as much as individual pile capacity.
