How the two differ in kind
Two ways to get a column's load into the ground, and they solve it in opposite directions.
A SPREAD FOOTING goes sideways. The load is distributed over an area large enough that the pressure beneath is within what the soil can carry, at a depth set by frost, by the need to reach undisturbed ground, and by whatever is below the surface. It is cheap, it is fast, it needs no specialist plant, and it depends entirely on the bearing capacity of the soil immediately beneath it.
A DRILLED PIER goes down. A shaft is bored and filled with reinforced concrete, carrying the load partly on its base — which can be belled out to increase the bearing area — and partly by friction along its shaft, to a stratum below whatever is near the surface. It needs a rig, it costs more per foundation, and it is indifferent to what the topsoil is doing.
So the question is what the shallow soil is. Where there is a competent stratum at footing depth, the footing is the obvious answer and a pier would be paying to bypass ground that was perfectly adequate. Where the shallow soil is soft, variable, filled or compressible over something better, a pier reaches past it — which is precisely what it exists for.
The property most often overlooked is UPLIFT. A spread footing resists upward force only through its own weight and whatever soil sits on it. A drilled pier resists it through friction along its whole shaft as well, which makes it the natural foundation for a column under net wind uplift, for a sign or pole or light standard, and — the case that catches people out — for structures in EXPANSIVE CLAY. There the problem is not the load coming down but the ground swelling and pushing UP: a shallow footing is simply lifted, while a pier extended below the active zone is anchored by friction in soil that is not moving.
The factors that actually differ
Show
| Spread footing | Drilled pier | |
|---|---|---|
| How the load is carried | Spread over an area, into the soil immediately beneath. | Down the shaft — end bearing on a deeper stratum, plus friction along the shaft. |
| What it depends on | The bearing capacity of the shallow soil, and nothing else. | A competent stratum within reach, or enough shaft friction to develop capacity. |
| Cost and plant | Low. Excavation, formwork, reinforcement, concrete — no specialist equipment. | Higher. A drilling rig, spoil disposal, and a specialist contractor. |
| Uplift resistance | Its own weight plus the soil on top of it, which is modest. | Substantial, through shaft friction — which is why poles, signs and wind-loaded columns use them. |
| Expansive clay | Lifted by the swelling soil. A shallow footing is in the active zone. | The standard answer — the pier extends below the active zone and is anchored in stable ground. |
| Variable ground | Poor. Differential settlement follows the soil's variation across the building. | Good — each pier reaches the same stratum regardless of what is above it. |
| Footprint | Wide. On a heavily loaded column in weak soil, the required area can become impractical. | Narrow, which matters on a constrained site or close to a boundary. |
| Excavation and spoil | A shallow open excavation, often with battered sides and the spoil placed nearby. | A narrow bore. Much less spoil, and no open hole for people to fall into. |
| Groundwater | A shallow excavation below the water table needs dewatering or support. | Manageable with casing or a support fluid, which is routine for the technique. |
| Verification | Inspecting the bearing surface before pouring — which is the check people skip. | Inspecting the base of the bore, plus integrity testing on a proportion of the piers. |
Which one, and when
Choose spread footing when…
- A competent bearing stratum at shallow depth — the common case, and where a pier would be paying to bypass good ground.
- Modest loads on ordinary ground, where the required footing area is reasonable.
- No specialist plant available or no access for a rig, which on a domestic site is frequently decisive.
- Where the ground is uniform across the building, so differential settlement is not a concern.
Choose drilled pier when…
- Soft, filled or variable shallow soil over a better stratum — the case piers exist for.
- Expansive clay, where a shallow footing is lifted by the ground rather than loaded by the building.
- Net uplift on the foundation: a wind-loaded column, a pole, a sign, a canopy.
- A constrained footprint, close to a boundary or an existing structure, where a wide footing will not fit.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why does expansive clay lift a spread footing?
- Because the clay changes volume with moisture, and a shallow footing sits inside the zone where that happens. Expansive clays swell as they take up water and shrink as they dry, and the movement is seasonal and can be substantial — enough to lift a lightly loaded footing, heave a slab, and crack the structure above. The depth over which that occurs is the active zone, and below it the moisture content is stable. A drilled pier extended below the active zone bears and develops friction in soil that is not moving, which is why it is the standard foundation in expansive-soil regions. The critical detail is the shaft within the active zone: the swelling clay grips it and tries to lift it, so the design either provides enough anchorage below to resist that or isolates the shaft with a void form or a slip surface.
- What is a belled pier?
- A drilled pier whose base is enlarged into a bell or underream, increasing the bearing area without increasing the shaft diameter — so a modest shaft can deliver a much larger end-bearing capacity. It is formed with a belling tool on the rig, and it is used where end bearing rather than shaft friction is the dominant mechanism and the stratum at depth is good. Two constraints govern whether it is available: the soil must stand unsupported while the bell is cut and the concrete placed, which cohesive soils generally do and granular ones do not; and the inspection of the base becomes more important, since the bell is where the capacity is. It also does nothing for uplift — a bell increases downward capacity, and uplift resistance still comes from shaft friction and weight.
- What is the check people skip on a spread footing?
- Inspecting the bearing surface before the concrete goes in. A footing's capacity depends entirely on the soil immediately beneath it, and the excavation is the only opportunity anyone has to confirm that the soil is what the design assumed — the right stratum, undisturbed, not softened by water standing in the trench, not loosened by the excavator, and free of soft spots or made ground. It takes minutes and it is the single most valuable inspection in a shallow foundation. The failures it catches are ordinary: a footing founded in fill rather than natural ground, a base softened by a week of rain, and a pocket of soft material that a trial pit elsewhere on the site did not find. Once concrete is in, none of that can be established and none of it can be corrected.
- How does a pier resist uplift?
- By friction along its shaft plus its own weight, and the friction term is usually the larger. As the pier is pulled upward, the soil in contact with the shaft resists through the same shear mechanism that provides its downward friction capacity — so a long shaft in competent soil develops substantial uplift resistance without any enlargement at the base. That is why piers are the natural foundation for anything where the wind can lift the structure: a canopy, a sign, a light pole, a wind-loaded frame with net uplift at its columns. A spread footing's only uplift resistance is its own mass and the weight of soil sitting on it, which is why resisting uplift with a footing means making it heavy — a much larger volume of concrete for the same capacity.
- How is a pier verified after construction?
- By inspection during construction and testing afterwards, because nothing about a completed pier is visible. During construction: the bore is inspected for depth, for the stratum reached, and for cleanliness at the base, since debris left at the bottom is soft material between the concrete and the bearing stratum. Afterwards: integrity testing on a proportion of the piers, typically by a low-strain method that sends a stress wave down the shaft and interprets the reflection, which detects necking, inclusions and a shaft that did not reach its designed length. Load testing on a sample establishes capacity directly where the design warrants it. The proportion tested is specified up front, along with what happens when a test fails, because that conversation is much harder after the rig has left.
- Can the two be mixed on one building?
- Yes, and it is common where the ground varies across a site — piers where a soft zone runs through, footings where the shallow stratum is competent. The detail that needs designing rather than assuming is the TRANSITION, because the two foundation types settle by different amounts: a pier on a firm stratum settles very little, while a spread footing on ordinary soil settles more, and the differential between adjacent foundations is what cracks the structure between them. The design either accommodates that movement with a joint, or reduces the difference by designing the footings for a settlement compatible with the piers. Mixing the two without addressing it is a recognised cause of cracking in buildings whose individual foundations were each perfectly adequate.
- What decides the footing's depth?
- Several requirements at once, and the deepest governs. Frost depth, in climates that freeze — the base must be below the depth to which frost penetrates, or the soil beneath it heaves. Reaching undisturbed natural ground, which on a site with topsoil, fill or made ground may be considerably deeper than frost requires. Bearing capacity, since some soils improve with depth. Adjacent excavations and services, where a footing must not be undermined by something dug later. And the influence of trees on shrinkable clay, where root-induced drying extends the depth at which moisture is stable — a requirement that frequently governs in clay areas and that is set by guidance relating tree species and distance to depth.
- Which is cheaper?
- The footing, where the shallow ground is adequate, and by a wide margin — ordinary excavation, no specialist plant, no mobilisation, and a trade that every contractor has. Piers carry a rig, a specialist subcontractor with a mobilisation cost that a small job spreads over few piers, spoil to dispose of, and a testing regime. The comparison inverts where the ground forces it: on soft or variable soil, a spread footing large enough to work may become impractical, differential settlement across the building becomes the governing risk, and the alternatives — excavating and replacing the poor material, or ground improvement — cost more than piers. And in expansive clay or under net uplift, the footing is not a cheaper option but an inadequate one, which is a different conclusion.