Foundations are the part of a building nobody sees, argues about most, and can least afford to get wrong. The choice between them is not a matter of preference. It is decided by what the ground can carry, how much the structure weighs, and how much differential movement the building will tolerate.
Start with the investigation, because everything else depends on it
A site investigation is a small percentage of the substructure cost and it governs every decision that follows: bearing capacity, water table, the depth of any fill, whether the soil is expansive, and whether the site has been used for something that left surprises behind.
Skipping it does not remove the risk, it just moves it to excavation stage where changing your mind is far more expensive. The commonest expensive discovery is uncontrolled fill — old excavation spoil or rubbish levelled and built over — which looks fine until it is loaded.
Pad foundations
Individual pads under columns, sized so the load spreads to a pressure the soil can take. They suit framed structures on reasonably competent ground at shallow depth, and they use the least concrete of the shallow options.
They stop making sense when the required pads grow so large they nearly touch, which is the ground telling you it cannot take the pressure. At that point a raft is usually cheaper and better behaved.
Strip foundations
A continuous strip under a loadbearing wall. Standard for masonry housing and low-rise loadbearing construction, simple to set out and simple to build.
Their weakness is differential settlement along the length of the wall — one end on firm ground, the other on softer material, and the wall cracks in between. Where ground varies, a reinforced strip that can span local soft spots is worth the extra steel.
Raft foundations
One slab under the whole footprint, spreading load over the largest possible area. Rafts earn their place where bearing capacity is low, where the ground is variable, or where the structure needs to move as one piece rather than in parts.
They use more concrete and steel than pads, so on a straight material comparison they look expensive. The comparison that matters is against the alternative the ground actually permits, which is often piling.
Piled foundations
Where competent material is deep, piles carry load down to it, through end bearing, friction along the shaft, or both. They are the answer for heavy structures on weak ground, deep fill, or high water tables.
Piling brings specialist plant, specialist supervision and a testing regime, so it is the most expensive option to mobilise and the one most sensitive to programme. Where piling is likely, it needs to be settled early: it sits at the very front of the critical path and everything waits on it.
The conditions that change the answer
A high water table
Water changes excavation from a digging problem into a dewatering problem, affects concrete placement, and can float lightweight structures. It usually pushes the design towards shallower solutions or a raft, and it needs to be known before anyone prices the works.
Expansive clays
Soils that swell when wet and shrink when dry impose movement on anything founded in the active zone. The usual responses are founding below that zone, or a stiff raft that rides the movement without distorting the structure.
Sloping sites
Slopes bring stability considerations as well as bearing ones, and stepped foundations where levels change. The step detail is a common source of cracking when it is improvised on site rather than designed.
Comparing options honestly
The cheapest foundation is rarely the one with the least concrete in it. A fair comparison includes:
- Excavation volume, and what it costs to get spoil off a constrained site
- Dewatering, temporary works and shoring
- Programme time, including any specialist mobilisation
- Risk, and who carries it if the ground is not as assumed
A raft that costs more in materials but removes two weeks of dewatering and a settlement risk is often the cheaper project decision, even though it is the more expensive line in the bill.
What to insist on before work starts
- A site investigation proportionate to the structure, with a written report
- Design bearing pressures stated on the drawings
- A procedure for what happens when excavation finds something different
- Inspection and sign-off of every formation level before concrete
That last point is the one that gets skipped when the programme is tight, and it is the one that matters most. Once concrete is on it, nobody can see what it was founded on.
