Every structural load eventually has to reach the ground, and the foundation is what makes that handoff safely. Choosing the wrong type isn’t a minor inefficiency — it’s the kind of mistake that shows up later as settlement, cracking, or worse. Here’s an overview of the main foundation types and the conditions that point toward each.
The big split: shallow vs deep
Foundations fall into two broad categories based on depth relative to width. Shallow foundations (sometimes called spread footings) are founded near the surface — generally where the founding depth is less than the footing’s width and less than around 3 metres. They work when surface soils are strong and stiff enough to support the load directly. Deep foundations sit too far below the surface for the bearing capacity to be affected by surface conditions (usually beyond 3 m), and are needed when surface soil is weak, loads are very heavy, or the structure is tall.
Shallow foundation types
Isolated (pad) footing — supports a single column, spreading its load over a square or rectangular base. The default choice for residential and light commercial buildings on reasonably strong, evenly loaded soil.
Combined footing — supports two or more closely spaced columns under one footing, used when individual isolated footings would be too close together or would overlap.
Strap footing — similar in purpose to a combined footing, but the columns are built on separate footings connected by a strap beam rather than one continuous base. This is the go-to solution when a column sits near a property line and an isolated footing would need to cross it or become awkwardly eccentric.
Strip footing — a continuous footing running under load-bearing walls, distributing weight along its length rather than at discrete points.
Raft (mat) foundation — a thick, reinforced concrete slab spread across the entire building footprint, supporting all columns or walls at once. Rafts are used when soil bearing capacity is poor, when column loads are heavy or closely spaced enough that individual footings would interact, or when reducing differential settlement matters — the continuous slab resists uneven movement between loading points far better than separate footings would.
Deep foundation types
Pile foundation — long, slender members driven or cast into the ground to transfer load to deeper, stronger strata when surface soil can’t carry it. Piles work two ways: end-bearing piles rest their base on rock or a strong layer; friction piles transfer load along their entire length through resistance with surrounding soil. A pile cap links multiple piles together and spreads the building’s load across the group.
Pier foundation — an underground structure transmitting heavy loads that shallow foundations can’t carry, typically shallower than piles and common in multi-storey structures.
Caisson — large, hollow cylindrical foundations, often used in water-bearing soils or river crossings, particularly relevant to bridge construction.
Piled raft — a hybrid combining a raft slab with piles beneath it. Rather than either element carrying the full load alone, both share it: the raft distributes load over area while the piles reduce settlement and reach deeper, more competent soil. This is especially useful where surface soil is moderately weak (not so weak that piles alone are the only option) and controlling settlement matters as much as raw load capacity.
How the choice actually gets made
Selection isn’t arbitrary — it follows from geotechnical data and project constraints:
- Soil bearing capacity: higher capacity points toward shallow foundations; lower capacity pushes toward raft or pile systems.
- Building height and load: taller, heavier structures generally need deeper, more robust foundations.
- Groundwater level: high water tables complicate shallow excavation and can favor piled or piled-raft systems.
- Site constraints: a tight property line can force a strap footing instead of an isolated one.
- Settlement tolerance: where differential settlement is a real risk, a raft (or piled raft) outperforms separate footings.
None of this is a substitute for an actual geotechnical investigation and a structural engineer’s design — soil reports, bore logs, and load calculations are what turn this overview into a real foundation choice for a specific site.
Part of our Structural & RCC series. Next: load calculation basics for a simple building.