Deep vs Shallow Foundations Explained

Every building has to transfer its weight safely into the ground, and the part that does this is the foundation. Broadly, foundations fall into two families: shallow foundations, which spread load near the surface, and deep foundations, which carry it down to stronger soil or rock far below. Choosing a deep foundation over a shallow one — or the reverse — depends almost entirely on what the ground beneath the site can bear.

This article explains the difference between deep and shallow foundations, how each one works, and what conditions push an engineer toward one or the other. It is the ground, not the building above, that usually decides.

What shallow foundations do

Shallow foundations sit relatively close to the surface and spread the building’s load over an area of competent soil directly beneath it. The common types are:

Pad (or isolated) footings under individual columns.
Strip footings running under walls or lines of columns.
Raft (or mat) foundations, a single thick slab spreading the load of the whole building over the entire footprint — covered in detail in raft foundations explained.

Shallow foundations are simpler and cheaper to build, and they are the natural choice where firm, strong soil exists close to the surface and can carry the load without settling excessively. They work by bearing — the soil directly underneath pushes back up against the footing.

What deep foundations do

When the soil near the surface is too weak, too compressible, or too variable to carry the building, the load has to be taken down to a more reliable layer. Deep foundations do exactly this. The most common type is the pile — a slender column driven or bored deep into the ground — explained in pile foundation types.

A deep foundation transfers load in two ways, often together. End-bearing piles reach down to a firm stratum and stand on it, like a column resting on solid rock. Friction piles transfer load through the friction between the pile’s surface and the surrounding soil along its length. Many piles act partly in both modes. By reaching past the weak surface soils, deep foundations give a building a dependable support that shallow footings could not.

The ground decides

The single most important factor in choosing between deep and shallow foundations is the soil profile, which is established by a site investigation: boreholes, soil sampling and testing that reveal what lies beneath. The investigation tells the engineer how strong each layer is, how deep the firm ground sits, and where the water table is.

If competent soil is near the surface, shallow foundations usually suffice. If the upper soils are soft clays, loose fills or reclaimed land — common in parts of Singapore — the load must be taken deeper, and piling becomes necessary. No responsible foundation design proceeds without this ground information; assuming conditions is one of the most expensive mistakes in construction. Interpreting the investigation and selecting the foundation is central to what a structural engineer does.

Settlement: the quiet governing factor

Foundations must do more than not fail — they must not settle too much, or unevenly. A building can be perfectly safe against collapse yet still suffer if it settles unevenly, because differential settlement cracks finishes, jams doors, and can distress the structure. Soft, compressible soils settle more, and they settle for longer, which is another reason such sites often need deep foundations that bypass them.

Engineers therefore check foundations against two things: bearing capacity (will the soil fail?) and settlement (will it move too much?). On weak ground, settlement frequently governs the decision, pushing the design toward piles even where a raft might technically carry the load. Time matters too: some soils continue to settle for years as water is slowly squeezed out of them, a process called consolidation, so the foundation must be judged not only on its immediate behaviour but on how the ground will respond over the building’s whole life.

Cost, programme and neighbours

Deep foundations are more expensive and slower than shallow ones, and piling brings its own considerations: vibration, noise, and the effect on adjacent buildings. On tight urban sites, the foundation method has to respect the neighbours and the boundary, and bored rather than driven piles are often chosen to limit disturbance. These practical constraints sit alongside the geotechnical ones.

The decision is rarely about the building alone; it weaves together soil conditions, settlement tolerance, cost, programme and the surroundings. If you are at the start of a project and unsure what your ground demands, our structural engineering consultancy can help interpret the investigation and advise on the right foundation.

Frequently asked questions

What is the difference between a deep foundation and a shallow foundation?

A shallow foundation spreads the building’s load over soil close to the surface, while a deep foundation, such as piling, carries the load down to a stronger layer far below when the surface soils are too weak.

When is a deep foundation needed?

A deep foundation is needed when the soil near the surface is too soft, compressible or variable to carry the load safely, as with soft clays, loose fill or reclaimed land, so the load must be taken to firmer ground.

How does an engineer decide which foundation to use?

The decision is based mainly on a site investigation that reveals the soil profile, strength and water table; this, together with settlement limits, cost and the surroundings, determines whether shallow or deep foundations are appropriate.

What is the difference between end-bearing and friction piles?

End-bearing piles stand on a firm stratum deep in the ground, while friction piles transfer load through friction along their surface with the surrounding soil; many piles work partly in both ways.

Why does settlement matter as much as strength?

Because a foundation can be safe against failure yet still allow the building to settle, especially unevenly, which cracks finishes, jams doors and distresses the structure, so settlement often governs the design on soft ground.

Starting a project and unsure what your ground requires? Talk to our consultancy or get in touch.