Underpinning Foundations Explained

When an existing foundation can no longer carry its load safely — because the building is settling, because the ground beneath it has weakened, or because the structure above is being made heavier — the foundation has to be strengthened or extended downward. The process of doing this beneath a structure that is already standing is called underpinning. It is one of the more delicate operations in structural engineering, because the work happens directly under a building that must stay supported throughout.

This article explains why underpinning becomes necessary, the principle behind it, the main methods engineers use, and the precautions that make it safe. The emphasis is on the reasoning, since the right method always depends on the building, the soil and the reason the work is needed in the first place.

What underpinning is and why it is needed

Underpinning is the strengthening or deepening of an existing foundation so that it transfers load to a deeper, firmer or more extensive bearing surface. The original foundation may have been adequate when built, but circumstances change. The need typically arises for one of a few reasons.

Settlement or subsidence — the ground beneath the foundation has compressed or moved, causing the building to drop or distort.
Increased loading — extra storeys, heavier use or new structure raise demands beyond the original foundation’s capacity, a common driver in addition and alteration projects.
Adjacent excavation — a new, deeper basement next door means the existing foundation has to be taken down to match, so it is not undermined.
Deteriorated ground or foundations — changes in groundwater, soil washout or decay of an old foundation reduce the original support.

In each case the aim is the same: to re-establish a sound, reliable path for the building’s weight into competent ground. Because the foundation supports everything above it, underpinning is rarely a quick fix and almost always needs proper engineering design.

The principle: transferring load without losing support

The challenge at the heart of underpinning is that you must remove or modify the very thing holding the building up, while never actually letting it lose support. The solution is to work in small, carefully sequenced sections rather than all at once. At any moment only a limited length of the existing foundation is exposed or unsupported, with the surrounding sections continuing to carry the load. Once a new, deeper element is built and has gained strength, it takes over that portion of the load, and the next section can be tackled.

This staged, hit-and-miss sequence is fundamental. Excavating a long, continuous trench under a wall in one go would remove its support and risk collapse. By contrast, opening short bays in a planned order keeps the structure stable at every step. The same logic of maintaining a continuous, deliberate load path runs through all foundation engineering, and understanding it is part of what a structural engineer does on these projects.

Common methods of underpinning

There is no single technique. The method is selected to suit the ground, the depth required, the loads and the constraints of the site. The main families are described below.

Mass concrete (traditional) underpinning

The oldest and simplest approach. Short sections beneath the existing foundation are excavated in sequence down to firmer soil, then filled with mass concrete to build the foundation downward in a series of legs. It suits relatively shallow extensions of a foundation and modest loads, and it needs little specialist plant, but it becomes slow and impractical at greater depths.

Beam-and-base underpinning

Here a reinforced-concrete beam is constructed to span over a series of new bases or piers, picking up the wall load and redistributing it to widely spaced supports. It is useful where the soil immediately below is poor but firmer ground or suitable bearing can be reached at intervals, and it reduces the amount of excavation directly under the existing wall.

Piled underpinning

When firm ground is deep, piles are installed alongside or beneath the existing foundation and connected to it, usually through a new beam or cap, so that the building’s load is carried down to a deeper, stronger stratum. Mini-piles or small-diameter piles installed with compact rigs are common in tight, low-headroom situations such as inside an existing building. This method is often the right answer on soft sites, including Singapore’s deep marine clay, where competent bearing may lie well below the surface.

How a typical underpinning project proceeds

Sound underpinning begins long before any digging. The first step is investigation: establishing the existing foundation’s depth and condition, the soil profile through boreholes, and the loads involved. From this the engineer determines why movement is occurring, how deep the new support must go and which method fits. Monitoring is usually set up to track the building’s movement before and during the works.

The construction itself follows the staged sequence already described. Sections are opened in a planned order, new elements are formed and allowed to gain strength, and the load is transferred in increments. Throughout, the building is watched for any sign of unexpected movement, and the sequence is adjusted if conditions differ from those assumed. Because the works affect a structure that is already standing and often shared with neighbours, the design and supervision should sit with a qualified engineer, and in Singapore such works fall under BCA’s regulatory framework with the design endorsed by a Professional Engineer. The reasoning and recommendations are normally documented in a structural engineer’s report.

Risks, precautions and limitations

Underpinning is inherently a higher-risk activity than building a new foundation, precisely because the structure is already loaded. The principal risks are loss of support during excavation, inducing fresh movement or cracking in the building, and disturbing neighbouring structures or services. These are managed through careful sequencing, conservative bay lengths, temporary support where needed, and continuous monitoring.

It is also worth being realistic about what underpinning can and cannot do. It addresses the foundation, but if the real cause of distress lies elsewhere — a burst drain washing out soil, a tree drawing moisture from clay, or ongoing ground movement from an external source — the underlying problem must be tackled as well, or the trouble will return. This is why diagnosis comes first. Where underpinning forms part of addition and alteration works or a basement extension, it should be planned together with the wider scheme rather than treated as an afterthought. If you are unsure whether your situation calls for it, it is worth checking whether you need a structural engineer early.

Frequently asked questions

How do I know if my building needs underpinning?

Persistent and widening cracks, sloping floors, doors and windows that bind, and visible tilting are common warning signs, especially if they follow nearby excavation or appear after a change in loading. Only a proper investigation can confirm the cause and whether underpinning is the right remedy, since not all such symptoms stem from foundation movement.

Why is underpinning done in small sections rather than all at once?

Because the foundation is holding the building up. Excavating a long, continuous length beneath it in one go would remove its support and risk collapse. Working in short, carefully ordered bays keeps the surrounding sections carrying the load at every stage, so the structure stays stable throughout the work.

Which underpinning method is best?

There is no single best method. Mass concrete suits shallow, lightly loaded cases; beam-and-base spreads load to spaced supports; and piled underpinning reaches deep firm strata. The right choice depends on the ground, the depth to competent soil, the loads and site access, which is why the method is selected by an engineer for each project.

Is underpinning common in Singapore?

Yes, particularly piled methods. Because soft marine clay underlies many sites and competent ground can be deep, transferring load downward through piles is often the appropriate solution. Underpinning is also frequently needed where new, deeper basements are built next to existing buildings.

Will underpinning permanently fix settlement?

It can, provided the true cause is addressed. Underpinning re-establishes a sound load path to firmer ground, but if the underlying trigger — such as a leaking drain or ongoing external ground movement — is left untreated, problems may recur. That is why accurate diagnosis precedes any underpinning design.

Considering underpinning, a basement, or worried about an existing foundation? Talk to our consultancy or get in touch.