Structure and support · Foundations & Ground
Underpinning and Foundation Deepening System
A reference account of the only foundation arrangement formed under load, explaining what the pinning joint does, what governs the work, and why nothing about how it is carried out can be generalised.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What underpinning is
Every other foundation entry describes an element cast before anything stands on it, onto ground nobody is loading. This one does not. Here the structure is already bearing, the load path is live throughout, and a new foundation is formed beneath and beside the old one, so the path has to be interrupted and remade rather than established.
The pinning joint is the point of the whole exercise. A new mass or beam built beneath an existing footing does nothing at all until load is handed to it, and that handover happens through a packed joint - a dry-pack, a grout or a wedge - between the top of the new element and the underside of the old. Leave that joint unmade and the new element is present but idle, and the old footing arrives on it later by settling, which is the outcome the work exists to avoid.
What governs the work is not a pattern but a determination. How much bearing may be absent at any moment, what holds the structure while it is, and in what arrangement the work proceeds are decided together, for this structure, by a qualified temporary works and structural engineer. No sequence, order or open length can be read off another building, and none is stated anywhere on this page.
The ground at the deeper level is discovered rather than designed to. The formation the new element sits on has not been seen before the work exposes it, and what is found there can change the depth, the element and the support arrangement together. That is why an underpinning scheme is commonly written to be revisited once the ground is open.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- underpinning
- mass concrete underpinning
- foundation deepening
- traditional underpinning
- foundation underpinning
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Take an existing foundation down to a different bearing level without removing the structure standing on it.
- Keep the structure supported while the bearing beneath part of it is interrupted.
- Transfer load from the existing foundation onto the new element through a joint made for that purpose.
- Resolve what happens at each end, where deepened foundation meets foundation that has not been deepened.
- Reinstate the damp-proof, gas and drainage arrangements that the work cuts through.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportThe existing foundation and the structure on it
The starting condition, and the one thing here that is loaded from the outset. What it is, what it bears on and what it is carrying are established by investigation rather than assumed, because the whole scheme is built around not losing it.
- Primary supportTemporary support to the structure
Whatever holds the building while its bearing is interrupted. Its design is a discipline of its own, is specific to this structure rather than to this kind of work, and is inseparable from how much bearing is absent at any moment.
- Primary supportThe new element beneath and beside
The mass, beam or deepened footing formed at the lower level. It is cast against ground and against existing construction rather than into open formwork, and it is doing nothing structurally until the joint above it is made.
- Jointing and sealingThe pinning joint
The packed joint through which load passes from the old foundation to the new one: dry-pack, grout or wedges between the top of the new element and the underside of the existing. It is the smallest part of the work and the reason the work exists.
- SubstrateThe newly exposed formation
The ground at the deeper level, seen for the first time when the excavation reaches it. Whether it is what the scheme assumed is a question answered on site, and the answer can change everything above it.
- Edge and terminationThe junction with the un-deepened length
Where the new founding level meets the old one at each end of the work. It is a step in the foundation line that was never designed, and the wall above it is spanning a transition between two different bearing conditions.
- Control layerInterrupted damp-proof and gas barrier lines
The horizontal barriers in and beside the existing construction are cut through by the excavation and by the new element. Making them continuous again is part of the work rather than a repair afterwards.
- Service zoneExisting buried services and drainage
Drains, ducts and supplies commonly run at exactly the depth and position this work occupies, beside older walls. They are found rather than located, and what happens to them changes both the excavation and the new element.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The pinning joint is what makes the rest of the work mean anything. A new element formed perfectly at the right level, on sound ground, with the right reinforcement, carries nothing until the joint above it is packed. Until then the structure is still bearing where it always was, and the new foundation is a block of material waiting for the settlement that will introduce it.
Temporary support and how much bearing is interrupted are one question, not two. What holds the structure depends entirely on how much of its bearing is absent at that moment, so changing either changes the other. Both are determined for the particular structure by a qualified engineer, and neither is a general rule that can be transferred from one building to another.
Deepening one length changes the length beside it. The un-deepened neighbour now founds at a different level, so a step exists in the foundation line where none was designed, the wall above spans a change in bearing condition, and ground beside the new element has been unloaded and reloaded in a way it had not experienced before.
What is found at formation governs everything above it. If the deeper ground is not what the scheme assumed, the depth changes, the element changes and the support arrangement changes with them, which is why the design is written to be revisited on site and why a fixed scheme agreed in advance is a warning rather than a reassurance.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
New elements are commonly formed in these families, whether as mass concrete, a reinforced beam or brickwork in a traditional arrangement. What form the element takes is a structural determination for the particular structure and ground.
Jointing and sealing
Stiff mortars and grouts are commonly encountered at the pinning joint. What is packed there, and how, is specified by the engineer for the particular joint rather than chosen on site from what is available.
Primary support
Needles, props and shoring to the existing structure are commonly met in these families. Temporary works of this kind are designed by a temporary works engineer and are outside anything a general reference can describe.
Substrate
The existing footing and the wall above it are commonly built of these families, and their condition is part of what the new work is being joined to. Their assessment is an investigation question rather than a visual one.
Control layer
Barrier layers of these kinds are commonly encountered where the work cuts an existing horizontal line and has to leave it continuous. Which barriers are present, and what they were doing, is established before they are interrupted.
Protection
Applied protection of these kinds is commonly met on steelwork left permanently in the ground or built into the new work. What the ground is like chemically is a ground-investigation question rather than an assumption.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
The continuous footing being deepened
A strip footing is the element most commonly worked beneath, and its own entry describes it as cast before anything stands on it. Everything that makes it straightforward when new - open formation, no load, continuous pour - is absent here, and the differences are the reason this is a separate arrangement.
Read about Strip Foundation →The wall standing on the work
The wall above is the structure whose bearing is being interrupted, and it is also the element that has to span any length temporarily unsupported. Its condition, its openings and any existing cracking are part of what determines how the work beneath it can proceed.
Read about Load-Bearing Masonry →A floor bearing on the wall being worked
Where the floor inside bears on the same wall, the work beneath affects the support of both. Access from inside commonly means lifting the floor, and the void beneath it is where the ventilation and damp arrangements of the existing building live.
Read about Suspended Timber Ground Floor →Deepening carried out to create a basement
Where the reason for going deeper is to gain usable space, the finished condition is a below-ground box propped by its own floors, and the deepened foundation becomes part of it. The temporary condition and the permanent condition are two different structures on one site.
Read about Basement Structure →The interrupted barrier line at and below ground floor
The horizontal and vertical damp-proof layers of the existing building are read as one barrier, and this work cuts through it. Where the line is remade, and whether the new element is inside or outside it, is decided as part of the scheme rather than afterwards.
Read about Damp-Proof Continuity →Drains beside and beneath the work
Old drainage commonly runs at the depth and position this work occupies. Whether a run is diverted, bridged or left in place changes both the excavation and the new element, and a drain damaged during the work is a defect that appears months later.
Read about Buried Drainage Network →The adjoining structure and the boundary
Deepening beside another building changes the ground that its foundation is bearing on, whether or not anyone intended to touch it. Notice, consent, condition surveys and monitoring requirements in this situation vary by jurisdiction and are legal obligations as much as technical ones.
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Documentation
- What the existing foundation was, what was found at formation, where each new element sits and how the joint was made are recoverable afterwards only from records. A later owner, a later extension and a later insurer all depend on that record existing.
- Movement
- Work of this kind is carried out because movement has occurred, or in order to permit a change that would cause it. What movement is anticipated during and after the work, and what is being monitored, belongs in the discussion from the start.
- Moisture
- Excavation beside and beneath an old wall exposes construction that has been damp and stable for a long time, and it interrupts the barriers that were managing that. Changing how water moves around a foundation is a consequence of the work whether or not it was intended.
- Buildability
- The work happens in confined excavations, beneath a loaded structure, commonly with restricted access and alongside services nobody mapped. These are conditions that make the arrangement one for qualified contractors and engineers rather than a scaled-up version of ordinary groundwork.
- Interfaces
- Almost everything this work touches belongs to someone else's system: the wall above, the floor inside, the drains beside, the barrier line through it, and commonly the neighbour's foundation. Coordination is not a phase of the job; it is most of the job.
- Durability
- The new element is joined to construction that may be old, variable and previously unexamined. How the two are expected to behave together over time is a question for the engineer, and it depends on the condition of what was there as much as on what is added.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What has been established about the existing foundation and the ground beneath it, and by whom?
- What is the reason for going deeper - observed movement, a change of use, a new excavation nearby - and is that reason resolved by this work?
- Who is designing the temporary support, and is that the same party designing the permanent element?
- How is load intended to be handed from the old foundation to the new one, and who inspects that joint?
- What happens at each end, where the deepened length meets the length that is not being deepened?
- What is known about drains, services and the neighbour's foundation in the zone of the work?
- What monitoring is proposed during the work, and what would cause it to stop?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- This is not a foundation in the ordinary sense. Every other foundation is formed before anything stands on it; here the structure is already bearing, so the load path has to be interrupted and remade rather than created.
- The new element is not what supports the building. It supports nothing until the joint above it is packed, and a scheme where that joint was neglected leaves a structure that settles onto its new foundation later instead of being handed to it.
- It is not ground improvement. That approach treats the soil and leaves the existing foundation where it is; this one leaves the soil and moves the foundation.
- Underpinning does not by itself resolve why movement happened. Trees, drains, changed water regimes and made ground are causes that continue after the work unless they are dealt with separately.
- No part of how this work is carried out can be generalised. The arrangement, the support and how much bearing may be interrupted at once are determinations made for one structure by a qualified engineer, and nothing on this page states or implies them.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What investigation has been carried out into the existing foundation and the ground beneath it?
- What is the diagnosis of the movement, and what evidence supports it?
- Who is the temporary works designer, and how does their design relate to the permanent works design?
- How is the load transfer at the pinning joint specified, and how is it verified before the excavation is backfilled?
- What has been assumed about the formation, and what happens to the scheme if the ground found is different?
- What are the obligations towards the adjoining owner here, and who is discharging them?
- What record of the completed work will be provided, and in what form?
What this page does not do
- This entry describes what the arrangement is and what governs it. It states no sequence, bay arrangement, open length, depth or support scheme, and none can be inferred from this page.
- Work beneath a loaded structure is temporary works design and is carried out by qualified engineers and contractors; it is not an owner activity and it is not a scaled-up version of ordinary groundwork.
- Deepening beside an adjoining building carries legal obligations that vary by jurisdiction, and verify them against local requirements before any work is planned.
- The presence of this page is not a determination that deepening is the answer to any particular movement; diagnosis comes before remedy and belongs with a structural engineer.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Substructure · Substructure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Foundation and Ground Interface Systems
Assemblies where a building first meets undisturbed ground — strip, pad, raft and piled foundations, pile caps, ground improvement and ground gas protection.
Browse all foundations & ground entries →