Mass concrete underpinning, sometimes called traditional underpinning, extends an existing foundation down to firmer ground by excavating beneath the footing in short sections called bays, filling each with concrete, and dry-packing the gap between old and new. Bays are dug in a planned hit-and-miss sequence so the wall is never left unsupported along its length. It suits houses where suitable ground is at a reasonable depth, and the engineer sets the bay size, depth and sequence.
Why is mass concrete still the most familiar method?
It is straightforward, uses simple materials and relies on hand or small-machine excavation, so it suits many domestic jobs where firm ground is not far below the existing footing. Many older Merseyside homes, from Victorian terraces to inter-war semis, were built on shallow brick footings. Where the engineer decides those footings need to bear on deeper ground, extending them down in concrete is often the most direct answer within our foundation underpinning in Liverpool.
The key is control. Each bay is short, dug only when the bays either side are sound, inspected before concrete goes in, and packed tight to the old footing once the concrete has gained strength. Done in that order, the wall keeps its support throughout.
How does the bay sequence work?
Imagine the length of wall divided into numbered sections. Rather than digging 1, 2, 3 in turn, the engineer might specify digging every third or fourth bay first, then returning for the others once the first set has cured and been packed. The exact pattern depends on the wall, the loads and the ground.
- Set out the bays. Positions and sequence marked out from the engineer's drawing.
- Excavate the first bays. Each dug alongside the footing, then underneath it, to the depth the engineer has specified, with support to the excavation where needed.
- Inspect the formation. The base of the dig is checked, typically by Building Control and, where required, the engineer, before concrete is placed.
- Pour concrete. Concrete is placed up to a short distance below the old footing, leaving a gap for packing.
- Dry-pack. After the concrete has gained strength, a stiff mortar is rammed into the gap to transfer load from the old footing to the new concrete.
- Repeat. Next bays opened in sequence until the wall is complete, then backfill, reinstatement and crack repairs to the engineer's specification.
When is mass concrete the right choice, and when is it not?
| Usually suits | Usually points elsewhere |
|---|---|
| Firm bearing ground at a moderate depth | Very deep firm ground: mini-piled underpinning may suit better |
| Reasonable access along the wall | Very tight access or heavily obstructed ground |
| Single bays, corners or short wall lengths | Long walls where a beam and base design spreads load more economically |
| Ground the engineer judges stable at that depth | Shrinkable clay with large trees nearby, or deep made ground |
Bay windows are a classic case. Many Victorian, Edwardian and inter-war bays sit on shallower footings than the main walls, and they are sometimes partly underpinned in mass concrete. Our bay window subsidence repair page covers that in more detail, including the hard-spot issue at the junction with the main wall.
Underpinning is rarely the first answer. The RICS consumer guide says fewer than 5% of subsidence-affected properties need it. If an insurance claim is involved, the insurer decides whether underpinning is used. Every scheme we build is designed by a chartered structural engineer, and digging near or under foundations must never be attempted as DIY.
What permissions are involved?
The Planning Portal states that approval under the building regulations will normally be required if you propose to underpin an existing foundation. Building Control, either the council for your area or a registered building control approver, inspects key stages such as the base of each bay before concrete. Excavating near a neighbour's foundations can bring in the Party Wall etc. Act 1996, with a section 6 notice where a dig within 3 m goes deeper than their foundations, or within 6 m where it cuts a 45° line down from them. In terraces and semis, the party wall side of underpinning usually needs to be considered; a party wall surveyor handles it.
Which local homes does this suit?
Mass concrete is common on terraced and semi-detached homes where shared walls and modest depths make a controlled, hand-dug approach sensible. We discuss it often for bay fronts and rear corners in Aigburth and Childwall, and for older terraces in Widnes. The engineer still decides case by case.
What drives the cost?
Depth to firm ground, the length of wall, the number of bays, access for excavation and muck-away, whether internal floors are lifted, drains or services that need diverting, Building Control and party wall involvement, and making good. Our underpinning cost guide gives published national ranges and explains each factor.
Talk through traditional underpinning
Call 07777 659786 or use the form. Tell us what has been recommended and by whom, and we will call back to explain how we would carry out engineer-designed underpinning across Merseyside on your property.
Frequently asked questions
How deep does mass concrete underpinning go?
As deep as the engineer needs to reach a suitable bearing layer, or to get below the depth where trees or seasonal moisture changes affect clay. That depth is decided from trial pits and soil information, not set in advance. Where firm ground turns out to be very deep, the engineer may switch to a piled or beam and base design instead.
Can we stay in the house during underpinning?
Often yes, particularly when work is from outside only. Where floors must be lifted or rooms used for access, part of the house may be out of use for a period. If a home becomes uninhabitable during an insurance repair, the ABI says buildings insurance usually covers alternative accommodation; check your own policy and talk to your insurer.
What is a hit-and-miss sequence?
It is the order in which underpinning bays are dug. Rather than excavating along the whole wall, short bays are dug in a staggered pattern, so each one has supported masonry either side. A bay is concreted and dry-packed before neighbouring bays are opened. The engineer sets the bay length and the sequence for each wall.
Is partial underpinning a good idea?
Underpinning only the affected part, such as a bay or one corner, can be the right answer, but it creates a stiffer section next to an unaltered one. That hard spot can lead to differential movement and cracking at the junction. The engineer must design for it, sometimes with a movement joint or stitching across the junction.
How long does mass concrete underpinning take?
It depends on the length of wall, depth, ground and access. MyJobQuote says most underpinning jobs take about 3 to 5 weeks on site. Investigation, Building Control and party wall matters can add several months before work starts, and we agree a programme once the design is settled.