Why a house gets underpinned
There are two quite different reasons to underpin. The first is remedial: the existing foundations have moved, usually through clay shrinkage, tree influence, a leaking drain or historic undersizing, and the building needs a deeper, wider bearing. The second is constructional: the owner wants a basement, and the existing footings sit above the proposed new floor level. Remedial underpinning follows a structural engineer’s assessment and often an insurance claim; constructional underpinning follows a basement design. The technique is similar, but the surrounding process is not, and conflating the two is a common source of confusion at tender stage.
Traditional mass concrete, in bays
The standard residential method remains traditional mass concrete underpinning. The wall is divided into bays, typically 900mm to 1200mm wide, and worked in a hit-and-miss sequence so that no more than about a quarter of the wall length is unsupported at any moment. A bay is excavated to the design depth, the sides trimmed, reinforcement or starter bars fixed where specified, and concrete cast against the retained ground. The engineer sets the bay width, the sequence and the number of bays open at once. That sequence is not a suggestion.
Dry-packing is the step that matters
Concrete shrinks as it cures, so a cast pin does not, on its own, touch the underside of the existing footing. The gap is closed with a dry-pack mortar — a very low-slump sand and cement mix rammed hard into the void, usually 24 to 72 hours after the pour depending on the specification. This is what transfers load from the old foundation into the new one. Skimped dry-packing produces settlement that appears weeks later as cracking above, and no amount of decoration will hide it. It is worth asking any contractor how they dry-pack before asking anything else about their basement experience.
Party walls and neighbours
Underpinning a party wall, or excavating within three or six metres of a neighbouring structure depending on depth, engages the Party Wall etc. Act 1996. Notices are served, surveyors are appointed and an award is agreed before work starts. The award typically fixes working hours, the condition schedule for adjoining properties, the monitoring regime and the security for expenses. None of this is contractor work, but all of it sets the programme, and a start date agreed before an award is in place is a fiction.
Monitoring and trigger levels
On sensitive sites, movement monitoring runs alongside the works, with targets or prisms installed on the retained building and the adjoining properties and readings taken at an agreed frequency. Trigger levels — amber and red — are set by the engineer in advance, together with the action required at each. Monitoring is only useful if somebody is obliged to stop when a trigger is reached, which is why the reporting route belongs in the construction phase plan and not in a filing cabinet.
What goes wrong
Almost every underpinning failure we have been asked to look at comes down to one of four things: a bay opened out of sequence because it was convenient; a pin left open overnight; dry-packing that was rushed or omitted; or water. Groundwater and leaking drains soften the bearing stratum and undermine adjacent bays, so drainage is investigated before excavation begins, not after the first bay fills up.