Under the house
How a basement is built beneath a house that stays put. A new floor under an existing Victorian terrace, dug bay by bay beneath walls that stand on a few courses of brick footing. Scroll, and the ground comes out.
A slice through the house, side to side.
We cut through the walls this house shares with next door and take the front away to look inside. The house above stands throughout, though nobody lives in it while we build: a basement like this is normally one stage of a whole-house refurbishment, and the family moves out for the duration. Below, the work advances one stage at a time.
The floor comes out; banks of earth stay.
We break out the old cellar floor and the internal walls, then dig the ground down to a first level, leaving a bank of earth against every wall. The house still stands on the ground it was built on.
The walls stand on new pins.
One bay at a time, in small sections: cut into the bank, pour a concrete base, then shutter and cast the stem against the wall. Once each pin has cured, we ram the gap above it tight with dry-pack mortar. Neighbouring bays are never open together: every wall stands on its bank of earth or on new concrete.
The pins take the load, the middle comes out.
With the ring of underpinning complete and cured, we dig out the centre and take the spoil away. Steel props brace the hole as it deepens; they are omitted here for clarity.
Slab and walls become one structure.
We pour a reinforced slab across the bottom and tie it into new walls around the edge: one concrete box. From here the house is carried by what we have built, not by the ground it was built on, and the props come out.
Water has somewhere to go.
We lay a drainage channel round the edge of the box, running to a pumped sump. Studded membranes line the walls and floor, and a screed covers the floor, with flushing points to keep the channel clear. Any water that gets through the concrete runs behind the membrane and is pumped away.
A new floor, joined to an old house.
We build the stair in with the structure, not afterwards. Then the room is lined and finished, and the lights go on.
From the pavement, nothing has changed.
The cut closes and the street is as it was, apart from a lightwell at the foot of the bay window. What changed is a whole floor that was never there, under a house that stayed put.
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