Trench fill is the foundation type most of the UK's housing is built on. Instead of pouring a strip of concrete at the bottom of a trench and building masonry up from it, the excavated trench is filled with concrete almost to the top — typically to within a couple of brick courses of ground level — and the wall starts from there.
Why trench fill instead of strip
The logic is mostly about labour and ground conditions.
Less time in the trench. A traditional strip foundation needs bricklayers working below ground to bring footings up. Trench fill replaces that below-ground masonry with ready-mixed concrete, which means the trench is open for hours rather than days — less time for it to flood, soften or collapse, and far less time with people working inside it.
Deep foundations without deep masonry. On shrinkable clay, and anywhere trees are close enough to matter, foundations have to go deep enough to reach ground that seasonal moisture change and roots will not move. The NHBC's standards on building near trees drive foundation depths that would be slow and expensive in masonry but are straightforward to pour. That is why trench fill dominates on clay sites — much of the South East included.
The trade-offs are real: more concrete, a cost that climbs quickly with depth, and much less tolerance for a wandering excavator — the trench is the formwork, so the sides have to be cut clean and to line.
What happens before the pour
This is where the paperwork earns its keep, because a foundation is the definition of work that gets covered up.
- Services are proven before digging. Cable strikes are a routine cause of serious harm in groundworks; HSE's HSG47, Avoiding danger from underground services, is the working reference, and on a managed site the digging happens under a permit to dig with plans checked and services located first.
- The excavation itself is controlled work. CDM 2015 Part 4 sets specific duties for excavations — supported or battered where there is a risk of collapse, inspected before people work in them, kept clear of loading at the edges.
- The trench is inspected before concrete arrives. Building control (and the warranty provider on housing) expect to see the excavation — depth, bearing strata, freedom from water, roots and soft spots — before it disappears under concrete. Pouring before inspection is the classic expensive mistake: nobody can approve what they cannot see.
- The pour is recorded. Concrete specification, depths, and who checked what — the inspection record is the only witness the foundation will ever have.
From the approving side of the desk, groundworks packages are judged on exactly this: not whether the RAMS mentions "excavation" in general terms, but whether the sequence — locate services, dig, support, inspect, pour, record — is written for this site, with its trees, its clay and its neighbours.