Apartment Block Land Drainage

Groundwater was collecting against the lower elevation of this apartment block, where the ground level sits well above the basement floor and a run of ventilation grilles feeds the undercroft. Water sitting against that elevation is a problem twice over: it works at the masonry, and it finds its way to the vents.

The block is occupied, the affected elevation looks straight onto residents' lawn and planting, and there are live services running through the ground alongside the wall. So the brief was as much about how the work was done as what was installed: get a proper perimeter drain in, work around the existing ducts and cables without damaging them, and leave the grounds looking as though nothing had happened.

We excavated a trench the full length of the elevation using a mini excavator, working down past the ventilation grilles to below the level water was reaching. Existing ducts, cables and service runs crossing the trench were exposed by hand and supported as we went, and a laser level was used throughout to hold a consistent fall along the run rather than trusting the eye on a long trench.

The drain itself is perforated pipe in a geotextile filter sock, bedded and surrounded in clean pea shingle. The sock is the part that matters over time: it keeps fine soil out of the perforations, so the drain still runs freely in ten years rather than silting up in two. Where ducts and pipework penetrate the wall, we haunched around them in concrete to keep everything supported and to stop water tracking along the outside of the ducts and into the building.

With the shingle backfilled to the right level, we reinstated topsoil across the whole run and graded it back to the existing lawn. Residents got a working drain and a lawn that goes back to being a lawn.