Using a geotextile membrane under concrete

Back in 1999 I was called out by a client who had a stylish and rather expensive Haddestone ornamental pool - like this one - installed as a show-piece in the yard of a barn conversion.

The farm had previously been a livery stables and before that a working farm on arable land close to the village of Crondall.

The farm yard itself was a rather thick slab of re-enforced concrete that I would estimate was a minimum of six inched in depth.

The client had decided that the slab was too costly to remove so it was decided by all, including the architects, landscapers and the owner to use it as the base and all entry door levels, pathways and borders were determined from this.

It seemed like a perfect scenario for construction because this foundations was deemed perfect - except for one (or more) fatal flaw.

The landscaper had carefully prepared a full concrete raft for the pool to sit on and the Haddestone pool was constructed directly on the concrete slab; G4 pond sealant was applied to its interior.

The client first noticed a problem when the water level started to drop on a regular basis. Initially it was put down evaporation and splashing as it tumbled from the centre fountain.

However, after being away for two weeks and leaving the fountain off, the client returned to an almost empty pool. No water could be seen leaving any of the joints in the modular pond sections - we were baffled.

We agreed to dismantle the pond and rebuild it; in fact, it was ideal for the client because it was also decided to re-locate the pool into the centre of its section of yard - currently it was off-set.

We removed the fish to a holding container and drained the rest of the water and then removed the weed. The G4 had de-laminated and peeled back in two separate lines across the bottom. It was apparent that these two cracks were a likely source of the leak.

When the 20mm gravel was raked back away from the underneath the pool we detected two stress cracks in the original concrete and they were almost identical to the two lines in the raft base.

To cut a long story short, the concrete base was obviously expanding and contracting over the year and because the concrete raft had been poured into a form-work directly on top, movement was transmitted through the new (and probably still green) concrete.

Our solution was to follow the exact same principal of a raft of concrete but isolate the new base from any movement by sandwiching a sheet of plastic DPC between two sheets of geotextile membrane.

This left the old concrete base to do its own thing with the pond effectively floating on top without being push and pulled from below - in fact, I reckon if we had put four lifting eyes around the edge of the base we cold have lifted the pond up and moved it around.

Consider the same method if laying a patio on an old concrete base.