Installing the Soakaway

A new soakaway is required to disperse rainwater from the rear elevation of the house. This is to be sited in the back garden and, as Gary from G L Smith explains, this needs to be at least 5m away from the house and lower than the ground at the back of the house.

Unlike the systems of old where a pit was dug and backfilled with hardcore and gravel, this soakaway is to be built with modular soakaway cells. He goes on to explain that the way these, attenuation cells as they are called, work is that they provide a storage area for the rainwater to be held while it is gradually dispersed into the ground. They are obviously not suitable for clay soil which doesn't allow water to drain away. But, in good draining soil, a fair quantity of water can be held within the cells while it gradually drains away into the surrounding ground.

A hole is dug part way down the garden to a depth of about 1.2m, and 200mm bigger in both directions than the final size of the modular cells. It might be quicker to dig the hole with the mini digger but that is on another job just now and the time taken to bring it over and set it up wouldn't be worth it. So the hole is dug by hand – fortunately it’s a bright dry day and it’s not too hard going. The overall size of the hole is marked out and the grass is carefully stripped away. They cut this out so that it can be put to one side and re-laid later. Next to the area a tarpaulin is laid to store some of the decent excavated topsoil without damaging the rest of the lawn. This will be used later to cover the soakaway before relaying the grass. The rest of the excavated material is barrowed straight round to the skip on the front drive.

While the pit is being dug out, a trench is also dug from the rainwater discharge point at the back of the house to the new soakaway. As you can see from the pictures, the ground surrounding the hole holds up fairly well and there was no need for any shuttering to prevent it collapsing in on itself. Once the hole is deep enough, the base is blinded with a 100mm layer of gravel and we’re pretty much ready to start the installation.

A large sheet of specialist permeable fabric is laid out over the pit. This will form a ‘liner’ which allows water to pass through it but not fine soil particles which would otherwise gradually clog up the soakaway preventing it from properly dispersing the rainwater. The fabric is then pushed down into the pit so that it lays smoothly over the blinding and is reasonably flat against the sides. There is plenty of excess which will lay over the top later. The modular ‘crates’ are interlocked before being lowered into the hole. This process is reasonably easy as the attenuation cells are very lightweight. Had this been a much bigger soakaway, it might be more practical to assemble it one section at a time in the ground.

The trench for the drain is blinded with a layer of gravel before installing the plastic pipe running from the discharge point at the back of the house to the new soakaway. Each cell has a preformed cut out position and one of these, in the side of one of the top cells, has been cut out ready. The drain run is slotted in here.

The permeable fabric now gets pulled up around the cells and laid over the top as well so that it forms a complete cover. This has to be done carefully to ensure that there are no gaps through which soil can pass. Once completed, the gaps around the edge of the pit – 100mm or so all round - are backfilled with gravel. The top of the soakaway is then covered with around 100mm of gravel before backfilling with topsoil. This gets compacted down and before relaying the turfs which were saved at the beginning. There will be some natural settlement over the next few weeks but this can easily be remedied by spreading more topsoil and allowing the grass to grow up through it.

If you live in the Hertfordshire area and are looking for a professional building contractor, you can get in touch with G L Smith and Sons via their website: http://www.glsmithandsons.co.uk/