Due to the local authority not allowing connection to the 300mm SW pipe that runs across the site, the project had to look for another SW solution.
There is a large ditch running along the lower end of the site, which we were allowed to send the water into.
But the large deluge of water from the building and associated car park/footpath/play areas is to be controlled, a SW attenuation system was specified. This is basically a tank constructed from many 'crates' interlocked together which is then sealed by a bespoke welded heavy plastic envelope, which is sealed with 100mm wide tanking tape. The outfall of the tank then exits into a chamber which contains a hydro-brake, which regulates the outflow of the cell to the ditch.
Typical hydro-brake device, I didn't get pics of it as it was going in - too busy!
Area excavated to formation level.....100mm of non-angular chipping laid in bottom of excavation then 100mm of sand to provide a level base for the unit. This is essential, we found. It is essential not to over-dig the excavation, as you will just consume more selected fill material than is necessary.
Top chamber installed - SW connection from car park comes in from the left, roof and play area SW comes in from the bottom of picture. Proposed outfall from system discharges into ditch behind machine, behind hedge.
Close-up of top chamber before cell showing incoming SW pipework, prior to backfill etc
Excavation almost complete. Blue cells can be seen in the background - that's a whole artic-load. Ground conditions - clay - soakaway unsuitable.
Welded liner fitted and orientated correctly, get it right first time as it isn't easy to reposition!
Installation of crates begins......clipped together by some very well designed clips, worked a treat. We had this installed in 3hrs - 3 men.
All crates installed bar the last 6 - six short!! All that remains is to neatly fold the liner in over the cell and seal down all edges. Install a vent on top of the chamber to protrude above finished ground level.
Backfilling - a protective membrane sheet is placed between the liner and the non-angular 100mm stone fill around the sides of the cell - the top was then sanded 150mm and then 500mm (min) cover over the top, back to finished ground level.
Hopefully I'll have my camera handy when we go back to do the final drain test.
RW Gale Ltd - Civils & Surfacing Contractors based in Somerset
those crates are daylight robbery,i'm sure tony mentioned a while back that they would be getting cheaper,we pay about £55 +vat for the blues and £37+vat for the greens
adds a lot of cost to a soakaway
LLL
