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Posted: Thu Nov 11, 2004 8:23 pm
by ceefox
First time post so please bear with me! Background:I am a landscaper in Essex and currently we are carrying out a complete garden project on a new build house approximate garden size 1000 square yards. The house is the middle of 3 new builds and the surrounding water table is very high, within 300mm of top surface. We have just completed a 200sq yd patio at the rear of the house which did not require any dig out we actually raised the surface by nearly 200mm to gain correct level i.e 2 courses below dpc. And the next part of the project is to excavate hole in readiness for swimming pool herein lies the problem...as soon as we dig down past 300mm the hole fills with water! I know the house next door has just pumped out and filled in a very large natural pond which was at a lower level than this garden, although I have also checked with other neighbouring houses and they have a history of lying water in their gardens
Query: What would be the best way to go about building a sump drain with possible float switch and connected into mains drain further up the garden. Given lowest point of proposed swimming pool being 1.8metres, how deep should I dig. I am aware of potential consequence of draining the surrounding area but I do need a solution to this problem urgently??? Any suggestions please????
Posted: Fri Nov 12, 2004 6:26 pm
by Tony McC
It's difficult to explain just what would be the best way to construct a wet well pump, which is what you need for this project, as each 'solution' is unique to the site, but, very basically, the key points are:
1 - create a temporary sump point with a hole at least 500mm deeper than the lowest point of excavtion for the actual well. Install a pump and position this so that it can outfall somewhere that it won't be able to find its way back.
2 - excavate for the well and get the base in using ST4 concrete. We've found that the most effective way to work is to use manhole bracing and take the excavation down to 200mm below invert level of the well, link to the sump point, if necessary, then use a 50mm blinding of ST1 semi-dry before placing the ST4 at around 25-30mm slump, and getting the first section of the well chamber bedded into the concrete while it's still plastic.
3 - give that first section 24 hours for the concrete to undergo initial set, and then you can start building up the chamber as required by the design. It's standard practice to complete the chamber along with any backfill, and to get the rising main pipeline in place and tested before installing and commissioning the pump.
The biggest difficulty you face is calculating the potential flow through the system. Very often, installation such as this, that are primarily intended to alleviate waterlogging in the property owner's garden, end up draining all the neighbouring gardens at no cost to those lucky buggers!
You need to find some way of calculating water volumes so that the right pump can be fitted, and the correct diameter of rising main. It might be worth having a read of the BRE Digest on soakaway design to get some idea of how potential demand levels can be quantified, but, from experience, I'd suggest you work with a safety factor of at least 100%.
In your position as a contractor, it might be a better idea to hand over the actual design to a drainage engineer and let them do all the branewerks and assume all the responsibility for the success or otherwise of the scheme. Constructing a small wet well for a fixed area of less than, say, 100m² is all well and good, but for this type of project, I'd be calling up someone else to provide me with a answers and a plan of what I am to build.