The Brew Cabin

Posted: **Tue Aug 07, 2007 10:18 am**

Hi,

The house I've just moved to has downpipes that, helpfully, end approx. 2 inches below the surface. Funnily enough, this gives us some problems with the downpipes backing up when it rains.

Using the calculations on the site, Iâ€™ve worked out an approx 90m2 area needs to be drained, which comes to a 1.5m3 soakaway. Trouble is, that this would work out to a 3m deep excavation if I used the IC sections suggested. Thatâ€™s a lot to hand-dig, and would be tricky to get a machine big enough onto the site. That plus the extra messing about with shoring and suchlike? I just donâ€™t fancy my chances.

So, looking at the SUDS-type â€œcratesâ€� to create the required volume in a narrow strip next to the site boundary, but I donâ€™t like the lack of access to the chamber these give.

Whatâ€™s the life expectancy of these â€œcratesâ€�? Do they suffer from silting up? Should I buy\build a silt trap ahead of the chamber?

The other problem is that I *cannot* get fully 5m away from the existing building, going to end up at 4.5 max. Would it be sensible to wrap the â€œbuildingâ€� side of the chamber in non-permeable material to try and divert the water away from the building, or is it so close to correct that itâ€™s not worth the trouble?

(Soil conditions seem good, no waterlogging that I can find)

Cheers.

Posted: **Tue Aug 07, 2007 4:42 pm**

try here mate
regards LLL

Posted: **Tue Aug 07, 2007 10:56 pm**

Even I've learnt a thing or two on that page!

I now know what a "swale" is!

Posted: **Sat Aug 18, 2007 2:01 pm**

1) I'm hazarding a guess that you sized the soakaway based on some kind of rule of thumb approach. This may be okay for most situations but if you wanted to do it properly then you really need to get hold of a copy of BRE Digest 365:

http://www.brebookshop.com/details.jsp?id=844

2) The "5m minimum from foundations" rule was not derived using any sort of scientific method. I've spoken to the guy who was mostly responsible for that figure and he's admitted that it wasn't really based on a proper analysis, rather it was just something that he felt would give a good factor of safety under most cicrumstances based on his experience. Technically you would probably be okay with 4.5m but regulations say 5m....mmm.

3) Plastic crate soakaways should have very long life expectancies with regards to the service life of the actual materials. Since the incoming water is only from your roof then the silt/sediment load should be low (unless you live next to a brickworks factory or something). Using a silt trap before the soakaway will help to some extent but under most circumstances roof drainage does not contain a significant amount of sediment in any case.

4) As an alternative to a soakaway you could have an infiltration basin. This is basically a depressed landscaped area into which you direct the water. These can work but you have to have the right sort of site conditions, mainly: you have to be able to get the water to the basin, which means the on-site gradients have to be workable; the permeability of the ground had to be fairly high (no clay!); you have to have the space for the basin; you have to not mind that sometimes you will have a waterlogged section of your garden with standing water whilst it drains into the ground (possible safety risk if you have kids, probably low though).

If you want help calculating the required excavation volumes and materials required for either a gravel-filled soakaway or a plastic crate variant then I have a (free!) spreadsheet that can help with the calculations. See:

http://www.sudsolutions.com/freeware.htm

The Brew Cabin

Options on soakaway construction? - Well i dunno, i work in computers...