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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorEdF
    • CommentTimeMay 18th 2007
    We had a friendly vist from our building inspector today who was extremely helpful in suggesting options for our drainage. Next week a digger arrives to make a soakaway for the rainwater from the roof. I was going to do the standard thing around here and dig a big hole and fill it with gravel, but the inspector suggested making a herringbone soakaway. I've made one for field drainage, but not the other way around! I have no idea how long to make it but I suppose I use holey wavey coil pipe and put gravel around it.. Our land only just passed the percolation test, it has a hard layer of clay and stones in it so it's a case of 'suck it and see'. Any advice, anyone? The land slopes downhill from the house, which is handy, and I have unlimited space to use.
    • CommentAuthorchuckey
    • CommentTimeMay 18th 2007
    Herring bone soakaways are dificult to construct, because it is importent to lay the pipes all dead flat, if one bit has a dip in it, the remainder of the pipe wont get used until the water has pooled up in the dip sufficiencly to flow onwards. Like wise if one of the first "bones" has a downwards slope on it, all the water will run down that pipe, until the level of the water rises enough to flow down the spine again.
    I built a stepped soakaway because the land drops about 12' over 60'. Seems to work.
    In your case of a gravel filled hole, I would line the bottom and up the sides by about 12" with a material that will allow the water to trickle through but will stop earth from washing into the voids in the gravel. Then for the rest of the hole , including the top line with plastic. Again this is to stop the soil filling up your voids in the gravel, but the water will only soak out of the bottom.
    What you really want is a reserve volume to take a down pour, with a lot of percolation area at the bottom. So BIG stones are better, because the voids make up a greater part of the available volume, just top of with gravel, to stop the top subsiding under the plastic.
    If a tank is buried and then filled with water, the greater pressure of the water at the bottom, will push the water out into the soil also because the bottom has the "open" end into the soil, the bottom is vastely more effective then the top.
    Hope I have given you some thing to think about.
    • CommentAuthorEdF
    • CommentTimeMay 19th 2007
    Thanks for that, I take your point about the herringbone, with the lower part filling up first, then the next lowest, but is that a problem? All of our land slopes, so there would be problems laying it flat.. Not sure of your thinking behind a stepped soakaway.The original herringbone soakaway on the main septic tank here clogged up fairly quickly, so I'm not in favour of having yet another do the same thing. Also had a land drain clog with tree roots. My thoughts are that waven coil with gravel around it is designed to extract water from the soil for field drains - someone once told me the pipe design causes a very slight suction effect. If that is true, I can't see it being very effective dispersing water into the soil... . I have been looking at the Osma 'Aquacell' and other makes of the same thing, a plastic space-frame box in efect which should be covered with permeable membrane which acts as a tank. The membrane has to be sealed so as not to allow soil into it. Costs about £100 per cubic metre, which seems a lot, and one supplier wanted to sell me a 100ft roll of membrane to go with it for £95.... I can't understand why it isn't possible to put rainwater into a water course in a very rural area, it will end up there anyway eventually..
    • CommentAuthorchuckey
    • CommentTimeMay 23rd 2007
    If all your water (or too much) ends in one "bone" on a sloping site, there will be a wet patch on the surface downhill from it. I have seen soakaways like this, muddy patch on sloping field 20' downhill from a septic tank.
    A very cost effective membranes are used polypropolene(?) big builders bags, chat up your builders for them.
    • CommentAuthorRachel
    • CommentTimeMay 24th 2007
    Have you considered rainwater harvesting? Seems a shame to let all that water soakaway. You could flush your loo, do you clothes washing etc...
    • CommentAuthorfuncrusher
    • CommentTimeMay 28th 2007
    Why not create a pond or wetland feature? If not, then you need trapped gully drains or grilles of some kind to filter out the usual debris before they reach the soakaway. There are formulae for soakaways, according to expected water flow and ground percolation. Ultimately, all percolation systems fail due to suspended solids in the water, but with roof water these should be minimal.
    • CommentTimeJun 1st 2007
    Check out:
    and follow the links at the bottom of the page to find details of herringbone and other infiltration drainage schemes.
    • CommentAuthorEdF
    • CommentTimeJun 9th 2007
    Under the advice of the digger driver, he dug a big hole and filled it with 40mm stones. He dug down below the clay level into a sandy layer. Took about 5 tons of stones, and it was a big hole, roughly 2m x 2m x 2m, covered it in polythene about 30cm down, then top soil.
    Today I was laying 110mm waste pipe in a complex bathroom installation (bath, shower, basin and loo) and was perplexed how to cut the pipe lengths neatly, as no matter how carefully I measured and (hack) sawed the pipe, the blade was very difficult to keep straight. After some cogitation I came up with an idea. I removed the seals (and the small 'tongues' in the middle, inside) from a 110mm straight connector and slipped it over the pipe. This gave me a solid guide for the saw. As there was still a tendency for the saw to make a slightly spiral cut, I carefully sawed about halfway through, going all the way around the pipe, then kept going to finish the cut, again going all the way around slowly, rather than trying to cut downwards from one point in one go. Can't do this with a connector which has a solid stop in the middle. Perfect ends! Don't applaud, just throw money..
    • CommentAuthorwastetech
    • CommentTimeAug 11th 2009
    Your contractor has installed a type of soakaway which contravenes Section H2 of the Building Regulations on soakaways.
    If you look at http://www.wte-ltd.co.uk/soakaways.html you will see that Chuckey and your Building Inspector were correct with their herringbone suggestion.
    You can always get your contractor back and make him do it properly this time.
    • CommentAuthorjon
    • CommentTimeAug 11th 2009
    Posted By: wastetechYour contractor has installed a type of soakaway which contravenes Section H2 of the Building Regulations on soakaways.

    Why would it contravene the wastewater systems and cesspool regulations?
    • CommentAuthormarktime
    • CommentTimeAug 11th 2009
    @wastetech. Your link is to effluent from a septic tank whereas the OP is draining rainwater.

    If the digger has hit sand then the percolation problem has probably improved by a considerable amount. The hole itself can contain approx. 8000 l of water so there's a pretty good buffer if the percolation is average.
    • CommentAuthorCWatters
    • CommentTimeAug 11th 2009
    The BRE publish a guide to sizing and constructing soakaways... "Soakaway Design Digest 365"

    I thought you had to pay for their publications but there seems to be a copy online here..

    • CommentAuthorBrianR
    • CommentTimeAug 11th 2009
    To build a soakaway for rain water you can get one of the plastic cages from a drainage company. Wrap the cage in a semi-permeable membrane and then install this in a hole in the ground. I fitted one for my roof soakaway which was about 1 meter X 0.75 X 0.75.

    The soakaways built with rocks and tarpaulins can fill up with mud after a few years. Not sure how much better the semi-permeable membrane types are. Guess I will find out.
    • CommentAuthorCWatters
    • CommentTimeAug 15th 2009
    If you put a manhole on the top you can dig the mud out.
    • CommentAuthorTerry
    • CommentTimeAug 16th 2009
    nothing stopping you wrapping the rocks in a suitable geotextile filter to keep the mud out. Would have thought the cage was unnecesary ???
    • CommentAuthorCWatters
    • CommentTimeAug 18th 2009
    We have a rainwater tank supplied by downpipes from the roof. It has a fancy stainless steel filter that removes all leaves etc. Despite that it's amazing how much gunk still ends up in the tank. After two years there is two or three inches of extreemly fine silt in the bottom. When it reaches the level pf the pump I guess I will have to get in cleaned out.

    If it were a soakaway full of rocks I'm not sure what I would do.
    • CommentAuthormarktime
    • CommentTimeAug 18th 2009
    Well I would have thought that with a 2 x 2 x 2 m hole it would have been sensible to drop in a 5000 l tank, use that as a sediment trap and emergency supply of water for the garden, and let it gently overflow into the drain sequence.
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