We are thinking dig a hole 1.5m by 1.5m
With membrane you mean geo or teram right ? have you looked into the use of preformed concrete rings http://www.pavingexpert.com/drain08.html
or plastic, these really will save you loads of time.If you go along the brick build route still worth looking at a pre formed capping (biscuit) to make life easier and build your walls to suit the size,also you only need foundations to support the walls around the outside ,leave the inside base to allow water to soak downwards too.
hi again Nelliedee . are you sure you cant source the cells any cheaper £90 is allot . Osma aquacell make two different ones a green one and a blue one, the green is for areas classed as non traffic and blue being for more heavy duty, i have just priced these up with my local merchants for you and found them to be much cheaper than your price, green ones look like around £30+vat,blue £45+vat , granted i am a trade customer but by now with your projects you should be on good terms with your merchant ?given by your last post these have got to be easier for you to install.
Does the hole have to be 1.5m cubed i.e. each dimension 1.5m?
Or does it have to be 1.5 cubic metres i.e. each dimension approx 1.15m?
If the latter it could mean a saving on materials and labour
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