Quantity help needed (soakaway) - How much gravel/sand do i need?

[Bernie]

Afternoon all. I wonder if anyone out there can help on this one???

To sort out a bit of waterlogging at the bottom of the garden prior to laying new turf, I've dug a trench to install a very basic soakaway. the problem I have is, never having done anything like this before, I haven't got a clue how much gravel/rubble and coarse sand I need to fill the thing.
The trench is 24 feet long by 1 foot wide by 1 foot deep so I guess that's 24 cubic feet. Allowing for 3" of topsoil I want 6" of gravel/rubble and 3" of coarse sand, so that must come to 12 cubic feet of gravel and 6 of sand. Anybody know what that equates to in weight or bags???

[steve r]

Bernie, turn your tape over and measure your trench in metric then you can work out how much you need. Gravel 1.64 T/cubic metre and grit sand 1.65 T/cubic metre.
A trench of that size isn’t going to take much water to fill it up and stop it working, unless it drains to somewhere else. Might be an idea to look at this page http://www.pavingexpert.com/drain03.htm and this one http://www.pavingexpert.com/drain08.html

[Bernie]

Thanks for that Steve.

My plan is also to sink plastic pipes (the top ends protected by weed fibre/geo membrane to avoid 'em clogging up) down through the troublesome layer of clay - hopefully this will help it to drain quicker.

Having seen the stuff on this site re hydrostatic pressure and half perf'd pipe, is there any value in me rigging something up to divert the flow from the troublesome area along to the better draining area, or does the pipe solution only really work if it's all connected to a proper outlet (which is a non-starter for me)? After all, the last thing I want to do is just move the waterlogged area over a few metres.

[steve r]

Do you know how thick the layer of clay is?
To see if you can get through to a free draining layer you would need to dig a test pit, fill it up with water and see if it drains away.

[Bernie]

Thanks again Steve - that looks like my job for the weekend then!!!

The half perf pipe idea has gone west - I've kept an eye on the trench after it's been raining and there's no major difference in the drainage rate so it would be pointless.