Clay - the devil's doing - Help with sleeper decking and drainage

[cammackmartin]

Hi one and all. Having lived with a damp dirty unusable back garden (6.5m x 5.3m) for 2 years since we moved in, I finally decided to do something about it so the kids can play in it (and adults drink in it!). The house is at the end on a terrace that slopes down an incline with us at the top end and the whole collection of houses is built on a very thick layer of clay that has railway cutting on 2 sides. The garden used to have house bricks as a patio and on 2 sides their is a path outside the fence about 40cm above the level of the patio. The patio was surrounded by banked soil for flower beds also. Lastly the next garden along is slightly lower than ours. So as you can see there is good reason for water to have collected in our garden with higher ground on 2 sides and the third blocked with a mound of soil.
Never wanting to do anything conventionally, thinking decking would be good but having had rats in the garden before eating the bird food and not wanting them back living under our warm deck, I decided to deck the garden with new tanalised railway sleepers (2.4x19.5x9.5) directly on to the ground and create raised beds on 3 sides.

My question is this, in addition to a run off to the rear of the garden I was thinking of putting in 3 linear land drains (40cm x 40cm x 5 metres with 80mm pipe and filled with pea shingle) across the garden towards the next lower garden. I thought if these drain into a simple French drain along that boundary of the same dimension and this with a run off down to the bottom right of the garden, then any rain water should drain away from the patio area. Last I was going to have a geophysical membrane over all this with 75mm MOT1 or DTp1 sub-base and 35mm of sharp sand as a bed on top of the membrane.

Can anyone tell me, is this going to work? Is there a better way and is this overkill? Will the water just sit in the troughs and fester? Would it be better to abandon the linear drains and have a deeper sub-base and deeper layer of sharp sand as a flat run-off to the lower ground next-door?

Any help will really be appreciated.

Thanks

[GB_Groundworks]

as a start have a look here i recently got 50 new french oak sleepers from this guy and he has a whole section of projects using sleepers,

http://www.railwaysleeper.com/Custome....cts.htm

with the drains i would say make them as wide as your spade, makes it alot easier and they still will be as effective. i do mine 9 inch as thats the width of the bucket on my machine.

if you have enough geotextile left i would either wrap the pipe in it or the whole trench in it then back fill with your pipe and porous fill.

the french drain your connecting into do you know where its out-falls is or just into neighbors garden?

you will need to remove any top soil before you lay your membrane and geotextile and subbase. you want to find a layer of good hard ground to form the base for your patio.

[cammackmartin]

Thanks for that.

Had a look at the website and they have some great projects on there but haven't found any quite like this but useful anyway.

The French drain bordering the next garden will only drain down to the bottom right corner that will be under a raised bed eventually. I expect this will just drain into next door at some point.

I have plenty of geotextile and intend to enclose the troughs to prevent any soil getting in.

There's no problem with the ground underneath which is the original problem as it is solid clay. It's really firm.

[cammackmartin]

Just an afterthought. Am I mistaken in thinking that running the linear drains under the subbase and sharp sand with the sleepers on top of that will improve the drainage of the patio area or are these types of drains only effective under soil. Will the water pass through the sharp sand and subbase ok to get to the drains?

[GB_Groundworks]

yes they will drain the subbase, keep them narrow and filled to the top and cover with geotextile or the dust out of your mot will block the porus nature of the back fill and stop them working.

[cammackmartin]

Are 3 trenches 40cm square and 5 metres long across this area a bit overkill or about right?

[cammackmartin]

It seems a lot to me but I calculate that for an area of 6.5m x 5m to create a subbase of 7.5cm and bedding of 3.5cm I'll need 5.2T of MOT1 or DTp1 and 2.1T sharp sand. Also to fill 3 0.4x0.4x5m and one 0.4x0.4x6m linear drains I'll need between 3 and 3.5 cubic meters of pea shingle. Am I right anyone?

[matt h]

you are about right, best to over order to be sure

[seanandruby]

dont think its wise to direct it to your neighbours garden. is the railway cutting not in use and cant you direct it to there if its not? i expect you have discussed it with your neighbour about overloading his system?

[cammackmartin]

I'm going to direct the water down to the end of the garden under the raised flower beds so the neighbour shouldn't get any more water than usual. Draining to the cuutting rquires a lot of digging up of communal areas so out of the question sadly.

[cammackmartin]

P.S. Thanks Matt for that. Glad I am about rght.
With regards to the drainage. I've read all of the comments about drainage/soakaways in clay that they are just holes. For this project I just need the water away from the central part of the patio area and in doing the project I see that although the water pools at low points it does eventually drain away. Hopefully the water will drain down the gradient to the bottom of the garden under the raised beds and eventually either into the plants above or out to the surrounding soil.

[Pablo]

Sorry to go against the grain here but I feel certain that new sleepers laid directly onto the ground without any kind of subframe is a recipe for disaster. These sleepers are generally uk sourced pine or rarely Larch and have a very wide grain so rot fairly quickly. They are not perfectly machined and have a tolerance of <15mm and are never perfectly straight either. Because of this tolerance saturated sand will migrate into the gaps whenever they are stood on and eventaully they will start to rock and lift at one end etc. When unfixed damp wood is exposed to hot dry sunlight the exposed surface dries quicker and shrinks as the other side stays wet and expanded. The result is that they warp slightly. Oak sleepers won't do this but they will still move in the sand bed.

[cammackmartin]

Thanks Pablo. Certainly worth considering. These are pressure treated and tanalised. They are supposed to cope with this sort of treatment but I'm sure they may well warp a bit. My friend has a piece he had half buried in the earth for about ten years and it was still ok but it may have warped a little. I'll soon find out I expect! It'll be like walking a surfboard!!

[Pablo]

Par and tan wood of today isn't the same as 10 years ago the quallity is much worse I've seen the stuff turn to mush after 5 years in the ground. How do you intend on laying them as they won't be straight enough for a screeded sand bed and individual bedding on sand will mean they settle to different heights. Also not suitable for kids unless you sand them due to how rough they are . If you use a sand cement mix then the water will not get away. I know it's your money and you can do whatever you want but whats the point in wasting it when you have so many other options.

[cammackmartin]

Hi Pablo. Was going to lay on sharp sand and settle them in. But I'm open to suggestions. A garage full of sleepers says I'm happy to sand them. I like the chunky effect of the sleepers so happy with this option but if a small frame underneath is the way to go then perhaps I should consider it.