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Drainage Under Patio Built on Steep and Terraced Garden

Postby laurencewithau » Mon Oct 19, 2015 11:16 am

Hi everyone. My bungalow is at the bottom of a steep garden with a steep hill beyond it.
I have terraced the garden and have now had earth removers in to clear a space for a patio at the bottom of the garden, the patio being at the same level as the path around the bungalow.
About 100 cubic metres of heavy clay subsoil has been removed, leaving a rectangular space about 10 by 8 metres, and varying in depth because of the garden slope.
Where the garden has been cut away to form a cliff, this being the far side of the rectangle from the house, the cliff is about 1.5 metres high from what will be the patio.
My problem is that the clay floor of the rectangle is wet, despite weeks of fine weather, and the footings that were dug for the side walls and steps have pooled up with water.
I know about leaving holes in the walls I'll build, for the water from the cliff/bank to fall onto the patio and into plastic drainage channels running into the main drain.
What I'm not at all sure about is whether the wet clay and footings means that the patio will be wet from below the slabs, even with the drainage pipes for the cliff/wall installed.
I would guess not, because the newly dug ground is heavy clay, and I don't think that water from the water table is rising up through it.
More likely the water on the surface and in the footings trench is seeping from the cliff/bank. But I can't be sure.
My worry is that I'll lay the patio, which will become waterlogged, and will then have to re-lay it, perhaps after raising it up.
I could raise it up now, just in case, but then it would be above the house wall damp course.
Really, then, I'm asking how to install a drainage system under the patio.
I should point out that because the new patio, if not raised up, will be flush with the existing path around the house, and therefore with the drain cover, for the domestic waste water,I don't know where the water from the drainage system under the patio would run to.
Could I connect it to the drains by digging a channel and connecting it below ground level?
But how would I do it and how would I install a drainage system under a 10 metre wide patio? Thanks in advance.
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