Sunken Patio

[diyjoe]

My daughter lives in a flat area with a high water table level, on visiting her last September she had dug a rectangular hole in the garden 2ft deep, at first I thought she was excavating for a swimming pool only to learn it was for a sunken patio and she had given no thought of water from above or below. By Xmas she had a pond, this has now subsided due to the very dry weather.

Obviously decent footings and a retaining wall needs to be built with dpm and shingle behind with drainage outlets in the first course of bricks, my dilemma is what to do with the patio base? I am thinking of laying a dpm covered with 4" shingle to allow any water to drain to a sump pump. Has any one any suggestions please other than filling the hole in again.

diyjoe

 

[r896neo]

You need to be very careful with dpms around structures like that. Dpms with hydrostatic pressure can lift serious weights like concrete slabs etc. I once worked on a site where a lift shaft was a 1m deep pit with a butly liner with a 200mm reinforced concrete slab poured on it. The water pressure after heavy rain jacked one side up about 4".

You want to instead use geotectile membranes to separate granular fill like gravel. Don't try to stop water with dpms but rather use free draining channels and layers of material to let it take the path of least resistance into your sump.

 

[diyjoe]

Thank you for this information, after posting my query "in the garden" section I also posted the query in the "building" section as I thought perhaps it was more of a building project than a gardening one. I can see your point of view and I have observed a pond liner with gravel base and some remaining water being lifted by ground water level when being emptied for cleaning, obviously due to unequal water pressure. My only worry in adopting the line of least resistance for water to a pump/sump is the amount of water that will drain from the surrounding ground, perhaps the heavy clay soil will naturally retain a lot of the water? What size pump/sump would be required, I might also add that the pump will exit into the main sewer system as does all surface water from roofs etc in the area.

In the building section the advice I am given is to treat the project like the basement of a house in Salisbury flood area which I presume is full tanking?

On the lighter side perhaps with full tanking my daughter could have a unique floating patio which rises and falls according to ground water levels?

Thank you again for your advice.

diyjoe