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Flooded footings

Postby jamesjoewill » Tue Aug 12, 2008 5:49 pm

I am at present building a lean-to conservatory at the back of our bungalow. I have dug the footings to approximatly 1m deep x 550mm wide. My bunglow sits on clay & has a 5' basement area. The floor of the basement is about a 100mm layer of concrete & beneath that is the clay & a permenent layer of water in between. During heavy rain the basement can flood a bit through water rising up. This does not usually cause too much of a problem as i think the clay/foundation/water situation is pretty much in harmomy most of the time (& there is a floor drain). However the base of my footings must be about 150mm below the basement concrete level & when i hit this layer water spurted out from holes in the exterior brickwork & quickly flooed my trenches. The trench water level has now reached the same water level as the basement floor & risen no further. My plan is to pump the water out just before the concrete is poured, but of course it will just start pouring out of the basement layer again. I have no way of stopping the water under the house pouring into the trenches, but my question is that as the concrete is poured will it cause enough pressure to stop the water comming out & will the concrete cure in this kind of situation? Also some of the sides of my trenches have fallen in a bit. Can i just pour the concrete to fill them in as well or will this cause unequal loading on the foundations. My conservatory has a 600mm dwarf wall but due to slope in existing house ground level, will also have a 700mm wall below that (1300mm max in areas). One final question is that the conservatory floor will be a suspended timber floor with about a 600mm air space between the floor & the oversite. If i install hardcore, blinding & a DPM do i also need to have a 75mm concrete layer as well, bearing in mind nobody is ever going to walk on it?

Many thanks.
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