My house is constructed on the side of a slope where the top of the slope is built the road.

I'd like to dig out the basement which is not high enough to stand in mostly, and is just clay. As part of the repiling work on the outside, and most downhill area of the house, we dug down 3 m and hit the water table probably about 2 m down. Overnight this fills with about 2000 litres of water which we have to pump out. We dug this hole to get access to a sewer.

My question is, is it safe enough to dig out half a metre or so with the water table another 1.5 m or more below that to build a room? I guess the builder would put down a water proof membrane and polystyrene insulation before laying the concrete slab for the basement.

And as for the hole that we've dug out, before we refill it with hard fill, should we bother about getting some drains laid to take the water away faster? It is flowing down the section 3.5 m or so, and seeping out of the slope into a small surface drain which is then draining into a stream.

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