I have worked on basements and underpinning before. The best idea is to use waterproof concrete and ensure that all concrete joints have a water-bar fitted so that any water that gets through the concrete can't get through the rubber/plastic water-bar and then inside. Even with all this most of the basements I have worked on have used a system where any water that does get in collects in special channels under your finished floor and runs to a sump hole which has a pump inside and a float switch which only switches on when the water reaches a certain level (or hopefully its never needed). By doing it this way you have all the bases covered.
Ideally the less joints you have in the concrete the better but you will have at least one, between the floor and walls but if proper care is taken with the water bar then it won't be a problem. It would be best to do the concrete in 2 pours, first the floor then fit the water bar and then your walls should go in one continous poor. Being that you are high up I would think you shouldn't have a huge problem, the last one I worked on was 4m below the water table!!
Depending on your existing foundations and how close you intend to go to them you may get away without underpinning ( putting foundations underneath your existing foundations because you remove the strength in the originals when you dig too close). If you do have to go with underpinning I suggest you get a really decent guy as I have seen them go wrong before and it really doesn't pay to skimp on this particular aspect. Do you really want a cowboy digging under you foundations?!?!
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Topic: Opinions please - Basement under new side extension (Read 4813 times)