Sondage underpin or not to underpin that is the question!

You may remember we are planning a renovation and were hoping to excavate out part of the ground floor of the house (which is already a third excavated). Well we've since had an Etude de Sondage carried out which has identified shock horror that there is 'earth' underneath the house. Ok so not sure what else they were expecting to find? A gold mine would have been nice.

We were quite surprised when we looked in the garage and saw where they'd done the etude - right in the middle of the garage floor and nowhere near the walls.

The architect seemed disappointed that he'd found earth - and it appears he was hoping for some nice open spaces...aah really? Come on, did you really think it would be empty under there?

Anyway on the basis of this he concludes that we should not excavate as we may need to underpin all the foundations and that will be extremely expensive and risky. So he has proposed a set of plans without the excavation. Mmm not quite what we were after. Wasn't the point of the Sondage to find out how deep the foundations were? Apparently not. He was looking for empty spaces, we were looking for an answer as to whether we could excavate and at what cost. We now realised we should have specified the exact positions of the sondage and not left it to them as they still don't have the answer to the question we asked.

So - Monsieur Sondage will be returning to redo the Sondage and tell us how deep our foundations are so we can finally get an answer as to whether we can excavate without underpinning. Monsieur Architect seems to think that underpinning will completely blow our budget - has anyone got any ideas of prices of underpinning per m2? He's talking of 2250 euro per m2 and hence is recommending we do a new build extension rather than excavate. Any one else undertaken underpinning type renovation work in France? Should we steer well clear or push on? I'd love to hear your experiences and thoughts.

I did some. We had one wall and one wall only not built on bedrock directly. Lo and behold it is on the course of an old spring that has been diverted down a sink hole and feeds our well eventually. However, there is a wet fault along the old bed of the spring and since it must have flowed for hundreds if not thousands of years, it has cut a kind of trench through the rock, which being limestone is soft. So, seeing some subsidence I had a bit of a dig. Bloo** Hell, said I, it followed most of the line of the wall. Whoever extended the house at the time had apparently used the trench to put the wall into deepish, but had not thought a century or so ahead. I got a neighbour to help, we manually dug out sections on both sides of the wall, the old extension was a barn and this part had been used as a shelter/garage over the years and still has an earth floor, then underpinned section by section, about 80cm a time because digging out a metre and raising subsidence with jacks only left us that much between. It took us more than two weeks to do six metres and cost very little for most of the materials except the metal work which basically freaked me out, given it is only a bit of low quality steel that coming back out the scrap collector might give €10 for tops.

I had free labour by having a neighbour help, but if it had been two paid men for two weeks it would have cost about €2400 with the materials and probably hire of a mechanical digger and two or more pneumatic jacks. That was autumn 2010, no doubt it would be more now, so I don't think your estimate is that far out somehow.