who foots the bill when diggin foundations doesnt go to plan?

Have you a formal contract which sets provisions for 'unforseens', 'variations', and or 'extra overs'? Suspecting that you have not, I would say; 1. 'Reasonably Foreseeable' test - It may be held that the Contractor could / should have undertaken a suitable and sufficient scan of the site to identify potential cables etc. Even if agreed otherwise, the contractor would need to work safely, hence should have scanned etc. Following this, any suspect areas would usually necessitate a more gentle dig as it were, and the contractor could have carefully excavated the area. So sounds like the contractors liability here. 2. Sewage pipe, logic may have pointed towards their likely presence? So, again, soft dig and caution etc, same as 1. 3. Coming across concrete sounds like an 'unforeseen'. I was digging a trench for my build and came across masses of concrete, all unforseen. heck of a job to excavate it! It may be fair for this cost to pass to yourself. Can he do this you ask? yes he can, but you can reasonably challenge some of it as stated above. Hope that helps, thanks, Jason.

Yes of course as you have failed to identify these items. However, a decent tradesman would repair at costs