Part of the answer will depend upon the Option you are working under and what the specific risk is.
If the risk is the amount of repairs to the existing structure then then under Option A it is the Contractor's risk and yes he is liable to pay. If it is an Option B then you will need to review the bill of quantities for how the work is measured. If it is Option C then the risk is shared through the gainshare arrangements. If it is Option D then need to review the bill of quantities for how the work is measured.
If the risk is the amount of repairs to the existing structure then then under Option A it is the Contractor's risk and yes he is liable to pay. If it is an Option B then you will need to review the bill of quantities for how the work is measured. If it is Option C then the risk is shared through the gainshare arrangements. If it is Option D then need to review the bill of quantities for how the work is measured.