If the query is not related to any irrigation project or a road/rail construction contract, I would suggest to study the practice being followed in Metro rail projects being carried out in many cities where "tunneling" below the surface is common. I was associated for over 3 decades with 'U/g mining' in ultra depths ranging more than 13000 ft.below surface and tunnels of many thousands of kms interlinking the 'shafts' from all sides, ups & downs-raises & winzes, stopes, footwalls etc. The atmospheric temperature at work sites, generally high risk areas, used to be 50 deg.C. In these circumstances also we had no scheme to pay specifically 'tunneling allowance' but paid only U/G allowance ( a risk allowance as they have to bear uncomfortable working conditions braving risk of their lives) for those who are posted in such categories (miners) and for others who had to visit now and then at actual duty days on fixed lump sum. However Insurance policies to cover accident & death/injuries, loss of earning capacity etc apart from health insurance is a must. This can be, I would advocate, in addition to what has been assured under Employees compensation act.