If the query is not related to any irrigation project or a road/rail construction contract, I would suggest studying the practice being followed in Metro rail projects 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 13,000 ft. below the surface and tunnels of many thousands of kilometers 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 degrees Celsius. In these circumstances, we had no scheme to pay specifically for 'tunneling allowance' but paid only U/G allowance (a risk allowance as they have to bear uncomfortable working conditions braving the risk of their lives) for those who are posted in such categories (miners) and for others who had to visit now and then on actual duty days for a fixed lump sum.
However, insurance policies to cover accidents, death/injuries, loss of earning capacity, etc., apart from health insurance, are a must. This can be, I would advocate, in addition to what has been assured under the Employees Compensation Act.