My reply seems to be truncated.  Here's the rest.

I would be more concerned about the HAZMAT issues than the depth - with a trench of that potential depth you are not going to know what is being pulled up until it is on the surface so if the operator encounters some serious buried wastes you won't know until its up on the ground.  I would assume/prepare for the worst case, meaning working in protective suits.  How much environmental study has been done on this HAZMAT site?  You may want to consider having an evironmental assessment prepared via soil borings prior to your work, if one hasn't already been done.  I think given all of these issues/concerns, the most prudent first step would be to evaluate what could possibly be down there and how significant it might be.  I think it would have to be really significant to warrant going after it in 10-20 feet of hazardous waste.

Joe Joseph

the opinions expressed in this posting are my own and do not necessarily reflect the views of my company or any other organization with which I am associated.