In any case, Maryland's only experience with DDC was not entirely satisfactory. We used it to consolidate a land fill. Weight was 18 tons, drop was 60'. Patterns were in accord with the FHWA publication on the subject.

In dump material, the penetration seemed to be just under 20'. The road that we built has suffered some differential settlement.

Tim did not say how much fill was to be placed over the existing ground. If we were building a 4 lane project over mines as described, I think that we would be thinking along the lines of a geogrid reinforced fill over the mines even if we had to undercut and backfill in order to achieve it. This won't stop mine subsidence, but it provides warning in the form of settlement instead of catastrophic collapse. We would not rule out "compaction" grouting. I use the term in quotes because it is not compacting much, it is filling voids. We would be especially prone to use grouting methods for deep vertical shafts. We would only be comfortable with DDC for near surface tunnels and rooms.

Newer geophysical techniques might have a place here. The sales demonstrations for seismic tomography that we have seen imply that they can provide all the answers. I know that we have all heard that before, but they do keep getting better.

-David-

If dynamic compaction were performed on such an area, I would be concerned about incomplete collapse of mine voids causing future problems in known areas and new problems in previously stable areas

The features can not be easily investigated because they are choked with large boulders, tailings, trash, and debris. This makes them impossible to auger. The air voids in the "gaps" between the boulders quickly attenuate geophysical signals (GPR, EM), allowing little depth penetration. Much shallow reflection seismic was performed to search for shallow mines, and the bigger, obvious shafts did show up, but not the smaller ones we have left to deal with.