Underground clay and coal mining also occurred in the area.  Previous investigation found up to 50-foot thick backfill in the pits, with several low-strength zones.

In 2004, a sinkhole developed in West Campus Drive where the street passes near or over the old mine workings.  A year later, a sinkhole reoccurred in the same area, along with two smaller subsidence openings in an adjacent athletic field.  In addition, some settlement was observed both in a newly constructed retaining wall supporting West Campus Drive and in a storm sewer under the roadway.

Following the second subsidence event, CSM retained Shannon & Wilson to develop an approach and specifications for emergency repair of the sinkhole using compaction grouting techniques.  We also assisted CSM personnel with a pre-bid site meeting and contractor screening, and provided construction monitoring during the compaction grouting activities.  A total of 63 holes ranging in depth from 14 to 86 feet required compaction grouting.