In 2017-18, DA Smith (formerly Agapito Associates, Inc.) was commissioned by the U.S. Office of Surface Mining & Reclamation & Enforcement, Denver, to drill into and grout abandoned underground room-and-pillar coal mine entries and shafts to abate surface subsidence at five sites in Michigan. The historic mine workings were located approximately 100 feet below surface and actively subsiding at some sites at the time of drilling. Personnel tethers and other special safety equipment were utilized to prevent falling into voids.
Underground voids were grouted with different mixtures of cement, fly ash, sand, aggregates, and calcium chloride. Sinkholes on surface that developed while drilling were backfilled with imported material. Drilling was conducted in both residential neighborhoods and rural fields, requiring coordination with private property owners.
Over a four month campaign, a total of 10,845 feet were drilled with various subcontractors and over 1,985 tons of grout injected. Finished sites were restored upon completion of surface activities.