In 2017-18, DA Smith* was commissioned by the US 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 10,850 feet were drilled, 6,200 feet of PVC casing installed, and 320 tons of cementitious grout, 1,060 tons of flyash, and 882 tons of sand injected. An additional 490 tons of gravel were placed to fill in sinkholes and other surface depressions. Finished sites were restored upon completion of surface activities.
* Formerly as Agapito Associates, Inc.