This paper presents the results of a pilot project demonstration using foam technology for placing sand backfill to mitigate mine subsidence in Glenrock, Wyoming. The foamed sand backfilling approach consists of mixing foam with sand to create a flowable mixture which is then injected into the mine openings, rubble zones, and fractures remotely through boreholes to effect backfilling. For the pilot study, over 4,000 tons of ASTM C-33 concrete sand was injected into the mine workings located beneath an area of less than an acre. Subsequent verification drilling revealed that the sand backfill completely filled large openings tight to the roof, exhibiting an average SPT (N_1)_(60) blow count value of 5.9. The SPT samples also showed that the sand backfill material effectively penetrated and filled void spaces within blocks of caved and rubblized overburden material, including openings as narrow as 1/4-inch wide. The project serves to demonstrate the effectiveness of using foam to place underground backfill. Economic advantages of the foamed sand backfilling approach include the ability to use a wide range of low-cost backfill materials derived from near- or on-site sources or wastes from other industries, the use of no cement, wider borehole spacing, and low water consumption.
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