Mine stabilization with coal combustion by-products such as fly ash is common throughout the United States. However, the use of the dry scrubber ash as opposed to fly ash has created some unique challenges in permitting and monitoring. The dry scrubber ash is a mixture of fly ash and residue from a dry scrubber unit designed to control air pollutants. The dry scrubber ash contains ammonia, and ventilation during mine stabilization can become a major issue. An abandoned room and pillar limestone mine in Sugar Creek, Missouri, which is owned by Lafarge North America, Inc. (Lafarge), is being backfilled using between 100 tons (90,700 kg) and 900 (816,300 kg) tons per day of dry scrubber ash slurry generated by two nearby power plants. Mixing the dry scrubber ash with water at the site creates the slurry. The slurry is injected into the mine through 8- to 10-inch (0.2 to 0.25 rn) diameter cased boreholes drilled through as much as 175 feet (53.3 m) of soil and rock overburden. Because of previous subsidence, the Mine Safety and Health Administration (MSHA) refused permission to enter the majority of the mine and also did not allow construction equipment to be placed on top of the unstable portions of the mine. Planning and permitting for this project started in the mid 1990's. Prior to beginning injection activities, Lafarge obtained both Local and State permits for the dry scrubber ash project. The various permits required monitoring the extent of the underground areas stabilized, the progress in filling subsided areas, the volume of fly ash placed for stabilization, and potential environmental impacts (including ammonia emissions). Because of the restriction placed by MSHA on entering the majority of the mine, all observations of injection to date have been conducted via remote video photography or ultrasonic distance measuring sensor. The mine stabilization began on December 19, 2001. As of May 2003, approximately 165,000 tons (149.7 million kg) of dry scrubber ash slurry have been injected into the mine.