Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 386, 1991 Jun; :1-2
Objective: Develop a method to evaluate the effectiveness of coal seam fire surface seals so that potential problems may be corrected at an early stage, before the integrity of the surface seal is lost. Background: Surface seals are the primary coal seam fire abatement method being used in the western United States. Surface seals smother a fire by denying it oxygen. The seal is usually constructed by placing a mantle of so four to ten feet thick, over a coal seam fire area to exclude air from the fire. The soil is compacted to increase effectiveness, and a soil cement may be used to augment the soil. In some cases, burning coal may be excavated and separately smothered or otherwise extinguished in another area; in other cases, a trench may be excavated and backfilled with incombustible material to isolate a fire. More often, a surface seal is simply applied over and around the entire known burning area. In the 1950's, the Bureau of Mines administered a large program to treat numerous coal seam fires across the western United States. Methods used in this program were described in Bureau of Mines Information Circular (IC) 7932, dated 1959. More recently however, various coal seam fires have been treated with surface seals under the direction of state Abandoned Mine Land Reclamation authorities. It has frequently taken years to evaluate the effectiveness of surface seals. By the time problems become evident, it has often been too late to take corrective action within the scope of the original abatement project. For this reason, the Bureau of Mines developed a means for tracking temperature trends at coal seam fire projects to obtain a better and earlier handle on the effectiveness of surface seals. Approach: Under an Abandoned Mine Land (AML) research contract, the effectiveness of coal seam fire seals was evaluated at thirteen sites in Montana. Nine of these sites were sealed during the 1950's and four were sealed during the 1980's. Underground temperature data and soil test data collected for the sites were correlated to determine information that might serve as indicators of succcssfu1 surface seal methods. A change in the rate of underground temperature change at a given borehole was found to be an indicator of the local effectiveness of a surface seal. How the Evaluations Were Made: To conduct the evaluations, boreholes were drilled to install temperature monitoring probes. The temperature monitoring probes consist of a themocouple sealed with Teflon fluorocarbon polymer tape and heat shrink tubing. The probes were lowered into the boreholes, connected to the surface with extension wires, and backfilled with grout or soil to exclude oxygen. Temperatures, read with a potentiometer, are influenced by a relatively small area within three to four feet of the probe. Test Results: Temperatures were monitored over time to determine temperature trends. The measured temperatures were also compared to the baseline normal ground temperatures for the area and were correlated to visual indications of possible ongoing fire activity. For the sites treated during the 1950's, present temperature readings were far below active burning temperatures, but in some cases were still somewhat above normal ground temperatures. This indicated that the seals had been successful in containing the fires but that complete extinguishment may not yet have been achieved. For the sites treated during the 1980's, temperatures were monitored during thecritica1 period of a year pr two after the seals had been installed. It was found that a decrease in the rate of temperature rise or an acceleration of temperature reduction was an early indicator of a successful surface seal.
Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 386