VERIFICATION OF A PREDICTIVE MODEL FOR LOCATING SUBSIDENCE-PRONE UNDERGROUND COAL MINES, SOUTHWEST INDIANA

There are 186,000 acres (291 mi²) of underground coal mines in southwest Indiana located in 17 of Indiana’s 92 counties. Ten mineable coal seams occur in Pennsylvanian rocks that dip gently westward into the Illinois Basin. Underground, room and pillar mining, begun in the early 19^th century, has resulted in many subsidence features; pit-like structures caused by collapse of the abandoned mines. 1179 features were mapped in these 17 counties. The 2^nd author presented a winning paper at the North Central AEG student paper contest entitled “Prioritizing Grouting Operations for Abandoned Underground Coal Mines, Southwestern Indiana”. An expanded version has been submitted for publication in the Environmental and Engineering Geosciences journal. Three factors, thought to influence mine subsidence, were used in the analysis (1) distance from a subsidence feature, (2) vertical distance to water table, and (3) age of mine. Mines dating from 1905 to 1935 had the greatest occurrence of subsidence. Based on these factors, mines were given scores from 0-100 and ranked relative to grouting need with ranges: little (0-25), some (25-50), significant (50-75) and strong (75-100). An existing site was selected to test the analysis. A grouting operation, supervised by the Indiana Division of Reclamation, was conducted in summer 2013, in Linton, Greene County, IN. Grouting below a city street in northeast Linton was observed. Fifteen feet of soil and 10 to 15 feet of shale/sandstone overlie the four-foot Springfield coal seam consisting of intact coal, voids, or fall down material. Grout holes were placed at 35-foot intervals, with grout takes up to 100 cubic yards per day. As this site was included in the previous analysis, it had received a score of 100 indicating that grouting was in order. The grout mix used was one-half flyash, one-half sand, 300 pounds of cement and 80 gal of water per cubic yard of grout. About 3,000 cubic yards of grout were pumped for the total project at a cost of about $800,000. Indiana Division of Reclamation projects are designed to prevent mine collapse below streets and highways, as subsidence insurance is not available for these structures.