Brutcher, D. F., B. B. Mehnert, D. J. Van Roosendaal, and R. A. Bauer. 1990.
Rock Strength and Overburden Changes Due to Subsidence Over a Longwall Coal Mining Operation in Illinois.
Paper in Rock Mechanics Contributions and Challenges (Proc. 31st U.S. Sym. on Rock Mech., Golden, CO), Balkema, pp. 563-570.
(posted September 4, 1996)
The overburden above an active longwall coal mining operation was characterized before and after subsidence using core drilling, geotechnical instrumentation and in situ testing. The analysis of mining-induced changes in the overburden provides a better understanding of the mechanisms which lead to the surface expression of subsidence and hydrologic changes.
-- The study site is located in the flat to gently rolling farmland of south-central Illinois. The Herrin #6 coal seam, which is approximately 9 feet thick in this area, is being mined at a depth of 725 feet. The overburden consists primarily of Pennsylvanian shales and siltstones overlain by 20-25 feet of glacial drift. An 85-foot thick, argillaceous sandstone aquifer (Mt. Carmel Sandstone) is located near the top of the bedrock. Two adjacent longwall panels were instrumented. Panel dimensions are 5000 feet long by 600 feet wide with double-chain pillars between panels. Adjacent panels are 200 feet apart.
-- Pre- and post-subsidence field investigations included geotechnical and geophysical logging of inclined core holes and the monitoring of survey monuments, piezometers and time domain reflectometry (TDR) cables. The sandstone aquifer was characterized with pump and slug tests prior to undermining and hydraulic injection tests before and after subsidence. In the laboratory, core samples were tested for unconfined compressive strength and modulus of elasticity.
-- The development of fractures in the overburden was documented by a decrease in RQD and shear-wave velocity and an increase in fracture frequency and hydraulic conductivity. TDR signals indicated shear deformation between strong and weak lithologic layers near the panel edge. Tensile deformation of the TDR cable at the centerline showed evidence of bedding separation.