The surface subsidence of the longwall section of an Eastern Ohio Mine was monitored. The program involved a network of 250 monuments (1) in the panels distributed in 3 cross-sections, (2) on the county road passing through one end of the panels, (3) on an active 30-in diameter gasline and (4) on two angle towers for power transmission lines. The angle of draw was approximately 33 degrees, subsidence factor ranged from 0.595 to 0.7. The subsidence profiles, subsidence profile and subsidence development curves were presented and discussed for all the structural elements monitored. The study indicated that depending on the locations of the surface structures, the surface structural damage can be minimumized and that most structural damages can be made to be temporary.
One of the biggest concerns in introducing modern longwall mining method into U.S. Coalfields was that it would cause considerable enviromental damage as a result of the large scale surface subsidence. Because longwall mining creates much larger openings than those by room-and-pillar mining. The problems of surface subsidence and its associated structural and enviromental damages have therefore received considerable attention by the coal industry. In addition, the Surface Mining Reclamation and Control Act of 1977 mandates that surface subsidence control plan be an intergral part of the underground coal mine design. Accordingly, many major coal companies that operate coal mines, with or without surface right, near population center, and/or under· areas wittl active surface structures are likely to have some type of surface subsidence monitoring program or programs (1).
This paper describes the surface subsidence monitoring plans sponsored separately by three different companies, i.e. (a) the coal mining company that performed illlderground longwall mining, (b) the power company that operated the high voltage transmission power lines across the mine property, and (c) the utility company that operated a gas pipeline across the mine property.
The objectives of the subsidence monitoring plans were (1) to obtain operating data concerning the characteristics of surface subsidence and its effects on surface structures, and (2) to monitor continuously the surface structural response such that immediate preventive measures could be taken to eliminate and/or reduce damages.
The mine is located in Eastern Ohio near the Ohio river. The Pittsburgh coal seam is flat within the mine property and is approximately 5 ft. 6 in. thick with an overburden ranging from 350 to 800 ft. The surface terrane varies considerably over the mine property. Most of the area are heavily vegetated. The borehole log in Fig. 1 shows the typical stratigraphic sequences over the Pittsburgh seam. The immediate roof is weak shale which caves in the gob as soon as the powered supports are advanced.
The subsidence monitor was conducted in the first longwall section which consisted of three longwall panels. The panel widths ranged from 465 to 500 ft. and lengths from 2200 to 3200 ft. Panel development consisted of three entries, each 20 ft. wide.