Protecting The Quinsam Coal Mine Access Road From Subsidence Damage

CIM MineSpace 2001
R. Pakalnis, K. Galovich,
Abstract The Quinsam Coal Mine, situated on Vancouver Island, recently extracted a panel beneath the mine access road. The depth of cover beneath the road varied from 56 to 129m. To prevent damage to the road, and adjacent powerline, it was necessary to establish a “protection zone” beneath these services. For economic reasons it was desirable to have the protective zone as small as possible, with the greatest extraction ratio, while not compromising long-term stability.

The extent of the protective zone was established using numerical simulations coupled with field measurements. Historically, angles between 15 and 36 degrees have been used to establish protective zones over coal mines. Recent numerical modeling work has shown that even 15 degrees may be very conservative for protective zones over shallow mining operations. Although the angle of draw may be in the range of 5 to 15 degrees, the angle of critical deformation is typically in the order of 0 to 5 degrees. The angle of critical deformation is the angle at which intolerable damage is sustained by a structure; it is this angle which should be used for protection of surface structures. Angles of 5 to 7.5 degrees were used at the Quinsam Mine; these values included an additional safety factor.

The maximum allowable extraction ratio was determined using the program ARMPS to establish pillar sizes needed for stability. Excavations spans to prevent caving (hence increases in pillar height and potential sinkhole formation) were determined by numerical modeling and historical experience at the mine. Extraction ratios between 54 and 73% were determined to be appropriate. The maximum span was determined to be 8.5m.
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