Horizontal Hydraulic Conductivity Estimates for Intact Coal Barriers Between Closed Underground Mines

Abstract

Unmined blocks of coal, called barriers, separate and restrict horizontal leakage between adjacent bituminous coal mines. Understanding the leakage rate across such barriers is important in planning mine closure and strongly affects recharge calculations for postmining flooding. This study presents upper-limit estimates for hydraulic conductivity (K) of intact barriers in two closed mines at moderate depth (75–300 m, or 250–980 ft) in the Pittsburgh coal basin. The estimates are based on pumping rates from these mines for the years ranging from 1992 to 2000. The two mines do not approach the outcrop and are sufficiently deep that vertical infiltration is thought to be negligible. Similarly, there are no saturated zones on the pumped mines' side of shared barriers with other mines, and therefore pumping is the only outflow. Virtually all of the pumping is attributed to leakage across or over the top of barriers shared with upgradient flooded mines. The length of shared barriers totals 24 km (15 mi) for the two mines, and the barriers range in thickness from 15 to 50 m (50–160 ft). K values calculated independently for each of the 9 years of the pumping record ranged from 0.037 m/d to 0.18 m/d (0.12 ft/d to 0.59 ft/d) using an isotropic model of barrier flow. Using an anisotropic model for differential K in the face cleat (Kf) and butt cleat (Kb) directions, results range from 0.074 to 0.34 m/d (0.24 to 1.1 ft/d) for Kf and from 0.022 to 0.099 m/d (0.072 to 0.32 ft/d) for Kb.