ESTIMATION OF HYDRAULIC CONDUCTIVITY FOR HORIZONTAL FLOW ACROSS COAL MINE BARRIERS: A LARGE SCALE FIELD TEST IN WEST VIRGINIA

Unmined coal barriers separate adjacent coal mines and form barriers to horizontal leakage between mines. Understanding the leakage rate across such barriers is important in planning mine closure and strongly affects recharge calculations for post-mining flooding. This study presents estimates for intact (non-compromised) barrier hydraulic conductivity (K) in 2 closed mines at moderate depth (100-350 meters) in the Pittsburgh coal basin. The estimates are based on field pumping rates from these mines 1997-2000 associated with leakage across their updip barriers from shallower flooded mines. Both the pumped mines were maintained nearly dry, and so had dry seepage faces downdip from flooded mines, allowing precise estimates of hydraulic gradient. These mines do not approach the outcrop and are sufficiently deep that vertical infiltration is thought to be negligible. Similarly, there are no wetted barriers facing on other mines, and therefore pumping is the only discharge. The length of barriers totals 24 kilometers for the two mines, generally ranging in thickness from 15 to 50 m, and so the K test was large-scale. These test conditions are relatively unique and ideally suited to K estimation. The two mines were analyzed using both isotropic and anisotropic models, assuming that K is independent of barrier thickness and spatially uniform. Cleat angles were estimated from published strike measurements. Pumping data were annualized; one leakage K value per year per mine was estimated. Estimates of K for the two mines ranged from 0.26 to 0.61 m/day face cleat and 0.14-0.19 m/day butt cleat, an anisotropy of from 1.8 to 3.2. The isotropic estimates were 0.04-0.10 m/day. The isotropic results were very consistent between all but one year, and it is hypothesized that pumping records for this year underestimated withdrawals. Excluding this year, the mean isotropic K is 0.091 m/day for both mines with standard error 0.005. The calculated K values are upper limits due to the assumption of zero infiltration. They only pertain to barriers in coal of bituminous grade and of thickness in the 15-50 m range. The values are slightly higher than those obtained in borehole tests for western coals. They indicate the validity of this range of horizontal K for barrier leakage tested at a scale much larger than any tests previously reported.