Semester

Summer

Date of Graduation

2010

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mining Engineering

Committee Chair

Syd S. Peng.

Abstract

The longwall mining method is a commonly used method of coal extraction that involves the complete removal of large, rectangular panels of coal. Since it causes subsidence through the overlying strata to the ground surface, the surface-and ground-water above the longwall panels may be affected and drained into the lower levels. The purpose of this study is to determine the effects of longwall mining subsidence on surface- and ground-water systems by using a groundwater flow model: Ground Water Vista (GWV).;To setup the GWV model, the Globe Information System software (ArcMap) was used to determine the geometry and boundary conditions of GWV model. AutoCAD software was used to draw local geological cross-sections and analyze geologic formations of overburden strata. The final and dynamic subsidence profiles were plotted to determine the surface strains during the water table fluctuation periods. The slug tests were used to determine hydraulic conductivities of overburden strata. The recharge rates and evaptranspiration rate were obtained from field tests as input parameters in the GWV models.;Subsidence monitoring monuments were installed across the longwall panels to measure the subsidence before, during and after the longwall face passed under them. Additionally, extensometers in three different borehole locations were installed to monitor the overburden strata movements.;For recording the water level fluctuation, three water wells were drilled down to the proposed deformation and fractured zones to monitor the different water level fluctuations throughout the mining period. These water elevation records were used to calibrate the groundwater flow models.;The hydraulic conductivity is a measure of the rock's ability to transmit water when subjected to a hydraulic gradient. It is an important quantitative parameter characterizing the flow of groundwater. Slug tests were performed to determine the rock hydraulic conductivity of pre- and post-mining conditions.;A GWV model was used to predict the water table contours for the periods of pre- and post-mining conditions. The field data from the three water wells were used to calibrate the model.;Based on the analysis of final subsidence, extensometer readings, water table fluctuations and numerical modelings, it was concluded that:;The water bearing zones of overburden strata occur at the bedding-plane openings between different rock types. The main water bearing zones in the study area occur in the lower and middle members of the Waynesburg Formation.;The water table drawdown contour lines after longwall mining showed that the maximum water table drawdown was 50 ft, which was at the edge of panel B5. In general, the water table drawdown was larger around the recovery room than the setup room. The water table drawdown was smaller over the streams than the other areas. The water table dropped about -5, -22, -28 and -41 ft for the W1, W2, W3 and W4 intermediate wells, respectively. The maximum influence distance of the post-subsidence water table drawdown in the northern side was 2,911 ft from the edge of panel B5, and 1,688 ft in the southern side (downside) from the edge of panel B6. It indicated that the influence of water table drawdown after longwall mining was localized and did not affect the water table more than about 3,000 ft beyond the mined out panels.;The hydraulic gradients of post-subsidence outside the panels became larger and smaller inside the panels than that of the pre-subsidence. The maximum water table drawdown was 50 ft at the tailgate of panel B5, near the recovery room of panel B5. The maximum drawdown in panel B6 was 35 ft at the recovery room of panel B6. It was concluded that the groundwater hydraulic heads above the longwall panel dropped a maximum 50 ft in panels B5 and B6. Therefore, the shallows wells that were less than 50 ft deep from the surface were affected by longwall mining subsidence. For the intermediate wells that were more than 85 ft deep (W2I), and strata movement acted as a unit during longwall mining subsidence, i.e., it was not disturbed by longwall mining subsidence, the water elevations might drop due to the passing of tension cracks associated with the dynamic subsidence but rebounded quickly after longwall mining subsidence. Therefore, there were little impacts of longwall mining on the intermediate wells in the study area. For the deep wells that were located below the Waynesburg Sandstone, the water was drained to the underlying strata or even into the mine gob (W2D, W3D).

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