Date of Graduation
Statler College of Engineering and Mineral Resources
Civil and Environmental Engineering
This research was performed to develop a method of determining the potential hazard of underground mining near surface bodies of water. Underground mining creates a void within the subsurface that is eventually filled by overburden material. This strain movement perpetrates through the strata layers through fracturing and bedding plane separation, creating a depression above the mined seam (subsidence). The effect of subsidence may occur rapidly over the course of several weeks to months (longwall mining, pillar extraction) or may occur slowly (room and pillar mining); in some cases over one hundred years. These strain movements may affect the physical properties of the overlying rock layers, specifically hydraulic conductivity as it pertains to this research. Hydraulic conductivity is the ability for a medium to transmit water when submitted to a hydraulic gradient. The change in hydraulic conductivity may permanently alter the local groundwater table, or create a pathway from a surface body of water into the mine void. This may lead to erosion around the reservoir rim, potentially causing uncontrolled water loss within the reservoir.;This research was performed to develop a methodology to determine potential seepage failure modes due to changes in hydraulic conductivity in overburden, caused by underground coal mining. The research is separated into four tasks: 1) literature review of mine subsidence prediction and empirical assessment, 2) a method to develop potential seepage failure mode analysis of a mine site, 3) numerical analysis assessing seepage at a field site, and 4) semi-quantitative sensitivity analysis of risk based events for seepage mode failure near reservoirs.;Analysis shows that subsidence due to underground mining affects the hydraulic conductivity of the overlying medium, affecting the localized groundwater table and creating a cone of depression where hydraulic conductivity is increased. The extent of the cone of depression from the mine void is referred to as the Angle of Groundwater Influence. If this angle intersects with a reservoir pool level, seepage from the reservoir may cause uncontrolled drawdown or erosion.;Computer model analysis was performed on a field site to show how this methodology is applied. It was analyzed for three different lateral offset distances based on various pool levels. The numerical modeling results show that the reservoir pool has minimal impact if it lies beyond the affected overburden of the mine. However, if the reservoir rim intersects the impacted area, the increased flow rate may initiate erosion in the subsurface potentially leading to a failure mode for the reservoir. Within the subsurface, the controlling factor is the rock layer with the highest initial hydraulic conductivity located above the fractured zone. At the modeled field site, the changes in groundwater flow rate below the reservoir rim increased beyond one order of magnitude at Probable Maximum Flood (PMF) pool level. The findings developed within the sensitivity and field site analyses were used to develop practical application of the methodology to aid in determining the potential hazard from underground mining on surface bodies of water.
Wachtel, Thomas K., "Potential Failure Mode Analysis for Mineral Extraction Near Reservoirs" (2016). Graduate Theses, Dissertations, and Problem Reports. 6883.