Characterization of the hydrogeology and stress state in the vicinity of the homestake mine, Lead, SD

Underground workings in fractured rock are common worldwide. They have applications in numerous areas and fields of study. These include mining operations, civil engineering projects like tunnels and underground facilities, and research projects that require underground laboratories such as the physics research being conducted by Sanford Laboratory at the former Homestake mine and Fermi Laboratory near Chicago (Bahcall et al. 2001, Elsworth 2009, Sadoulet et al. 2006, bge science@DUSEL, fnal.gov). These excavations can reach several kilometers in depth including the 3.9 km deep TauTona mine in South Africa, the 3 km deep LaRonde mine in Quebec and the 2.4 km deep Homestake mine in South Dakota. Large quantities of rock are removed when constructing deep excavations, for example Rahn and Roggenthen (2002) estimated the total volume of rock removed from the Homestake mine to be 2.1x10 7 m3. Removing large volumes of rock alters the local stress state and ground water flow, potentially increasing risks to workers and the environment (Kaiser et al. 2008, Blodgett et al. 2002, Lucier et al. 2009, Goldbach 2010, Kang et al. 2010). The objective of this research is to develop a better understanding of how deep rock excavations can alter groundwater flow, stress state, and deformation in the rock that envelopes them. The approach is to evaluate how the hydraulic head, flow paths and stress state have been affected by excavation at the Homestake mine in Lead, South Dakota, one of the deepest mines in North America. The Homestake mine was selected as a focus of this research because it has recently been evaluated as the site of a deep underground research laboratory where an understanding of the groundwater flow and stress state was needed to plan underground experiments. The investigation includes poroelastic modeling of the Homestake mine using available geologic and geophysical data and mine records. Results from the analyses indicate that mining and dewatering have changed the hydrology and stress state in the vicinity of the Homestake mine. Dewatering reduces the hydraulic head and changes the flow systems in the vicinity of the mine. Four major hydrogeologic zones are recognized: 1.) a Shallow Flow System in the upper few hundred meters that dominates recharge and discharge to streams, 2.) a Recharge Capture Zone where water that has entered the region as recharge since mining began is captured by the mine, 3.) a Storage Capture Zone where water from storage in the host rock around the mine is captured, and 4.) a Mine Workings Zone where rock has been removed. Water enters the system at the top of the Shallow Flow System and either discharges to the streams or flows downward and becomes recharge to the lower capture zones. The Recharge Capture Zone grows with time as regions of storage are depleted and new recharge enters, and eventually it is assumed that the entire capture zone for the mine will become the Recharge Capture Zone. Fluxes from the Shallow Flow System to the Recharge Capture Zone typically range from 1x10 -9 to 4x10-9 m/s. The largest recharge fluxes from the Shallow Flow System to the Recharge Capture Zone occur above the shallowest portions of the mine. Recharge flux also occurs above areas adjacent to the mine, and when projected to the surface the Recharge Capture Zone creates a roughly elliptical shape that is 6 km x 3.6 km. The Storage Capture Zone extends out beyond and below the Recharge Capture Zone and when projected to the surface creates a roughly elliptical region that is approximately 8.3 km x 6.6 km and extends down to depths of almost 5 km. Hydraulic heads and flow paths have been affected beyond the Storage Capture Zone but this water had not reached the mine by 135 years and therefore these regions are not included in the capture zones. The model was calibrated using in-situ stress data at various points in the mine to improve its ability to estimate the stress state and mechanical deformation around the Homestake mine. This was done by varying the rock density, Poisson's ratio, the effective Young's modulus of the workings region, and including initial stresses until predicted stresses best fit in-situ stress data. The changing mechanical properties in the workings and dewatering cause changes to the stress around the mine. The mining process typically causes increased compression laterally around the workings and decreased compression above, below, and within the workings. The greatest changes in total stress are near the base of the mine and reach roughly 40 MPa between the ore bodies and in the lower portions of the West Ore Body. The softening of the mine region because of material removal and decreased fluid pressure in the workings results in deformation in the vicinity of the mine. Subsidence occurs above the mine region and is greatest near the surface and decreases with depth; above the shallowest workings subsidence can reach approximately 0.18 m. There is also uplift along the footwall of the workings in the deeper portions of the mine that can reach up to 0.022 m. Horizontal displacements of as much as several centimeters occur around the mine and with displacement towards the workings region. Deformation in the vicinity of the mine results in tilt that is towards the workings with the greatest tilts near the surface. A fault that intersects the West Ore Body was considered as a location for an experiment into the mechanics of earthquake nucleation, so the stress state in the vicinity of this feature was of particular interest. This simulation shows that mining and dewatering reduce fluid pressure and change stresses along the fault. The shear stress along the fault typically increases along most of the fault and decreases in the region where the fault and West Ore Body intersect. Increased shear is typically on the order of 1 to 2 MPa but can reach as much as 5 MPa in areas around the intersection of the fault and West Ore Body. In the region along the fault intersecting the West Ore Body, the decrease in shear can reach -11 MPa. The total normal stress along the fault becomes more compressive along most of the fault and less compressive in the intersection between the fault and West Ore Body. The increase in total compression is approximately 2 MPa, and the reduction in compression in the intersection is approximately 10 MPa. The critical shear stress along the fault was calculated using Mohr-Coulomb failure criteria presented by Byerlee (1978), and the ratio of the estimated shear stress along the fault and the critical shear stress ( ts /tf ) was found to approximate the potential for slip along the fault. Mining results in a reduction in slip potential with values of ts /t f ranging from 0.66 to 1.1 before mining and from 0.22 to 0.67 after mining. This reduction in slip potential results from reductions in fluid pressure and increased normal compression caused by mining activities.

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