Investigation of shaft stability and anisotropic deformation in a deep shaft in Idaho, United States

During construction of a deep shaft at Hecla's Lucky Friday Mine, excessive anisotropic ground deformation led to significant damage to the shaft liner. This necessitated a major design change during construction, with the excavated shape of the shaft being changed from a circular to an approximately elliptical geometry. This study utilizes extensometer data collected during construction as well as a three-dimensional finite-difference model of stress redistribution around the shaft and a calibrated two-dimensional discontinuum numerical model to develop an understanding of the factors affecting the relative stability of both shaft geometries. Although the focus of the work is the deformation behavior of the foliated rock surrounding the shaft, the role of liner installation in suppressing ground displacements is also considered. The change in shaft geometry was found to be highly effective in improving stability of the shaft, with the maximum time-dependent displacement around the shaft decreasing by approximately an order of magnitude. Additionally, finite-difference models confirmed that increased stress concentrations around the shaft caused by adjacent level developments played a significant role in exacerbating ground deformation in the circular portion of the shaft.