Design, construction, and performance of a remotely-placed adit plug

An adit plug was constructed in the historic Pinnacle adit, located at the Homestake Mining Company Pitch Reclamation Project, using remote placement methods. The plug was constructed to promote resaturation of the Pinnacle Mine workings as part of the ongoing reclamation of the Pitch property. This paper addresses the geotechnical aspects of the design, construction and performance of an adit plug using remote placement methods. The Pitch Reclamation Project is located in the Sawatch Mountains, approximately 12 miles east of Sargents, Colorado. The Pinnacle adit was developed by the Pinnacle Mining Company, which mined uranium from the underground Pinnacle Mine from 1959 to 1962, predating involvement by Homestake Mining Company. Remote placement of the concrete plug was chosen because poor ground conditions made access to the partially collapsed Pinnacle adit impractical. The Pinnacle adit is located in close proximity to a major high angle reverse fault. Ground conditions reflect the tectonic history of the site. The rock mass along the alignment of the adit is characterized by vertical and overturned bedding, brecciation, and fault gouge. A site investigation program revealed an isolated zone of competent rock that represented the best ground available for siting the plug. The options of opening the existing adit, or driving a new adit to the plug location were unattractive from both a technical as well as a health and safety perspective. All construction activities were conducted from the surface by borehole access. Principal design considerations included structural integrity against shear failure and minimizing leakage across the length of the plug. The concrete plug was placed in a continuous pour, following placement and curing of an upstream barrier. The concrete mixes were designed to attain the optimal slump and compressive strength, flow characteristics, resistance to shrinkage, chemical degradation and thermal cracking. The rock surrounding the periphery of the adit was pressure grouted, using borehole access, to complete the hydraulic seal. Piezometers were installed to monitor resaturation within of the Pinnacle Mine workings. Resaturation of the workings upstream of the Pinnacle adit plug has exceeded hydrologic predictions.