The Nelson Tunnel is the lowest level of a mine complex including, but not limited to, the Nelson Tunnel, the Commodore Mine, the Amethyst Mine, the Happy Thought Mine, and the Last Chance Mine. The Nelson Tunnel was driven to serve as a drainage and haulage tunnel for the complex in the 1890s. Shortly after that, the owner of the Commodore Mine drove the Commodore Level 5 Tunnel to serve as a haulage tunnel for Commodore ore and to compete with the Nelson Tunnel.
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The Commodore Level 5 Tunnel is located above Nelson Tunnel and is connected by vertical mine workings. As the water levels periodically fluctuate in the largest mine pool of the Nelson Tunnel, water flows into the Commodore Level 5 Tunnel, allowing pressure relief on the largest impoundment before water returns to the Nelson further down gradient. In this way, the stability of the Commodore Level 5 Tunnel is critical the stability of the largest Nelson Tunnel impoundment. The Nelson Tunnel mine pools are heavily mineralized with elevated levels of zinc and cadmium.
DRMS monitored water levels and condition of the workings in 2016, 2017, and early 2018 and reported that conditions in the Commodore Level 5 Tunnel were deteriorating at an increasing rate. As the workings continue to deteriorate without intervention, further rockfall could completely block the flow path of water between the two tunnels, hindering the pressure relief that the Commodore Level 5 Tunnel currently provides. Such a blockage would also prevent any inspections of water levels beyond it, eliminating all knowledge of the amount of water impounded within the system.
The overall objective of the removal action is to prevent water from impounding in the Commodore Level 5 Tunnel, which will preserve current conditions and prevent further pressure from building on the Nelson Tunnel impoundments. Work will include:
- Stabilizing areas of Commodore Level 5 Tunnel to pervert further deterioration.
- Clearing much and debris such as rock fall from the tunnel roof or walls from the floor of the tunnel to prevent water from impounding.
- Maintaining and keeping vertical connections from Commodore Level 5 to the Nelson Tunnel
- Maintaining physical access between Commodore Level 5 Tunnel and Nelson Tunnel.
- Rehabilitating the portal gate structure and maintaining Commodore Level 5 Tunnel; walls and roof for worker access.
This removal effort will contribute to any future remedial effort at the Superfund Site by ensuring continued access to key portions of the Nelson Tunnel for remedial design and water level monitoring purposes. It is also intended to keep the Commodore Level 5 Tunnel open so it can relieve pressure on the Nelson Tunnel impoundments and thereby reduce the likelihood of a large, sudden, uncontrolled release of the Nelson Tunnel.
Note: throughout this site, locations inside the mine workings are annotated by "station" followed by a marking in this format XX+XX. This is a standard mining reference system, and is measuring a distance from the portal. For instance 1+60 is 160 feet inside, or inby ("in-bye"), of the portal.