Red and Bonita Mine Bulkhead Evaluation
The Red and Bonita Mine portal sits on the east side of Cement Creek eight miles north of Silverton, Colorado. The 5-ft-wide by 7-ft-tall adit divides into two main branches, with several exploratory drifts and stopes each, through the Burns Member of the Silverton Volcanic sequence. Ground support consisted of timber sets at the portal, Station 6+50, and at several other locations. The remainder of the adit was bald.
In 2013, after an assessment by the EPA, the Colorado Division of Reclamation, Mining, and Safety (CDRMS) entered the adit as far as possible to assess and document the rock, existing ground support, groundwater inflow areas, cross cuts, stopes and other connections to existing mine workings, map the geology, map the joints, and evaluate potential bulkhead locations. Schnabel provided a third-party review of the ground support, ventilation, and portal structure adequacy. Our work included a review of existing geologic and mining documentation, rock mass classification, and key block analysis. CDRMS designed a bulkhead that was installed in 2015 by EPA contractors during a removal action. We designed a high-pressure valve upgrade for the bulkhead.
Shotcrete support was added near the portal in 2016. From 2015 to 2017, we conducted a third-party evaluation of the potential effects of closing the bulkhead valve. We considered the impacts of a closure on the regional groundwater table, streams, and existing mines such as the Gold King, Mogul, Adams, and Sunnyside. Our evaluation focused on the nearby American Tunnel, with its three closed bulkheads, and recommended methods for monitoring water levels. We also evaluated the design, capacity, and potential failure modes of the tunnel’s bulkheads and installed a monitoring well. In 2020, we coordinated a bulkhead closure test with detailed monitoring or piezometers, seeps, and underground in several adits.