AN OVERVIEW OF UNDERGROUND MINE BULKHEADS IN A FRACTURED MOUNTAIN SETTING (Invited Presentation)

Underground hard rock mining tends to take place in fractured mountain settings for the simple reason that this is where the mineralization has occurred. While early stages of tectonic activity form mountains, later stages emplace minerals in the fractures through hot fluids or gas. The fractures that accumulate enough minerals of commercial interest become ore veins that are discovered and mined out. Historically, the mines are abandoned after the ore is mined out. Water then seeps into the fractures, mined out veins, and other mine workings where it combines with oxidized minerals to form sulfuric acid which then brings heavy metals into solution which then flows out as acid mine drainage (AMD). The AMD impacts the surrounding streams by providing a steady influx of acidic metal rich water.

Over time, partial mine collapses and sludge buildup can create underground dams that cause AMD to back up and form pools. The underground dams eventually fail and send a large surge of AMD and sludge out the mine portal. Such surges and sometimes all flows can be controlled by installing concrete plugs (bulkheads) in strategic locations. When designing a bulkhead, one has to consider both the specific bulkhead and the area it will impact. The bulkhead itself must be designed for the highest anticipated pressure without jacking the surrounding rock, shearing along the adit interface, structurally failing, dissolving, or letting excessive seepage past. This requires a detailed study and mapping of rock conditions, joints, and shears in the vicinity of the bulkhead. It also requires a comprehensive understanding of the mine workings, other nearby mines, veins, faults, and other fractures. Only after the effectiveness and regional impact of a bulkhead has been evaluated should it be installed.

This talk will address underground bulkheads in general and will then discuss the current rehabilitation work at the Gold King Mine and surrounding area as a case history.