The Gold King Mine Level 7 portal is located on the North Fork of Cement Creek, approximately eight miles north of Silverton, Colorado. After the August 2015 uncontrolled release of mine drainage, Schnabel prepared plans and specifications for mine stabilization and restoration. This included stabilizing the portal area with rock bolts and shotcrete, with backfilled corrugated steel arches in the future.

Inside the mine, we evaluated ground conditions and designed support. The initial support was comprised of rockbolts, mesh, and shotcrete. The final support design consisted of fully lagged steel sets on a concrete foundation, backfilled with cellular concrete. An interesting aspect of this project is the design of a low-pressure flow control structure. This structure was needed to prevent an uncontrolled surge of mine waste and limit the flows exiting the portal to levels that can be treated by on-site waste treatment systems. Due to the existence of a nearby old portal and the fact that hydrogeologic evaluations are not complete, this structure could not be a standard mine bulkhead. It would have to be a medium-term structure capable of holding back 30 feet of waterhead while providing machinery access through it for future work.

For this structure, we designed a stainless-steel frame with HDPE-backed, removable stop logs. The stop logs include valves for water control, instrumentation, and ventilation. During construction, we provided part-time construction engineering services and assisted in coordination between the EPA, surveyors, contractors, and other engineering firms.