The Wharf Mine is located four miles west of Lead in the Black Hills, a heavily forested, mountain range in western South Dakota. Since 1983, Wharf Resources Inc. (WRI) has been successfully operating the Wharf open-pit gold mine and heap-leach operation. The property consists of several areas of adjoining gold mineralization amenable to open pit mining with WRI holding title to the surface and mineral rights of the claims. In 2009, gold production from the Wharf Mine amounted to 68,000 ounces of gold. Positive exploration results have extended the mine life to approximately 2021. In addition, the mine has achieved major milestones in health and safety, as well as progressive reclamation of previously mined areas.
When WRI wanted access to a site located nearby, a tunnel solution was needed underneath State Highway 473 near Stewart Slope Road. After State Highway 473 was realigned, twin, round 43’ x 98.69’ BridgeCor® Structural Plate structures were installed for their ability to accommodate CAT 777 haul trucks.
“FMG Engineering (FMG) contacted Contech Engineered Solutions for ideas on a grade separation structure that was large enough to pass CAT 777 haul trucks under the highway,” stated Richard Sudmeier, PE, Senior Engineer with FMG. “The solution FMG and Contech agreed would be best was two 43’ diameter structure plates constructed side by side under the highway.”
BridgeCor enhances the established performance of 6” x 2” MULTI-PLATE by offering nine times the stiffness and three times the strength. Its bolted, segmental construction was economical, easily assembled by Plate Erectors (a contracted plate assembly company) — which was part of Contech’s bid package to Zandstra Construction — and was manufactured with the industry’s best equipment allowing for the longest laying length sheets in the industry.
During the installation of the BridgeCor tunnels, the site received 5” of rain as well as some snow. This caused some scheduling challenges at the beginning, but once the weather subsided, the installation of the twin structures was completed in just 14 working days.
Approximately 15,696 square feet of Hilfiker Mechanically Stabilized Earth Welded Wire Wall was utilized as a low-cost, long-term soil stabilization solution. It was chosen for its easy assembly (even in remote site access locations), significant cost savings (when compared to other solutions) and ability to utilize locally available select fill which was also compatible with the tunnels.
“Contech provided FMG with guidance for the design, installation and backfilling of the structures throughout the design and construction phases of the project,” added Sudmeier. “Their willingness to assist FMG was greatly appreciated.”