Barrick Gold Corp.’s Cortez Hills gold mine, located approximately 35 miles (56 km) from Battle Mountain, Nevada, USA, represents a major expansion project for the world’s largest gold mining company. The operation comprises a reopened surface mine and an all-new ramp-accessed underground mine. The reopened mine features major upgrades to all facilities and an upcoming mill renovation, with the new underground facility requiring a major multipurpose concrete and backfill plant.
The underground mine uses conventional concrete for structural construction and water control, and some pavement as required. Shotcrete is produced for overhead rock stabilization and tunnel support, and backfill (concrete made from unclassified waste rock material overburden/ore). Production of these concrete materials is expected to exceed 1 million yd3 annually; with the underground mine anticipated to produce 1.08 to 1.12 million oz of gold over the next 10 years.
For this particular multipurpose batching plant project Barrick contacted specialty design-build contractor Plant Architects + Plant Outfitters (PA+PO) to develop an early conceptual design as the mine was being staged for reopening. A year later the mine issued a prequalified bid call to three firms to supply the large-scale plant. PA+PO was chosen to design-build the entire facility based on a turnkey offer that included 100% of the structural steel and storage bin fabrication being completed onsite, at an area adjacent to the dual mine ramp portals.
Conceptual Design Considerations
Robert Ober, CEO of PA+PO, had been involved in the design and installation of several multipurpose backfill plants in northern Nevada over the past two decades. In the past, Ober had utilized various plant and concrete compulsory-mixer manufacturers for these installations. In the case of the Cortez Hills plant, he teamed up with his design partner of the last several years, Lloyd Garcia, on a plant that would actually be manufactured on the mine site by PA+PO welders and fabricators, and installed by the crews of PA+PO ironworkers, millwrights and electricians—a total design-build project.
Garcia and Ober assigned a design team that consisted of an architectural designer, architectural engineer and a registered architect. Rounding out the design team was a team of engineers including a registered structural engineer and a registered civil engineer, with several design engineers tending to the material handling systems.
The design team produced a rendering of the overall enclosure of the cold weather plant first, for a limited footprint located just aside the twin portals of the underground mine, and to assimilate the traffic pattern and access for the 20-ton articulating dump bodies and underground mixer truck chassis.
Access to the plant for three aggregates (a blend sand to 3/8 in., a classified 1-in.+ and the unclassified waste rock for backfill) was to be from the hilltop above and adjacent to the portals. The three aggregate bins would total 1,000 tons, and the dual powder silos would each be sized at 250 tons—requiring a large-sized enclosure capable of protecting the plant equipment: a 10.5-yd3 (8,000 liter) output twin-shaft compulsory mixer, 3.5 million Btu hot water heater, dual ramped wash-water weir pits, admixture storage, batching hopper and dual apron feeders for the offset two aggregates, with the sand gravity fed into the 12-yd3 cumulative aggregate batcher to a 60-in. (1,524-mm) high-speed transfer belt.
The all-weather enclosure required a heavy snow load design and wind load capability—not to mention the fact that the plant location was to be in close proximity to a fault line and the resulting seismic requirements. Garcia and Ober’s design team chose a sandwich panel siding of aluminum powder-coated exterior and stainless steel light colored reflective enclosure panel of 4 in. (100 mm). The plant was designed as cut into the hillside.
Undertaking an onsite fabrication of a large-scale plant such as the Cortez Hills plant required a crew of 55 personnel working seven days per week for five months. The onsite fabrication is a trademark of the PA+PO crews and was used here in lieu of a factory purchased option due to the totally custom nature of the design and the savings afforded by not utilizing bolt-together structures, but rather fully welded steel structures and plate sub assemblies, which would have required some 35 truckloads if fabricated offsite and transported to site.
Site Infrastructure Requirements of the Design-Build Team
The all-welded steel structure of the plant was designed to sit atop a floating monolithic concrete slab of four levels of 1-in. reinforcing steel with 2-1/2-in.-diameter anchor bolt embeds to affect the requirements of the seismic zone for both the mixer and powder silos tower structure, as well as the 700-ton aggregate structure. PA+PO construction crews together with selected excavation and concrete subcontractors completed the infrastructure required in less than three weeks time working 14 hour shifts seven days per week. The concrete work was hurried prior to the early arriving winter weather in this high desert locale.
The plant was originally to be placed into a cut made into the highwall above the portals of the underground mine; however, shortly after the project was started in fabrication Barrick decided it would be best to have PA+PO fabricate a steel structural bridge as integral to the plant, with a concrete deck for the dual 988 Caterpillar loader feed of aggregates from the topsides of the plant arrangement. This required PA+PO undertake a 16-ft-deep (4.8-m) concrete abutment for slope stabilization and bridge anchoring, and a 24-in.-thick (600-mm) concrete bridge deck. PA+PO fabricated a 75-ft (23-m) structural steel bridge that its engineering department completed in-house and sent out to a third-party structural engineering firm for certification. This loader access bridge was designed to also offer topsides access from the load-out level below via a series of stairs and catwalks. The topsides elevation is 68 ft above the plant discharge level below.