Award of Excellence: Tailings Management at Greens Creek Mine

The Hecla Greens Creek Mining Company’s polymetallic mine (silver, gold, zinc and lead) is located on northern Admiralty Island in Alaska. A portion of the mine facilities located within Admiralty Island National Monument.

The island is well known for having one of the largest populations of brown bears in the world, and it is home to many species of salmon, birds, whales and deer, which makes it a popular destination for tourists.

To have a licence to operate in such a sensitive area, mines have to have a safe tailings disposal method. The solution at Hecla Greens Creek Mine is the world’s first large dry stack filtered tailings disposal system. The approach has several advantages: it reduces the footprint of the tailings storage by over 50%, with no tailings pond; it allows wildlife to roam freely without risk; and it allows for progressive and continuous reclamation of the land. As well, dry stacks can withstand static and seismic forces.

This was the first mine in the world to adopt the filtered tailings disposal method and the facility has been operating the longest.

Klohn Crippen Berger’s involvement with the tailings disposal facility dates back to before the mine was built, to the pre-feasibility and conceptual design stages in the early 1980s.

The mine began operations in 1989 and constructed the first stage of the dry stack facility, which was designed by others. Following a brief shut down due to low ore prices in 1993 the mine re-opened in 1996 and Klohn Crippen Berger was asked to assist with further development of the pile, including an expansion to the tailings disposal area. Further expansion of the mine up to an additional 50 years is important to the economy of Southeast Alaska and the performance of the tailings area to date is a key determinant in the permit application.

To help manage the design, construction and operation of the expansion, KCB had to determine whether dry stack tailings could withstand a major seismic event. They had to develop a tailings placement plan that could be implemented in a harsh and remote climate while meeting strict regulatory requirements, including federal and state air and water quality standards. And they had to develop solutions that would coexist with wildlife and would have a reduced visual impact.

During the course of this work, major advances were made in understanding the seismic behaviour of filtered tailings, and the methods for design and construction of such facilities in a cold, wet environment.

Thanks to the technical advances, demonstrated excellence, and environmental stewardship of the pile development and expansions from 1996 to 2012, the mine operators were able to build the confidence of the regulators to permit future additional expansions.

Water diverted from stack

The work on the tailings disposal facility included installing geomembrane-lined cells, upgrading the water management system, and constructing infrastructure to support the mine operations.

New water management structures were designed to intercept and divert surface water and groundwater (non-contact water) away from the tailings stack. The tailings stack also incorporates internal drainage structures to promote low phreatic levels. Water coming in contact with the tailings is collected and treated to meet permitted water quality standards, and ultimately discharged to the environment.

Innovative construction techniques were developed so that tailings can be placed in small areas during all but the wettest of weather. The surface is roll-sealed and gently sloped to prevent water infiltration. Runoff is collected in settling ponds designed for storm flows. Water from the ponds is then sent to a water treatment plant for eventual discharge to the environment.

Conventional methods of in situ testing were not conclusive in regard to the seismic performance of the tailings, so it was decided to undertake a sampling and laboratory testing program. To obtain the high quality samples needed for laboratory testing, a new apparatus was developed for taking undisturbed samples. The samples were carefully transported to a special laboratory at UC Berkeley in California. The design work, construction observations and innovative testing demonstrated that further expansions could go ahead with confidence.

Greens Creek is a flagship project and an example for other operators of the application of best practice tailings storage. The mine is able to safely coexist with one of the densest brown bear populations in the world and bears can regularly be seen roaming unharmed over the landform. Also, during site development, careful attention was paid to sight lines, which means nearby passing cruise ships can still have uninterrupted views of the wilderness.

The testing information from the project is being further studied by Klohn Crippen Berger in collaboration with the University of British Columbia to benefit tailings management practices across the industry. cce

Name of project: Innovative Tailings

Management, Greens Creek Mine, Alaska

Award-winning firm (prime consultant, structural engineer): Klohn Crippen

Berger, Vancouver (Len Murray, P.E.,

Rick Friedel, P.E., Lowel Constable,

EIT, Daniel Klassen, EIT, Howard Plewes, P.Eng., Al Morrison, P.Eng.)

Owner/client: Hecla Greens

Creek Mining Company

JUROR COMMENTS

The project successfully overcame many challenges to achieve a unique solution to tailings management. Driven by a desire to be environmentally responsible and address issues of seismic stability, the project shows a leadership in this area that makes it worthy of recognition.