One Giant Remediation Project
The Giant Mine in Yellowknife produced gold from 1948 until 1999 when it went into receivership and the mine was transferred to Indian and Northern Affairs Canada (INAC).
Another company, Miramar Giant Mine Ltd. bought the mining operation on condition that it would not be responsible for the clean-up. In 2004, Miramar finished working the site and it officially became an abandoned mine.
INAC remains responsible for the site and contracted Deton’Cho Nuna Joint Venture to continue the required maintenance and environmental management activities.
The pollution problems are numerous; chief among them is the massive store of poisonous arsenic trioxide. Approximately 237,000 tonnes of the dust was produced over the mine’s life and stored underground in 15 purpose-built chambers and mined-out stopes. It must be prevented from contaminating ground water. To accomplish this, the dust will be frozen in place by first installing pipes below and around the chambers and stopes, and then pumping a coolant through the pipes. The technology is similar to that used to create hockey rinks. It has also been used to prevent groundwater inflows to other underground mines and, at a smaller scale, to isolate areas of contaminated soil.
Once the dust and the surrounding rock are completely frozen, the freezing system will be converted to thermosyphons. These are tubes filled with compressed carbon dioxide gas that act as completely passive heat pumps, so they will continuously cool the ground without any input of energy and only minimal maintenance.
There are eight pits on the site, five of which are substantial in size. One of the largest pits, named B1, must be backfilled to allow for the installation of a ground freezing system. All contaminated soils from other areas on the site will be in the portion of the pit that will ultimately be within the frozen zone. Waste rock, quarry rock or clean demolition waste will be used to fill the remainder of the pit. The entire backfilled area will then be covered with soil and revegetated. The other pits will be surrounded by berms or fences to prevent inadvertent public access.
More than 100 old mine buildings remain on the site. Many of these buildings pose a hazard to the public so all structures are being removed. Any arsenic-contaminated materials will be placed in the empty chamber 15. As well the public highway through the site was relocated to keep traffic away from the demolition, soil cleanup and ground freezing activities.
Yellowknife’s Giant Mine Remediation Project got underway as early as 2006 when SRK Consulting was hired to develop a remediation plan. By May 2007, SRK issued its report.
Early estimates to cover all deconstruction and remediation expenses, including design engineering, ongoing and long-term maintenance, amounted to more than $900 million. That figure is presently being revised and will likely be higher.
Total project expenditures to date are approximately $335 million, including human resources in the order of 20 to 100 people per day depending on the season and the work taking place at the site.
In 2014, the roaster complex had to be dismantled.
“It was a very contaminated building complex and had to be painstakingly taken down piece by piece,” says Natalie Plato, deputy director of the Giant Mine Remediation Project.
Engineering firm Parsons was hired to do the Roaster decontamination and deconstruction. Since then Parsons has acted as interim construction manager for the Project. Parsons contracted local joint venture firm Det’on Cho Nahanni Construction Ltd. to conduct surface diamond core drilling during 2016 to gather information about near-surface underground voids. Previously Golder Associates did some of this drilling work in 2015 and they sub-contracted local firm McCaw’s Drilling and Blasting to assist with the work.
Also in 2016, the Project team found a number of other structures had deteriorated: the vintage 1945 A-Shaft head frame, curling club and assay lab. The buildings were dismantled in September, 2016. The contract to deconstruct the structures was awarded to RTL Construction Ltd. in July 2016.
Work will be ongoing with the full remediation expected to take six to 10 years. That work should begin in 2021 says Plato. In the meantime, INAC is readying to hire a main construction manager.