The Det’on Cho Nahanni Construction (DCN) Ltd. won the $11.6 million contract for Interim Underground Stabilization in August 2014 through a competitive process. Under the two-year contract, DCN engaged in a design-build contract with support from a full range of professional consultants and contractor services.
About the Project
The work included stabilization of old mining stopes, some of which consisted of caverns the size of 12-storey buildings. DCN assessed the empty chambers, determined the requirements necessary to stabilize these chambers, and developed the plan to backfill the voids with paste material. The backfill paste was created with reclaimed tailings allowing for an on-site source of material, and reducing the need for trucked in aggregate. This paste was used in the stabilization of each underground chamber and reduced further degradation of both the crown/sill and rib pillars. Paste material was delivered to each underground stope with use of boreholes from the surface. A Nahanni-designed quality control program included the use of a wireless network of live-feed cameras as well as laser cavity scans where necessary. This technology was implemented to ensure the backfill activities were completed as designed.
The Challenges
The Giant Mine site is located within the city limits of Yellowknife, the capital of the Northwest Territories and home to almost 20,000 people. Arsenic trioxide dust that accumulated over 46 years of mining activity at the site was pumped underground for final storage. These areas needed to be stabilized to ensure chambers are kept intact to protect the environment and nearby community. There are significant health and safety considerations that go along with working on the site that include contaminants, silica dust, unstable ground, and dated supporting structures. There is a care and maintenance operator at the mine and often several other contractors working at the site. There is a significant amount of coordination necessary to operate effectively at the site and communication with contractors and on-site staff was important for project success. Timely supply of equipment, materials, and supplies is essential to ensure that DCN meet our scheduled commitments. Being situated in a northern location, Giant Mine exists in a climate that creates limitations and constraints in performing the paste production and delivery. The site is regulated by the Mackenzie Valley Land and Water Board and a water license is in place for the site that must be adhered to. This project involves filling the required voids while keeping other parts of the mine open and unhindered by paste. This is an operation that is complicated by drilling, monitoring, and underground distribution logistics on a busy legacy mine site.
How We Helped
DCN engaged the support of external professionals and subcontractors who have experience with similar projects. DCN implemented a safety program for our site operations and in the fall and winter of 2014, excavated, relocated, and stock piled 45,000 m3 of tailings. In the spring of 2015, monitoring cameras were set up throughout the underground mine and within the stopes. Underground fill barricades were constructed to contain paste within the areas to be backfilled, and processed tailings were turned into cementous paste by combining the tailings with water and binder in a custom designed and built batch plant. Six stopes (four non-arsenic, and two arsenic) were filled with paste to provide stability and prevent flooding to other areas of the mine.
Interesting Facts
- Batched and backfilled over 60,000 m3 of paste using reclaimed tailings.
- DCN created an underground wireless communication network for viewing live camera feeds and communication during backfill operations, with remote access/off-site viewing capabilities.
- First project of its kind backfilling over arsenic trioxide using tailings – in the world.
- Nahanni crews were the last to set foot in the underground areas that have since been filled with past; spaces left void from mining since the mid-1900s.