Nahanni Construction has been engaged on several projects related to underground stabilization work related to the remediation of Giant Mine in Yellowknife, Northwest Territories, from 2014 through 2022. Several contracts for underground stabilization and backfill projects have been awarded to Nahanni; leading to NCL becoming a key contractor in the Giant Mine Remediation. Throughout this work this unique project has provided the opportunity for Nahanni to become an innovator in the specialized area of underground concrete stabilization.

About the Projects

Initial Works (2014)

Work in 2014 – undertaken as a joint venture operating as Detoncho Nahanni Limited (DCN) – included stabilization of old mining stopes, some of which consisted of caverns the size of 12-storey buildings. Nahanni 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. This technology was implemented to ensure the backfill activities were completed as designed.

C5-09 Stope Complex Stabilization (2018 – 2019)

In 2018 – 2019 Nahanni returned to execute the underground mine backfill for the C5-09 Stope Complex Stabilization at Giant Mine, Yellowknife, NT. This involved deploying a non-segregating paste material, formulated on-site using reclaimed tailings. Nahanni managed the paste design, which included processed tailings, water, binder, and chemical additives. The comprehensive backfill program also included a 10-meter thick Self-Consolidating Concrete (SCC) formulation.

NN2111 – Early Works Drilling (2021-2022)

In 2021 -2022 Nahanni was engaged to lead the Giant Mine Early Works Drilling which involved intricately drilling multiple paste delivery and monitoring boreholes to access complex underground historical mine works. Notably, many of the borehole starts were in challenging areas such as pit bottoms, highwalls, and zones with organic material which required innovative solutions and tactical management of resources to ensure project success.

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 with hazards such as silica dust, arsenic trioxide, unstable ground, and dated supporting structures.  There is a care and maintenance operator and often several other contractors working at the mine site, which means a significant amount of coordination is 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 and the associated logistics on-site for delivery and staging were essential to ensure we met 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. The scope of work for stabilization projects included filling the required voids while keeping other parts of the mine open and unhindered by paste. This is an operation that is complicated by challenging drilling, monitoring, historical underground structures, and complex logistics on a busy legacy mine site.

How We Helped

During the 2014 program Nahanni engaged the support of external professionals and subcontractors who have experience with similar projects. Nahanni implemented a safety program for our site operations and in the fall and winter of 2014, excavated, relocated, and stock piled 20,000 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.

In 2018-2019 Nahanni returned to Giant Mine to complete further stope stabilization work. A unique slag cement design – designed by Nahanni – was used to meet the project requirements and support creation of very large monolithic concrete structures to stabilize historical underground mine works. This program also included highlights such the pour of a 10 meter thick, manufactured concrete plug within the C5-09 stope that creates a bridge within the void to stabilize the area. This monolithic pour was unique and never completed previously. Nahanni was very successful in completing this engineering feat and challenging scope of work.

The 2021 – 2022 drill campaign took place in the height of the COVID-19 pandemic where the Northwest Territories had significant travel restrictions and public health measures. Nahanni was able to manage within this complex environment to enhance it’s already strong safety program to include COVID-19 considerations, leading to zero outbreaks on site during this challenging time.

Interesting Facts

  • Nahanni received an award from the Slag Cement Association for the innovative SLC design and placement.
  • Monitoring and survey have been a key part of all underground activities. Nahanni has created a comprehensive monitoring program that includes traditional survey (total station, gps), down-hole surveys, lidar, drone survey, and 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 in the world, backfilling over arsenic trioxide that is stored in underground chambers using lightly cemented tailings
  • Nahanni crews were the last to set foot in some underground areas that have since been filled with paste; some of these spaces have been left void from mining since the mid-1900s.

By the Numbers

  • Batched and backfilled over 60,000 m3 of paste using reclaimed tailings during 2014 project.
  • A total of 112,000 m 3 of paste material was delivered from surface to the underground during the 2018 stope stabilization program, during the C5-09 Stope program 32,944 m3 of paste was delivered from surface along with 20,986 m3 of self levelling concrete were delivered into the stope in a monolithic pour.
  • During the 2021 – 2022 drill program:
    • 28 Paste delivery boreholes were completed for a total meterage drilled of 1264.5 m.
    • 17 monitoring boreholes were drilled from surface with a total meterage of 1485.2m.
    • 5 Underground monitoring holes were completed in the winter months using drilling rig and crew that could be mobilized to various portions of the mine using a 1 yard scoop.
    • A 370 meter pilot borehole was drilled with an accuracy of 0.8% utilizing a diamond drill and continuous wedging steering equipment, this completed structure will be used for future dewatering of the mine pool.
    • All boreholes were verified to be in a usable state; none required abandonment due to drilling outside required accuracy.