By Michael Dickey, P.E., Director, Presto Geosystems
Mined materials are essential to our everyday lives. We use these valuable minerals in nearly every sector of the economy—they are necessary to construct roads and buildings, manufacture vehicles, build computers and generate electricity. Additionally, the mining industry stimulates economic growth by providing employment opportunities and generating tax revenue that helps fund vital public services, such as hospitals and schools.
As the mining industry navigates environmental, social and governance (ESG) issues, there is a sense of urgency to adopt sustainable or green mining practices. Green mining can be defined as technologies, best practices and mine processes that are implemented to reduce the environmental impacts associated with the extraction and processing of metals and minerals.
The mining industry faces unique challenges
When it comes to sustainable development, operation and closure of mines, the industry faces myriad challenges—not the least of which are poor soil conditions, weak subgrades and other geotechnical challenges that can complicate miners’ efforts to meet ESG goals.
In this regard, some of the more common geotechnical challenges that mine operators must contend with include:
- Constructing and maintaining heavy-duty haul roads
- Stabilizing and protecting slopes
- Tailings management and site reclamation
Compared to conventional methods, integrating geosynthetics into designs can help achieve a more sustainable approach to overcome many of these types of geotechnical challenges.
Heavy-duty haul roads
Haul roads are an integral asset to a mining operation. They are a critical component of mining infrastructure, and they directly influence the operational efficiencies of mine sites. Haul truck payload capacities range from 36 tons (33 tonnes) up to a staggering 450 tons (408 tons) (for ultra-class trucks), so these roads require special design considerations to withstand heavy and continuous traffic. However, extreme weather conditions and soft subgrades present challenges to both the construction and long-term performance of heavy-duty haul roads.
At a gold mine in Latin America, owners adopted a value engineering approach to optimize haul road design and minimize long-term costs associated with recurring maintenance and repair. They accomplished this by using the GEOWEB Cellular Confinement System to stabilize and confine road-base materials. The geocell system allowed for the beneficial use of abundantly available coarse river sand for the placement of base materials.
Due to the improved bearing capacity of the system, there was also a substantial reduction in overall section thickness requirements. This resulted in construction savings of more than $150,000 per kilometer. Additionally, because of improved road stability, mine operations saw a 65% reduction in maintenance costs as fewer grader passes were needed, and requirements for frequent replacement of wear course materials were substantially reduced.
In addition to haul-road applications, geocells can be used on slopes to resist sliding, prevent severe erosion from surface runoff and facilitate the construction of steep slopes.
The New Caledonia mine in the South Pacific—one of the largest nickel mining sites in the world—faced the formidable challenge of protecting the mine slopes from erosion due to highly erodible soils, mountainous terrain and extreme weather, including cyclones.
The site’s owner ultimately selected a slope protection strategy consisting of GEOWEB geocells within the GEOWEB Slope Protection System, infilled with waste aggregate. The system was designed to perform as a robust protective veneer over the existing terrain. The beneficial reuse of readily available on-site materials meant direct cost savings to the owner compared to previous, less successful methods.
Tailings management and site reclamation
Tailings are waste products that result from mining, crushing, grinding and chemically treating ore. Tailings storage facilities typically contain crushed rock, water and chemicals, and are designed to prevent the uncontrolled release of material into the environment during operation and closure. These facilities are often set up as settling ponds that are later capped in place during site reclamation activities.
At the Moon Creek abandoned mine site in Idaho, for example, reclamation efforts included the construction of a tailings repository for on-site disposal of 88,000 cubic yards (67,300 m3) of flotation tailings, contaminated soils and waste rock. Project objectives were to reduce the release of heavy metals to Moon Creek, rehabilitate the site to limit human health and ecological exposures, and improve trout habitat.
The tailings repository was built on a special foundation, allowing its construction over soft, saturated in situ tailings. Foundation elements included an impervious below-grade scour-protection berm, gravel and limestone wick drains, and a GEOWEB mattress filled with waste rock to provide a surcharge over the tailings and lateral load reinforcement.
Following the placement of waste materials, the repository was covered with an engineered cap composed of a geosynthetic clay liner, overlain by a gravel drainage layer and 2 feet (0.6 m) of soil. Lastly, an erosion protection mat was installed over the surface of the cap to provide root matrix reinforcement for the native grasses used in the seed mixture.
Design support and resources for mining applications
The engineering team at Presto Geosystems works closely with civil engineers, offering free project evaluation services and on-site support for mining haul roads, slope protection, basin containment, channel armoring and tailings protection.