Closure design for mine openings requires careful consideration of many factors, including but not limited to strength, installation method, transport and longevity.
Following the conclusion of mining activities (including decommissioning and remediation) responsibility for the site is typically returned to the government to be managed in perpetuity.
As a result of this process, one of the bigger design factors related to mine closure is longevity. Or, how long will the measures taken to close the mine remain safe and effective? This is a critical question because there is no time horizon on mine closures.
One of the more vital, yet vulnerable aspects of mine closure is sealing openings to the mine’s underground workings.
Unsecured openings to the underground represent a significant public safety risk and as such, a robust closure design is required. Accordingly, the Saskatchewan Research Council (SRC) set out to develop better closure solutions for the dozens of abandoned uranium mines dotting the province’s north.
These mines operated from the 1950s to the 1980s and the remediation of the sites is now managed as a part of Project CLEANS (Cleanup of Abandoned Northern Sites), an initiative of the Saskatchewan government that is being executed by SRC.
Most of these mine sites are in the Uranium City area. These closures are an important aspect of public safety, to prevent unauthorized access and to keep people and animals from falling into the openings.
Due to the location, logistical and economic challenges, SRC needed to develop something better than that prescribed in legislation, and the industry standard for vertical mine openings, which are generally constructed of monolithic poured concrete.
Building a concrete cap, a third of a metre thick, for an opening seven metres by three metres, requires six cubic metres of concrete, placed in one continuous pour. Transporting the materials to a remote location, constructing forms, and hand-mixing concrete on site to meet specifications, is both difficult and costly.
To address the aforementioned considerations, a clear definition of the design criteria was jointly undertaken by SRC and McElhanney Consulting Services. They then evaluated several closure materials including ultra high-performance concrete (UHPC), aluminum, weathering steel, galvanized steel, stainless steel and polyurethane foam (PUF).
Based on the design criteria, structurally reinforced stainless steel plate, custom-fitted to each mine opening, was chosen as the preferred method for the sites included in the 2013 field season. The caps were manufactured off-site to ensure that appropriate quality control measures were available as well as to eliminate shipment of extra material to the project sites. Several of these caps have now been installed.
The design procedure starts with a site investigation to obtain the dimensions of the opening and to determine the attachment points of the cap. The data points
describing the size and shape of the mine opening and surrounding ground are collected with a reflectorless total station. Due to the remoteness of the sites, return field visits to pick up missed information are not economically feasible if some information is missed. With this in mind, traditional measurements are also collected with a note pad and tape measure.
The design team then develops a 3-D image of the mine opening. This allows the team to optimize each cap by reducing the volume of material needed and determine the best way to fix it to the attachment points surrounding the openings while maintaining a well-defined load path from the cap to the ground.
The caps are then fabricated at a qualified shop in Saskatoon and inspected for compliance before being shipped to site. One of the design criteria is to minimize field fitting. However, allowances are made to modify the bedrock around the opening.
The stainless steel caps, weighing from 730kg (1600 lbs.) to 2800kg (6200 lbs), are generally trucked to Uranium City via the Athabasca ice road or by barge across Lake Athabasca in the open-water season. From Uranium City, the caps are transported to each mine site with a rock truck or semi-trailer depending on access conditions.
During installation the caps are positioned with an excavator, using pre-installed and balanced lifting points. Following positioning of the cap, the anchor plate holes are drilled in the bedrock, the stainless steel anchors are installed with epoxy cement, and columns are cut to length and welded to the cap and anchor plate. Once the epoxy cement has cured, the anchors are tightened and rendered tamperproof. Following installation and compliance inspection, skirting is installed and the stainless steel cap is pickled to improve its corrosion resistance.
The general construction of the caps consists of 6.4mm (1/4 in) sheet stainless steel, reinforced with stainless steel beams, which are supported with stainless steel columns to bedrock. A requirement to prevent people from crawling under the caps and into the openings is skirting to the ground. The skirting is non-structural and is constructed in the field from 4.8mm (3/16 inch) if installed horizontally, or 3.2mm (1/8 inch) if installed vertically. Fastening to the bedrock is done with threaded stainless rod, cemented into the rock, using epoxy.
Once closures and other remediation activities of the mine sites are completed, the sites will be entered into the Saskatchewan Institutional Control Program for decommissioned mines, which is managed by the Saskatchewan Ministry of the Economy.
The properties of stainless steel make it an ideal candidate material for engineered mine closures worldwide. The basic strength properties, relatively simple handling and manufacturing practices, and the inherent corrosion resistance makes stainless steel ideal for climate conditions encountered anywhere in the world. With any material there may be specific metallurgy required in certain regions. An example would be if there is a corrosive environment, such as coastal areas near salt water.
Future efforts will be to continue to reduce material use, fabrication and installation costs, and the environmental impact during installation, as well as provide closure solutions for increasingly remote sites.
There are also on-going design efforts for a component-based closure method, using pre-manufactured parts which will be field adjustable to suit any shaped mine opening in competent or fractured bedrock.
Each cap is unique in order to fit each mine opening. However, each cap shares similar requirements to reduce public safety risks posed by unsecured mine openings. Based on this requirement, stainless steel currently is the material of choice to provide long-term protection to the public, as well as the natural environment, from the hazards of legacy mines.
Information for this article provided by Jonathan Lambert, Dianne E. Allen and Christopher Reid. Jonathan Lambert, P.Eng. is a Structural Engineer and Project Manager in the Terrace BC office of McElhanney Consulting Services Ltd. Dianne E. Allen, M.Eng., P. Eng., PMP is a Remediation Project Manager with the Saskatchewan Research Council and Christopher Reid, EIT is an Associate Geo-Environmental Engineer with the Saskatchewan Research Council.