The liner is at the Kittilä Gold Mine in northern Finland.
By Bertrand Breul, Mikko Huru, Anton Palolahti
This article will describe the mine in general, including the opening date and the years of operations, and the purpose and construction of ponds to retain pollution.
The structure of the bituminous geomembrane (BGM) is more than 5.10m (5.6yd) wide and has different uses in hydraulic applications and environmental protection. The design and structure of the two ponds is described, including the weather variations during installation, as BGM allows an extension of the installation period. Also considered is the interest of the client in terms of schedule and the maximum aggregate size that the BGM allows, offering cost-efficiency for the client. Construction details, BGM installation and quality controls, maximum output, and installation are detailed.
This article will review and identify areas of cost and schedule savings due to the installation during bad weather and at low temperatures.
The Kittilä Gold Mine in northern Finland is owned and operated by Agnico-Eagle Mines Ltd.
Permission for mining operations was granted by the Northern Finland Environmental Permit Authority in 2002. The mine started its operations after three years of construction with an open pit at a depth of 150m (492ft). Subsequently, a tunnel was opened to a depth of 450m (1,476ft).
Key statistics at this mine:
- Mine area: 860ha (2,124 acres)
- Annual ore mined: 1,000,000 tonnes (1,102,000 tons)
- Annual siderock mining: 4,000,000 tonnes (4,408,000 tons)
- Annual gold production: 5 tonnes (5.51 tons)
- Roads: 4km (2.48mi)
- Pipelines: 15km (9.3mi)
Construction at the Kittilä mine (on the Suurikuusikko deposit in northern Finland), approximately 900km (558mi) north of Helsinki and 40km (25mi) from Kittilä (Figure 1), was completed in 2008. The Kittilä mine, named after the nearby community, is now an open-pit operation, with underground mining via ramp access at a later date.
The current mining operations feed a 3,000 tonnes ( 3,306 tons) per day to a surface processing plant. The mine opened in the summer of 2008 and currently the estimated lifetime is 15 years.
The Kittilä Mine site lies within the Arctic Climatic Region where daylight reaches a minimum of one hour per day in winter and a maximum of 24 hours per day in summer.
Temperatures are cool (Figure 2), with an average mean monthly temperature in July of 15°C (59°F) and in January of -15°C (5°F). The mean annual air temperature at the site is approximately 0°C (32°F). Winds are moderate and generally from the west. Average wind speeds are about 25km/hr (15.5 mi/hr). Snow falls in every winter month, although rain generally only occurs between May and October.
A watertight liner was required for the environmental permit from the Ministry of Environment. The purpose of these ponds is to store water produced from the mining process. Water from the ponds is then circulated back in, making it practically a closed system.
Construction at the mine site started in June, when the topsoil defrosts enough for excavation, and continued into November in 2007.
This project proceeded in two phases:
- The first phase in 2007 built the smaller pond and began the bottom of the bigger pond.
- The second phase in 2008 completed the big pond, including slopes, by mid-2008.
Dam walls were made from blasted rock with compound structure, meaning the main dam wall is made from maximum 600mm (236in.)-diameter grain size blasted rock.
The “wet” sides of the dam walls are made also from blasted rock, maximum 300mm (11.8in.) grain size; and gravel, grain size 55mm (2.2in.). These layers are also compacted with an excavator equipped with a compacting plate (Figure 5). The idea for these structures is to prevent the dam walls from breaking. The grain sizes differ so that they don’t get squeezed in from the water pressure. If there is a leak in the dam walls, the water will spread throughout the dam walls and come out like a simple percolation of water instead of making a big hole in the dam wall and causing a tidal wave.
The mineral sealing layer is constructed under the BGM layer. This layer is made by using locally excavated moraine that is compacted. The thickness of the mineral sealing layer is 1,000mm (39.4in.) with water permeability less than 5x10E-8m/s. The mineral sealing layer and the BGM act as a composite liner.
The bituminous geomembrane offers several advantages for this job:
- It can be installed directly over the prepared mineral sealing bottom layer. This provided a large cost saving for the ponds.
- The low thermal expansion coefficient allows keeping the liner exposed and flattened directly on the mineral sealing layer, which is a main point in the design. The combination of both moraine and BGM liner has a long watertightness lifetime because the liner is always flat and in contact with the glacial till support, which is the passive barrier (Figure 6).
- It can be installed at low temperatures such as -30°C (-22°F) for the elastomeric grade of the BGM liner.
- Installation is relatively fast and is less dependent on weather conditions, extending the construction season for BGM liners. The quick construction period means that the equipment used for the ponds can be used sooner for mine construction. For investors, this means that the mine operations are up and running much sooner, with the financial impacts potentially huge.
To optimize the cost for the design of the lining system:
- The composite barrier system is made when BGM is installed directly on top of the mineral sealing layer.
- For the bottom, the 3.5mm (0.14in.) bituminous liner was used and for the slopes 4.0mm (0.16in.).
- At the beginning of construction in the summer, the contractor used a blown bitumen impregnated membrane (called NTP grade), which could be used until 5°C (41°F). In September, when the temperature, mainly in the morning, could be below 5°C, a liner impregnated with elastomeric bitumen (called ES grade) was placed, which could be used to -25°C (-13°F).
The design, and especially the use of bituminous geomembrane, was subject to the authorization of public authorities affected by this project (from local to national decision-makers). The approval process took three months and included a public survey.
- a nonwoven polyester geotextile, with mass per unit area of 200–400 grams/m2.
- a glass fleece reinforcement that provides stability during fabrication and contributes to the strength of the geomembrane.
- a bituminous mastic consisting of a blown 100/40PEN bitumen and filler. This mastic impregnates the entire structure and provides the waterproofing capability; in addition, it ensures the longevity and the high resistance of the product.
- an industrial film bonded to the underside when the membrane is hot, which prevents penetration of the membrane by plant roots.
- a coating of fine sand on the upper surface to provide greater traction on slopes, giving greater operator safety and security, and to give protection from the degrading effects of UV radiation.
The bituminous geomembrane used is composed of a combination of needlepunched, nonwoven geotextile, glass reinforcement, and bituminous impregnation. This system results in a long life expectancy due to its resistance to high levels of mechanical stress and negligible ageing characteristics.
Various grades of the material are available, appropriate to the end uses, in thicknesses from 3.5mm (0.14in.) to 5.6mm (0.22in.). This reflects the wide range of uses for this geomembrane, from landfill lining and capping to the protection of groundwater from contamination in hydraulic, environmental, or transportation applications.
Bituminous geomembranes are used in many applications and mainly for the protection of environment:
- solid wastes
- confining domestic, industrial, and mining wastes
- storage of liquid wastes, tailings dams and ponds
- ponds for recycling water
- biogas barriers
- hydraulics: dams, canals, and protection of aquifers and rivers
Prep and welding
In preparation for the installation, two workers removed sharp gravel and natural organic fill from the surface with rakes and another worker operated a compactor.
The installer used a hydraulic beam for the slopes and a mechanical beam for the bottom. Two workers unrolled the geomembrane and one other operated the excavator. Two welders were needed to weld the joints and one assistant to roll and seal the joints after welding (Figure 8).
One worker was needed to visually inspect the quality of the seam and finish the joint. One worker followed the quality manager to do the patches.
- On average, approximately 5,000m2 (5,980yd2)/day was installed with two welding teams.
- In some instances, mineral sealing layer work was delayed because of heavy rainfall.
One lab technician from the general contractor was trained by a supervisor from the distributor to follow the quality control plan. All seam testing and destructive samples were documented daily and recorded in a computer-generated report.
An ultrasound testing probe was used to check and ensure quality on the entire 20cm width of the seam (Figure 9). Ultrasound waves are able to detect imperfections in the seam and to measure the width of the defect; patching was done accordingly.
As a result, there was good compliance with the QC/QA requirements despite the sometimes difficult conditions. This procedure was straightforward, and the contractor and consultant on-site were trained and certified by the distributor/supervisor at the project to do this procedure on their own.
The completed result was the successful construction of two ponds. The use of bituminous geomembrane provided several benefits, including cost savings, efficient installation even in cold weather, and fully documented QA/QC from start to finish.