Agnico Eagle Finland's Kittilä mine is Europe's largest primary gold producer. In 2018, a €160 million (US$180 million) expansion project began, with the aim of increasing production to two million tonnes per year. Work included sinking a 1,044m shaft to access deep deposits and raising a large tailings dam to increase waste capacity to 15 million tonnes.
The rockfill dam was raised by 5m to 242m. A total of 15,000, 700mm diameter dry soil mix columns, an average of 15.3m long, were installed by KFS Finland to improve the soft tailings beneath the dam, to mitigate the risk of failure of the upstream raise and to ensure safety and stability over the lifetime of mine operations. Consultants were Geobotnia and Golder Associates.
In situ shear vane testing was used to verify the undrained shear strength of the soil mix columns and the surrounding ground. However, results showed undrained shear strengths of just 80kPa, significantly lower than expected.
Either ground improvement was not performing as anticipated or there was a problem with testing. A solution was needed - and quickly - the costs of delayed dam construction and under-performing ground improvement could run into tens of thousands of Euros.
Lankelma was approached to provide more accurate undrained shear strength assessment, using CPT and in situ shear vanes. A tracked truck rig was mobilised quickly, arriving three weeks later. A total of 124 CPTs and 28 in situ shear vane tests were carried out over five weeks.
Temperature effects are the largest potential source of uncertainties in CPT
"A key challenge was dealing with the temperature difference between the surface and soils at depth," explains Joe Hobbs, Lankelma's technical manager. "As the columns cured, they emitted heat, raising the ground temperature. Temperature effects are the largest potential source of uncertainties in CPT, affecting load cells and strain gauges, particularly in weak soils, such as those at Kittilä."
To combat this, Lankelma used its own system that considers ground temperature during testing. This, along with a temperature stabilisation unit to control cone temperature, delivered more accurate data.
"We also used our vacuum chamber to rapidly saturate the piezocone, increasing the reliability and accuracy of pore pressure measurements, which was used to determine undrained shear strength directly," Hobbs adds.
Lankelma used its ‘push-on' vacuum chamber to rapidly saturate the piezocone, increasing the reliability and accuracy of pore pressure measurement
"This delivered far more accurate results than before, when a very small number of CPTs and vane tests were carried out - which had meant designers were having to rely upon a ‘minimum measured value', or ‘worst case scenario', approach.
"In contrast, the large volume of high-quality data we provided allowed a more rigorous statistical approach, leading to far more efficient and economical design. We also performed downhole seismic testing simultaneously, giving an alternative evaluation of liquefaction risk."
Results were available almost immediately, allowing Agnico Eagle to assess ground improvement performance.
Michel Julien, vice president, Environment, Agnico Eagle Mines, says: "We collected a lot of very good data and interpretation indicated the soil mixing improved the behaviour of the tailings significantly." This allowed work to continue, reducing delays and associated costs.
Agnico Mines engineer of record, Edouard Masengo, adds: "Lankelma's approach, and the design efficiencies that resulted, changed the way we specify and carry out CPT. We recently used the methodology on two Canadian sites."
Hobbs says, that, while Lankelma regularly carries out testing of deep soil mix columns, this was the first time it had done so in Finland. "We were really pleased this success, which has led to a number of other projects, including an investigation for another Finnish mine tailings facility."