Throughout the years the mining company Boliden Mineral AB have had difficulties producing a working fine filter for their tailing pond at the Aitik copper mine. The problem with the fine filter occurs when it gets placed on the tailing pond wall. Coarse grains break down into fine grains and the entire composition of the fine filter is changed. The material that is used to produce the fine filter comes from the mine’s own waste rock supply. The primary waste rock in Aitik comprises of heterogeneous gneiss and pegmatite. To determine why the waste rock isn’t holding together well enough the mechanical strength of the rock is investigated. Huge differences for the mechanical strength both exist between the different rock types, but also in the different kinds of the gneiss. The effect of the explosions, used to mine the ore, and the crushing machine also impacts on the mechanical strength of the rock. Good mechanical strength is found in rock that has a high amount of secondary transformation like epidote and bad mechanical strength from foliated rock. To get the best mechanical properties it is suggested to exploit epidote transformed rock found in deformation zones, adjacent to the ore deposit or the pegmatite intrusions. It is also recommended to use less powerful charge when blasting rock material that will be used for production of the fine filter.

The Aitik Mine is the largest copper mine in Europe and every year, 15 – 20 Mt waste rock is deposited in dumps located close to the mine. This waste rock can contain sulfides, which if exposed to oxygen can cause leeching and formation of acid rock drainage. To prevent this from happening, Boliden has designed a dry cover system to prevent leaching from occurring. The purpose of this study was to characterize existing stockpiles of till at Boliden’s Aitik Mine site in the context of requirements for the closure cover system. The till was evaluated primarily of its suitability for use in a highly compacted layer (HCL), since Boliden has a cover system design that has to provide the necessary control on oxygen diffusion rates to underlying waste rock material over the long term. The sampling took place on three of the stockpiles and a total of 31 test pits were excavated. A total of 27 samples of soil were collected and analyzed for particle size distribution (PSD) testing. Following review of PSD results, select samples were tested for hydraulic conductivity and compaction testing. The PSD results showed that all materials were fine-grained sand with some gravel, cobbles and silt. Results also show the amount of clay particles (0,004 mm) were relatively low across the three stockpiles. The result for the hydraulic conductivity were higher than Boliden’s prescriptive criteria. The results indicate that the sampled till would require addition of sodium bentonite to be able to construct a HCL with a hydraulic conductivity not exceeding Boliden’s criteria.