Hydro-geotechnical and geochemical properties of self-weight consolidated paste backfill

9th International Symposium on Mining with Backfill
Omar El Aatar, Mostafa Benzaazoua, Bruno Bussière, Erol Yilmaz,
Abstract Paste backfill technology is becoming a common practice in underground hard rock mines worldwide, particularly in Canada. Cemented paste backfill (CPB) is composed of mill tailings which are mixed with water and binding agents such as Portland cements, lime, pulverized fly ash, and smelter blast furnace slag. The use of CPB not only provides ground support to the pillars and walls, but also prevents caving and roof falls and enhances pillar recovery, which improve productivity. However, CPB remain a complex material whose hydro-geotechnical and geochemical properties vary during curing. For example, it was observed in some Canadian underground mines that the uniaxial compressive strength (UCS) of in-stope cured CPB samples was 2 to 4 times higher than the UCS of in-plastic mould cured CPB material prepared in lab from the same mix recipe. The reason behind this important strength variation can be partially attributed to the CPB hardening conditions that are different in the field than in the laboratory. The in situ hardening conditions are influenced by various factors such as stope dimensions, characteristics and geometry, bleeding, drainage of part of the pore water, and the self weight-consolidation settlement.
The main objective of this study is to better understand the effect of underground CPB placement conditions on its microstructural, hydro-mechanical and geochemical properties. To reach this objective, three PVC columns, each 3 m high, were built and were filled with CPB material. The columns allow simulating undrained (UD), half-drained (HD) and fully-drained (FD) conditions along with the measurement of resulting self-weight consolidation settlement of the CPB. Bleed and drainage waters were also collected from each column for geochemical analyses (pH, Eh, EC, sulphates content). After 90-day of curing time, a total of sixty CPB samples were cored from the backfill columns and subjected to uniaxial compression tests followed by mercury intrusion porosimetry (MIP), along with a physical characterization (Gs, void ratio, water content, degree of saturation, specific surface area). The results showed that the total drainage water and the maximum observed settlement occur mainly within the first 72 hours after the columns are filled. The corresponding volumetric strains suggest that in situ backfilled stopes simulated behave in a similar way to the fully-drained (FD) or the half-drained (HD) conditions. The MIP test results show that the pore structure varies between the top and the bottom of each column.
Keywords: Paste backfill, Backfill settlement, MIP Porosity, Column Tests, Backfill drainage
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