Étude des processus chimio-hydro-mécaniques dans un massif rocheux fracturé perturbé par une exploitation minière :

Abstract

This study on the chemical, hydraulic and mechanical processes and their coupling, was conducted at the Bouchard-Hébert Mine, located in the Abitibi Mining District in Québec, Canada. In this mine environment, the acidic water migrating from the old mine workings to the new ones, in combination with the perturbed geomechanical stress field caused by the excavations, affect the rock mass permeability. Both field work and laboratory experiments were conducted in order to investigate these processes, particularly in the rock mass portion located between the two mine workings (drift at level 6A).Results of discontinuity mapping show that at the western part of the drift at level 6A (from 10600E to 10900E), the rock mass contains two sets of fractures: one set is subhorizontal (dip between 0° and 30°) with a trace length between 1 and 1.5 m, and the other set is subvertical (dip between 75° and 90°). This subvertical set is divided into two groups depending on their trace length: one group includes fractures with a trace length between 0.5 and 2 m, and the second group comprises fractures with a trace length between 3 and 3.5 m. The fracture density of these two sets of fractures varies from 1 to 6 m" 1 and their veins are 2 mm in width. At the eastern part of the drift at level 6A (from 10900E to 11200E), the rock mass contains the same two sets of fractures observed at the western part, plus a set of fractures with a moderate dip (between 15° and 45°) with NE trend. This later set of fractures is less frequent and has a variable trace length between 0.5 and 2.5 m; their veins have a width of 2 mm. The fracture density of all the fracture set is between 1 and 4 m' 1 . Although the rock mass at the eastern part of the drift at the level 6A contains three sets of fractures, it is less permeable than the one located at the western part considering its lower fracture density. Consequently, grounwater flow would be more intense in the rock mass at the western part of the drift at the level 6 A of the mine.The results of hydraulic testing in boreholes in the rock mass located immediately under the drift 6A and over a lengh of 48 m, have shown that in the eastern part of this excavation, the hydraulic conductivity of the rock mass is low and constant, between 1 xlO" 10 and 1 xlO " n ms"1 . In the central part of this drift, located just under the old excavations, the rock mass hydraulic conductivity is higher and increases with depth from 1 xlO" 11 and 1 xlO ' 6 ms" 1 , which is in accordance with the higher fracture density in this part of the drift.Hydrogeochemical results show that waters sampled from boreholes at the mine are of neutral pH and contain high concentrations in ions, such as Ca, SO4 and metals. Calcium originates from the dissolution of calcite which is in abundance in fractures. Sulfate and metals originate from sulfide mineral oxidation present in fractures as well as in the rock matrix. Waters at a bulkhead at the base of the old excavations and the mixing waters at the surface, pumped from all ...