DESIGN OF A COMPOSITE SOIL COVER FOR AN EXPERIMENTAL WASTE ROCK PILE NEAR NEWCASTLE, NEW-BRUNSWICK, CANADA

The Heath Steele Waste Rock Project was initiated in 1989 under Canada 's Mine Environment Neutral Drainage (MEND) Program to develop and tes t strategies for managing acid-generating waste rock. The multiphase p roject involved the identification and selection of a few waste rock p iles for field evaluation at the Heath Steele mine site located at abo ut 50 km northeast of Newcastle, New Brunswick. As part of the evaluat ion, a 0.25-ha acid-generating pile, pile 7/12, was relocated and reco nstructed on an impermeable synthetic membrane by end dumping from the perimeter and pushing into the middle section. The pile, which contai ns about 14 000 t of mine waste rock, has been producing an acidic see page characterized by high dissolved iron (3.5-13.5 g/L) and sulphate (12.7-43.4 g/L) concentrations. Following the definition of the baseli ne acid-generating characteristics of the pile and laboratory investig ation of potential soil cover materials in the vicinity of the site, a three-layer cover design is proposed. The design calls for a 60 cm th ick saturated impermeable cover sandwiched between a 30 cm thick sand base and a 30 cm thick, overlying granular layer. The principal object ives of the design are to obtain a low gas diffusion coefficient to mi nimize oxygen fluxes and, also, to attain a low hydraulic conductivity to reduce infiltration into the pile. Both objectives can be achieved by compacting the impermeable cover at a density of 95% of Modified P roctor or greater and a water content slightly higher than the optimum value. The design of the cover and the anticipated resulting low gase ous-oxygen fluxes are confirmed by one-dimensional diffusion modelling . The potential for the impermeable layer to remain nearly fully satur ated, even under an evaporative flux, is demonstrated by flow modellin g. It is noted that the assessment of the durability of the cover with respect to variable climatic conditions (drying, freezing, and thawin g) is a critical component of the performance evaluation.