Design of a composite soil cover for an experimental waste rock pile near Newcastle, New Brunswick, Canada

Author:

Yanful Ernest K.,Bell Alan V.,Woyshner Mark R.

Abstract

The Heath Steele Waste Rock Project was initiated in 1989 under Canada's Mine Environment Neutral Drainage (MEND) Program to develop and test strategies for managing acid-generating waste rock. The multiphase project involved the identification and selection of a few waste rock piles for field evaluation at the Heath Steele mine site located at about 50 km northeast of Newcastle, New Brunswick. As part of the evaluation, a 0.25-ha acid-generating pile, pile 7/12, was relocated and reconstructed on an impermeable synthetic membrane by end dumping from the perimeter and pushing into the middle section. The pile, which contains about 14 000 t of mine waste rock, has been producing an acidic seepage 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 baseline acid-generating characteristics of the pile and laboratory investigation 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 thick saturated impermeable cover sandwiched between a 30 cm thick sand base and a 30 cm thick, overlying granular layer. The principal objectives of the design are to obtain a low gas diffusion coefficient to minimize 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 Proctor or greater and a water content slightly higher than the optimum value. The design of the cover and the anticipated resulting low gaseous-oxygen fluxes are confirmed by one-dimensional diffusion modelling. The potential for the impermeable layer to remain nearly fully saturated, even under an evaporative flux, is demonstrated by flow modelling. It is noted that the assessment of the durability of the cover with respect to variable climatic conditions (drying, freezing, and thawing) is a critical component of the performance evaluation. Key words : acid rock drainage, soil cover, capillary barrier, oxygen flux, infiltration.

Publisher

Canadian Science Publishing

Subject

Civil and Structural Engineering,Geotechnical Engineering and Engineering Geology

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