Construction and monitoring of a composite soil cover on an experimental waste-rock pile near Newcastle, New Brunswick, Canada

Abstract

A 130 cm thick composite soil cover was constructed on an experimental waste-rock pile at the Heath Steele mine site near Newcastle, New Brunswick. The cover consisted of a 30 cm thick sand base, a 60 cm thick compacted glacial till, a 30 cm thick granular layer, and a final 10 cm thick gravel layer for erosion protection. The till was compacted on the sand base in three finished lifts each of 20 cm thickness. Results of a preconstruction pad test indicated six passes of a 5-t vibratory compactor were required to attain the design specifications of 95% of the Modified Proctor maximum dry density at a moulding water content of 2–3% wet of the optimum. These compaction specifications also ensure that the till has a degree of water saturation of at least 95%, which is required to reduce oxygen and acid fluxes in the underlying pile. Quality control measures were taken during the construction to ensure the specifications were followed. Monitoring instrumentation was installed during the construction of the cover. Results indicate a reduction in gaseous oxygen concentrations in the pile from 20% before cover to about 3% after cover placement. The decreased oxygen penetration implies reduced oxygen flux and acid production. Volumetric water contents averaged about 32% in the till both immediately following cover installation and 7 months later. The water-content data are corroborated by soil-suction measurements. Temperatures in the pile have decreased following cover installation but appear to be more influenced by climatic variability than by a decrease in heat production and hence sulphide mineral oxidation. Observed discharge from two lysimeters, installed below the cover, indicates infiltration of 2–2.5% of precipitation during a 55-day period when rainfall was heavy. The quality of seepage from the pile has not changed since cover installation. Further monitoring will be required to confirm the reduction in acid production. Key words : waste-rock pile, acid generation, soil cover, suction, oxygen flux, percolation.