Engineering
Design and construction Report Phase IV - Composite Soil Cover
Acid waste Rock Study Heath Steele Mines
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.31.1b
November 1994
Summary
A 130 cm thick
composite soil cover was constructed on an experimental waste-rock
pile at the Heath Steele Mines site near Newcastle, New Brunswick.
The cover consists of a 30 cm thick sand base, a 60 cm thick compacted
glacial tilt 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 preconstructed pad test indicated six passes of a 5 tonne vibratory
compactor were required to attain the design specifications of 95
percent of the Modified Proctor maximum dry density at a moulding
water content of 2-3 percent wet of the optimum. These compaction
specifications also ensure that the till has a degree of water saturation
of at least 95 percent, 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
percent before cover to about 1 percent after cover placement The
decreased oxygen penetration implies reduced oxygen flux and acid
production. Volumetric water-contents averaged about 32 percent
in the till 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
indicate infiltration of 2 percent of precipitation during a 55
Day period when rainfall was heavy. The volume of the seepage has
been reduced but the quality from the pile has not changed since
cover installation. Further monitoring will be required to confirm
the reduction in acid production.
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