Evaluation
of Field-Scale Application of a Shotcrete Cover on Acid Generating
Rock
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
2.34.1
September 1996
EXECUTIVE
SUMMARY
Northwest
Geochem, in conjunction with Powertech Labs Inc. has developed and
tested a cementitious material which incorporated mine tailings
as a cover for acid generating waste rock at Westmin=s Myra Falls
Operation. The project was initiated in 1992 under the MEND (Mine
Environment Neutral Drainage) program to assess the performance
of a cementitous cover. The project was funded by Westmin Resources
and the Canada Centre for Mineral and Energy Technology (CANMET)
through the British Columbia Mineral Development Agreement and was
implemented in three phases. The initial laboratory phase indicated
that the mixtures exhibited good mechanical strength and low permeability.
Leach testing indicated that metal release from the encapsulated
tailings materials was not a concern. The second phase of the research
focused on incorporation of flyash and polypropylene fibres into
the tailings mixtures and resulted in a product with good compressive
strength, good ductility, and low permeability to water.
In
the third phase of the study, presented in this report, a large-scale
field application of a shotcrete cover on a waste rock dump was
conducted. The primary purpose of this phase was to evaluate the
long term stability of the shotcrete in the field environment. Additionally,
a large scale test provided an opportunity to develop and use the
best practicable technology to install the shotcrete cover on reactive
waste rock.
The
shotcrete test was conducted on a 3500 m2 area of the
main Lynx waste rock dump at Westmin=s Myra Falls Operation. The
dump was recontoured to a grade of 22E and compacted prior to the
shotcrete application. A wet-mix shotcrete application was applied
in August 1992 using a remotely controlled robotic arm mounted on
a rubber-wheeled carrier. Mixes utilizing imported aggregate and
mine tailings were tested. Laboratory and field monitoring were
conducted through 1995 to determine the mechanical properties of
the shotcrete and to evaluate the long term performance of the cover.
The
results of the field trial indicated that the robotic application
system produced a good quality application with high rates of productivity
and a uniform placement of material. Some difficulties were encountered
which have led to suggested design modifications for the robotic
system. Visual inspections of the shotcrete cap over a three-year
period have indicated that the overall durability of the material
was good. No frost damage was evident and no movement of the cap
was detected by surveys conducted. Some cracks were observed and
appear to be correlated with areas where the shotcrete was applied
at less than the 75 mm thickness specified for the test. Some plastic
shrinkage cracks were observed in the shotcrete immediately after
application due to the high rate of evaporation before initial set.
The
results of the laboratory testing indicated that the compressive
strength of the mixtures exceeded the design objective. The toughness
index and flexural strength were lower than standard values for
shotcrete. Some reduction in compressive strength was observed in
the tailings mix after 400 days. It is believed this loss in strength
is a result of oxidation of the sulfide minerals in the tailings
material. Permeability of the shotcrete ranged from 10-14m/s
in the aggregate mix to 10 -10m/s in the tailings mix.
An assessment of the cost of the shotcrete application indicated
that the transport of the aggregate to the mine site is the largest
cost component. If a local aggregate was used, such as coarse tailings,
the total cost could be reduced by more than 30 percent. Modification
of the robotic spray boom and the delivery hose would increase the
rate of production by at least 30 percent.
The
results of this study have led to various recommendations for future
research requirements. Determining the effects on the shotcrete
cover due to the placement of overburden and vegetation is proposed
as the next study phase. Also of primary importance is the determination
of the effectiveness of the shotcrete cover in restricting acid
generation in waste rock. This study did not address this issue
and it is recommended that a controlled field scale test be conducted
to monitor acid production products in a capped dump. Due to the
high sulfur content of the presently available tailings material,
there is a need to evaluate other local material sources which could
be utilized as an aggregate source. Other recommendations include
the development of a more versatile robotic spray boom which can
manoeuvre on steep slopes, and modifications to the method of batching
and placing the shotcrete. Finally, the long term success of the
dry cover depends on the stability of the waste rock dump. Geotechnical
studies are required to estimate any movement of the final design
slope.
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