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 22
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.