REDPATH MINING – RIO TONTO
Project Overview
The Kemano Special Area Grouting Project was located in the T2 tunnel of the Rio Tinto-owned hydro
generating station in Kemano, British Columbia. The Tunnel 2 known as T2 was started in the 1950’s but
was abandoned halfway through its construction. Tunnel 1 known as T1 runs parallel to T2 and is the only
form of water transportation used to transport water from the Nechako basin to the hydro-generating
turbines located in Kemano British Columbia. T1 is at high risk of collapsing due to a granular layer of
ground surrounded by Talc. This bad area of ground intercepts T2 in the same location and in order to
commission T2 to transport water, the bad ground area known as the special area needed to be
supported using a reinforced shotcrete liner and grout.
Project Scope
Tunnel Constraints
The work was located 800 meters from the entrance portal. The tunnel constructed in the 1950’s is 4.7
meters in diameter. In order to produce large quantities of grout within the confined area, special
equipment was designed and built by Multicrete Systems Incorporated (MSI) in order to perform
continuous batching.
Since the tunnel was 4.7‐meter diameter, the work was located at the bottom valley of the circle.
Special consideration was taken for mine rescue and safe working area requirements in order to create a
flat platform for equipment and workers to work free of tripping hazards and with clear emergency exits
(figure 1).
Pressure Constraints
The reinforced shotcrete tunnel liner had a maximum engineered working pressure of 100 psi. The grout
injection refusal criteria required sustaining 100 PSI for 5 minutes with no more than 1 liter per minute
of grout flowing through the injection port. To adhere to this criterion, MSI manufactured a special grout
header that communicated with the pump and batch plant located 50 meters away.
Special Grout Header
The grout header monitored the injection pressure at the injection location and slowed the pump down
to maintain 100 PSI maximum pumping pressure. The grout header included an electronic pressure
sensor protected by a gauge-saving device, two manual pressure gauges for the grouting specialists to
locate plugged lines, and a digital electronic flow meter capable of reading grout flow through the line to
the nearest tenth of a liter. All components of the grout header were attached using high-pressure cam
locks to allow easy access to the inside components for cleaning.
Grout Mix Design
This special area had a water infiltration measured at around 800 Liters per minute. In order to prevent
wash out of grout MSI designed and employed a water stop grout that was capable of being mixed
through a colloidal mill and injected into the heavy water flow areas to push the water to higher
elevations and eventually stop the water. The compressive strength was specified low at 20 MPa, which
was easily achieved with MSI’s special water stop grout.
Productivity
The mobilization of equipment, set up in tunnel and dry run training on the equipment, took a total of
three days. With some minor issues related to compatibility of equipment with the mine infrastructure,
the equipment was ready to batch in two days. Standard batching productivities were limited to the
maximum injection pressure of 100 PSI, in order to continuously pump the material through the line and
maintain the pressure, the pump needed to be slowed down as the holes began to close off.
Another limitation to batching was the ability to deliver bulk bags to the work location. If there was a
safer, easier and faster way of delivering bulk bags to the work site located 800 meters within the
tunnel, productivity could have increased by 100%.
Productivity Projections
Actual Average Production – 15 yrds/shift
Optimal Material Delivery Process – 30 yrds/shift
Relaxed Injection Pressure Criteria – 150 yrds/shift
Final Product
The final product of the special area grouting was that the grout injection took 3.5 weeks instead of the
scheduled 6 weeks. MSI demobilized the equipment out of the tunnel which allowed the client to access
more work areas sooner to save weeks on their schedule. The special area grouting was confirmed with
inspection holes and down-hole bore cameras which showed that the grout encapsulated all granular
materials and filled the void entirely. Water was allowed to drain from its source area behind the wall
and into perforated pipes, the water reduced to 50 liters per minute out of the drain pipe and it was
believed that there was no hydrostatic pressure behind the grout liner.
