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The crossing for the gravity-fed trunk sewer also came with strict line and grade tolerances -- an upward slope of 0.126% was needed, while the line needed to be within two inches due to the limited size of the receiving pit. The requirements meant that the contractor needed a tunneling machine that could be steered continuously.
Volker Steven and In-Line ultimately chose a Robbins Double Shield Rockhead with a breasted plate cutterhead for potentially abrasive ground and 11.5 inch disc cutters to effectively excavate potentially hard rock. The project is the first instance of this particular machine type being used in Canada. “The Rockhead was the most economical option for us. Though we believe microtunneling would have worked as well, the microtunneling setup is much more costly,” said Rafih.
The sentiment was echoed by Bill Stace of In-Line: “In these particular project conditions, the Rockhead can not only equal but exceed the efficiency of microtunneling.”
Excavation in fractured ground
The launch site, consisting of a 20-foot wide by 35-foot long pit, was excavated deeper than expected due to the geology. “We had anticipated an open pit excavation, but once we started excavating, we found that the bedrock was quite fractured. Instead we now have a 23-feet deep shoring and shaft replacing the open pit,” said Dwayne Giesbrecht, Calgary Water Resources Department.
The Rockhead was launched in December 2009 with a pipe jacking unit for initial thrust at startup. The short pit length required that the machine be launched as a Single Shield machine, without a rear shield and gripper cylinders. After boring ahead 20 feet, the final gripper shield was bolted to the modified machine, consisting of a front shield, thrust body and shortened conveyor. Crews then added the remainder of the conveyor section, making the machine’s total length 39 feet.
Crews worked eight to 10-hour shifts, managing wet ground conditions. “At the start of each shift, ground water is an issue, as it collected overnight. We slowed the rate of machine advance to allow the friction of the cutterhead to dry out the tunneling face. This allowed the material to move through the cutterhead and onto the machine conveyor without clogging the hopper area. The method worked well as long as we took our time during startup each morning,” said Scott Fisher, SBU field service manager.