Toronto tunnel project helps meet increased demand for power
The need to replace aging underground infrastructure has led Hydro One Networks Inc. and Toronto Hydro to join forces to upgrade the existing 115 kV transmission lines in the midtown power corridor in Toronto. The project involves installing an additional 115 kV circuit between the Leaside Transformer Station and the Birch Ave. junction.
It was decided that the best solution for the 2.4 kilometres of new cabling required between Birch Ave. and the Bayview Junction just west of Bayview Ave. was to install the conduits in a tunnel. This, however, proved to be a challenge since the route crossed the Balfour Park and Moor Park ravines which were 20 and 30 metres deep respectively and considered sensitive urban ecological areas. The only viable option proved to be sinking the tunnels down into the shale bedrock which lies about 40-50 metres deep in that part of Toronto. In order to get sufficient unweathered rock cover above the tunnels it meant the tunnel had to be constructed at a 60-80 metre depth below the ground.
Construction of the main shaft and tunnel
Rock tunnelling is fairly straightforward but this project was to be constructed in a densely developed urban area affording very small working areas with residential dwellings immediately adjacent to them. Additionally, the tunnelling shafts had to be excavated through the overburden consisting of cohesionless soils well below the water table. The ground would have flowed like sand on the beach beside the lake if it wasn’t stabilized in advance of the excavation. Such measures are complicated and expensive to execute but essential to get the shafts safely down to the tunnelling elevation without causing damage to nearby structures.
Innovative freeze unit installed
The $33 million contract for the work was awarded to Technicore Underground Inc. of Newmarket Ontario in 2011. Technicore had an innovative way of sinking the shaft through the wet soils – by freezing them. In simplified terms, they drilled a series of holes in a circle just outside of the shaft excavation and installed pipes in the holes that could circulate brine through them to take the heat out of the ground. A freeze plant was installed on the surface and essentially a vertical hockey rink was created. Over the course of 6 weeks, the ground froze solid to -10 degrees C creating a frozen “donut” around the shaft.
Technicore was then able to excavate the soft centre of the donut down to the bedrock elevation in stable conditions. The freeze plant was kept on until the final concrete shaft Technicore designed and built its own 3.4-metre-diameter Tunnel Boring Machine (TBM) to mine the rock tunnel. Even that was not straightforward as the Consulting Engineers, MMM Group, had designed the tunnel with switchback 32 metre curves to get the TBM onto the Yonge St. right-of-way and then back onto Birch Ave. to hit the exit shaft. “We knew it was going to be difficult,” said Technicore CEO, Tony DiMillo, “and we designed this TBM with a double articulation system to give it a chance to get around the curves. We made best efforts but could only make the one first curve.” At this writing, Technicore is mining the 40 metres back from the Birch Shaft with a Dosco Roadheader to extract the TBM and finish the rock excavation for the project.
Concreting of the tunnel to a 3.0 metre diameter finished size is ongoing with separate sets of forms being utilized in the two tunnel headings. Technicore’s work is expected to be completed late summer of 2014 following which Hydro One’s conduit contractor will begin installing racks and cables in the finished tunnel.