In Toronto, Canada, Hydro One, the electricity company, has reached the halfway mark on its US$39.97M project that features a 2.1km long tunnel to carry a new 230kV high voltage transmission circuit beneath the city. The tunnel will improve the supply reliability by linking the east and west of the supply grid for backup switching.
Contractor Dibco Underground Ltd started the tunnel in July 2005 and is expected to complete work by December this year. The 3m diameter tunnel is being driven 30m below the surface through shale by TBM with a muck conveyor set through the back-up to a three car train. Each car can transport 6t of spoil, and are also used for material supply into the tunnel.
Hydro One told T&TI that this was a new approach for installing cable circuits in highly congested city areas, as it usually favoured open cut excavation. The route of the tunnel follows public property, i.e. the city’s roads, to minimise the need to negotiate multiple boundary crossings. The resulting alignment includes a 90° direction change and reports described how the TBM and 70m long conveyor had to be hinged in five sections to accommodate this.
Progress was slowed when the team encountered a 13m wide seam of clay. Ribs and lagging had to be installed to support the ground, which cost a week of programme time. By mid-May the TBM was reportedly back into the shale.
The tunnel will have cross-linked polyethylene cable embedded with fibre optic strands for temperature sensing installed from December, with the circuit due to be operational one year later. Six 115mm diameter cables will be installed, although the tunnel design provides space for future expansion.
