Award of Excellence: Sheppard Subway Tunnels

Category: Transportation

HATCH MOTT MacDONALD

The Toronto Transit Commission’s new $934 million Sheppard Subway is expected to go into service at the end of 2002. The 6.4-kilometre line runs east from the existing station on the Yonge line along Sheppard Avenue East to Don Mills Road.

Because the subway is located in a high-density urban location along one of the busiest arterial roads in Toronto, there were many design and construction challenges in relation to adjacent residential areas and businesses. Another concern was the effect on underground utilities in the corridor.

The engineers recommended using bored tunnels for constructing the subway due to cost issues and their reduced impact on the community. The cut and cover method was used only where the alignment was off the street and the structure was required to be very near to the surface, as at the five passenger stations.

The total length of the twin tunnels is 4.4 kilometres, with 3.9 kilometres of this constructed by tunnel boring using two earth pressure balance (EPB) tunnel boring machines (TBMs) in two drives. Hatch Mott MacDonald was the prime consultant responsible for the detailed design and design support during construction of the tunnels. Under a separate contract they were also responsible for the design and design support during fabrication of the precast concrete tunnel liners.

The transit corridor has very complex geology with interbedded layers of glacial tills and many boulders. A river valley crosses the alignment and most of the line is below the ground water table.

The engineers had to develop a single tunnel lining design that addressed the range of ground loadings and stiffnesses, was easy to install and yet was economical. The system selected is a precast concrete lining made up of segmented rings that are bolted from ring to ring. The connections between the rings are designed to yield at a certain load level, allowing the rings to act as more flexible, independent units. The tunnel diameter is 5.2 metres. The linings are made of high performance 60 MPa concrete with very low permeability and diffusivity. The design life of 100 years is achieved by the concrete quality rather than by epoxy coating of the reinforcing steel or by cathodic protection. The lining segments were equipped with standard EPDM gaskets designed for 4 bar of hydrostatic pressure, and were bolted radially and longitudinally with straight diagonal bolts. All the rings were tapered to provide tunnel direction control.

The development of earth pressure balance tunnel boring machines has solved the problem of having to pressurize tunnels during construction to prevent their collapse and the ingress of water. Hatch Mott MacDonald had recent experience using such machines for the St. Clair Tunnel.

The TTC had procured the boring machines from Lovat, a manufacturer located in Toronto. When significantly more boulders were found in the ground than had been expected, Hatch Mott MacDonald recommended replacing the machine’s cutting heads with new heads designed both for cutting rock and for excavating in soft ground. They also recommended adding protection devices to prevent large boulders entering the machine.

The boring machines have a 5.9 metre outside diameter, with mixed face cutterheads that have disc cutters for rock cutting, and rippers for soft ground excavating. The gasketed tunnel lining is installed within the boring machine and the space between the lining and the ground is continuously grouted as the machine moves forward. A chamber behind the cutting face holds the excavated material under pressure by regulating the speed of an Archimedean screw conveyor used to remove material from the chamber.

This was the first use of earth pressure balance tunnel boring machines in the Toronto area. Foam, which is a very recent development in this technology, was used, first to modify the ground to allow pressuring of the chamber, and second to lubricate the muck in order to reduce friction and wear on the tools in the machines.

Critical elements of the project included dealing with difficult launch sites for the tunnelling boring machines, and designing underpasses below an operating railway and below a 4.5 metre diameter tunneled storm sewer. The engineers also developed a cross passage for emergency exits, using special steel lining segments installed by the boring machine.

The tunnels and liner were substantially completed in August 2000, within one month of the initial 35-month schedule. The $100-million construction contract was well within budget, and included only 3% for contract changes.

Name of project: Sheppard Subway Twin Tunnels & Tunnel Linings