Toronto’s International airport is the busiest in all of Canada. The facility averages some 443,000 flights a year and more than 41 million passengers annually.

The Region of Peel awarded a contract to a joint venture of Dibco- CRS in April 2015 for twinning the last remaining section of the existing Etobicoke Creek Sanitary Trunk Sewer at Toronto Pearson International Airport.

The proposed gravity sewer pipeline alignment ran parallel to the existing trunk sewer (west side) and crossed underneath a major runway, among other critical airport facilities.

Dibco-CRS completed the drive of 575m of 1,800mm ID tunnel with minimal impact to the busiest runway in Canada, in mixed ground using a slurry MTBM. It’s one of the first projects for manufacturer Akkerman's new face access machines.

The contract was worth CAD 16.7M (USD 12.95M) and also included the construction of four new maintenance holes, modifications to two existing maintenance holes, and all associated appurtenances and fittings, and temporary bypass pumping as necessary to complete the works. Hatch Mott MacDonald designed the project.

Sensitive surroundings

Part of the alignment passed below active Runway 23 – the longest of the airport’s five runways and the busiest.

Approximately 570m of the sewer was to be completed by microtunnelling pipe jacking methods at depth ranging from 5m to 12m.

“Microtunnelling methods were selected mainly to minimize disruption to airport operations and preserve the integrity of operating surfaces at the airport,” says Neville Zuzarte, project manager with CRS Tunnelling

“To reduce the impacts of microtunnelling construction, shaft sites were strategically positioned where they would have the lowest impact to ‘airside’ operations at the airport.”

The launch shaft was located in an existing Creek valley (regulated wetland) close to an existing connection stub.

The reception shaft was located midway between an active taxiway and a service road for local access. Shaft construction compounds were within restricted “airside” areas of the airport and movement was generally restricted at all times, he explains.

“Extreme care and monitoring had to be taken to ensure there was no impact to these operating surfaces resulting from the tunnelling operations,” Zuzarte said.

The other major concern was to ensure that the machine continuously mined without any risk of getting stuck or encountering boulders that would impede the advance.

The geology along the tunnel alignment generally consists of fill materials associated with airport developments overlying a thin layer of non-plastic till, which were underlain by shale bedrock of the Georgian Bay formation.

“Granular till and riverbed deposits were also noted near the entry shaft,” he explains. “Based on project borings and the sewer inverts at the tie-in locations, the tunnel was expected to be completed in mixed-face conditions of native soils overlying completely weathered shale bedrock overlying fresh and or competent bedrock."

In addition, boulders and or cobbles of glacial origin were anticipated to be encountered in the native soils along the alignment. As such the biggest concerns with the ground conditions were related to the inherent natural variability in the ground – mixed-face/change in face condition – and the risk of encountering boulders and cobbles along the tunnel axis.

Excavation from the ground surface down to the tunnel alignment to remove an obstruction was considered an impractical option. Face access was particularly important if at any point cutting tools needed to be changed, and access to replace these tools would be required through the cutter head with changes from the rear.

And there wasn’t solely Runway 23 to worry about. Above the alignment there were also two taxiways, a high-speed taxiway exit, two service road and critical airport utilities. Furthermore, about 30m away was a creek, and parallel to the sewer was the existing trunk sewer, to which the JV was twinning, at a distance of about 15m.

Face access

Akkerman launched a range of face access machines last year, the first of these to be tested on a project is the MTBM used for the Etobicoke Creek twinning project. The new generation of peripheral drive machines are electric over hydraulic drive, which offers higher torque breakout forces. They are for larger sizes between 72 inch and 114 inch (1.83m – 2.8m) outside diameters.

According to the company face access allows for cutter head tool replacement to extend drive lengths. Back-loaded tooling mounts on the cutter head simplify the process of accessing and replacing worn tools.

"In high groundwater pressure the access can be done via an airlock seal to the tunnel," says Akkerman tunnel engineer Brad Wheeler. "Everything was ready for that on our Toronto project but in the end it was not necessary."

ASI Marine provided the hyperbaric airlock system. “The hyperbaric chamber was designed to be accessed through a bulkhead at the launch shaft when equalizing pressure to the tunnel face pressure," Zuzarte explains.

"The intention was with the pressure equalized, the MTBM face could be safely accessed by divers who could perform the necessary work at the MTBM front end.

“While being available onsite for the duration of the tunnelling operation, the hyperbaric chamber was not required for the tunnel drive, due to both the robust design of the MTBM and the skill of the operators.”

Akkerman has also launched a new guidance system, offering the capacity for curved drives for the first time and capable of longer drive lengths. Zuzarte says, “the tunnel guidance system utilized was particularly helpful as the alignment was considerably long for a pipe jacking system."

Intermediate pipe jacking stations (IJS) were installed along the alignment. In the event that jacking forces encroached the maximum available from the pipe jacking station, the IJS could be utilized.

The drive was done from a secant pile shaft heading to a reception shaft built with steel ribs and wooden boards. Tunnelling began in October 2015.

Jacking went smoothly and once the first few sections were in, round-theclock operation was possible to speed work and reduce any airport impact.

The average penetration rate was approximately 1.6m per hour. Tunnelling production rates ranged from 9.5m/ day to 11.6m/day (two 12-hour shifts).

Dibco-CRS holed through on December 3, 2015. The tunnel has an excavated diameter of 2,275mm, with a finished ID of 1,800mm (nominal), and is lined with precast reinforced concrete microtunnelling pipe supplied by Decast Ltd based in Ontario