Twinning of the West Trunk Sewer Project

Project Highlights

Role: General Contractor

Project Delivery: Design-Bid-Build

Tunnel Equipment: 3.6m EPB TBM & 1.8m MTBM

Primary Support: 3m Precast Concrete Tunnel Liners (PCTL), 1.5m Microtunnel pipe

Ground Conditions: Shale & Soft ground

Project Description

The project took place in a very busy urban area in Mississauga, Ontario. The work was complete in areas with very limited footprint and located at the intersection of two arterial roads and immediately adjacent to an environmentally sensitive creek under the Credit Valley Conservation Authority’s jurisdiction. The tunnel was mined majority in shale ranging from 15 to 40 meters dep with approximately 750 meters in soil conditions that included water bearing sand and silty sand, clayey silt, silty clay, all of which had a very high content of cobbles and boulders. The project included the construction of approximately 3800 meters of 3.0 meters internal diameter sanitary trunk sewer installed in a single-pass using precast concrete tunnel liners (PCTL). Toxic hydrogen sulfide gases were encountered during tunneling operations and shaft construction, therefore a high-density polyethylene (HDPE) protective layer was installed to protect the concrete from corrosive gases.

Technicore utilized two 3.6 meter EPB TBM, designed and built by TUC to complete the project. Multiple drive lengths were completed at 2028 meters, 1120 meter and 688 meters in length in a single-pass using PCTL installed behind the machine and grouted in place. The project also involved completing an alignment curve that was a 123m radius. Since the tunnel was mined mostly in the Georgian Bay shale formation, the project involved the use of a unique, proprietary compressible grout to fill the annual space of the outside tunnel lining to deal with the time dependent deformation (TDD) after excavation. The compressible grout not only eliminated the deterioration and damage to the tunnel liners, it also secured the assembly in place and allowed for continuous EPB TBM thrust and advancement. The soft ground sections of the tunnel was completed in EPB mode. The project also encountered HS gases which required the use of a corrosion resistant lining HDPE liners installed for 250 meters of the tunnel. Five sanitary manholes were construction utilizing a combination of cast-in-place and C300 water-tight structures that were able to withstand 50psi and prevent infiltration and exfiltration into the tunnel.

Additional scope was added to include the construction of two additional tunnels approximately 805 meters in length utilizing a 1.5 meter single-pass MTBM and pipe jacking and provided a cross connection of the West Trunk Sewer to another existing trunk sewer on Mississauga Road.

On this project and as part of the original contract scope, Technicore also constructed approximately 450 meters of 300 mm diameter watermain utilizing trenchless (HDD) and open cut subcontractors. Work was completed within very environmentally sensitive area and crossing Loyalist Creek without any major issues.

Owner: Region of Peel

Start Date: July 2015

Completion Date: August 2021

Drive Length: 3800 meters

Diameter: 3000mm

Consultants: WSP Canada Inc.

Contract Amount: $57,415,798

Ground Conditions

The tunnel was mined mostly in shale with some sections of mixed face and soft ground conditions.

There were two notable sections of soft ground at approximately 110 meters and 750 meters in length throughout tunnel construction which included water bearing sand and silty sand, clayey silt, silty clay, and mixed face, all of which had a very high content of cobbles and boulders. Hydrostatic water pressures ranged from 12 to 20psi. Tunnelling through this section was completed in EPB mode with the use of various soil condition agents and without the need for cutterhead intervention despite the unusually high rock content attributed to glacial deposits section within the sedimentary bedrock formation.

The Georgian Bay Shale that makes up 80% of the tunnel, is known to exhibit time dependent deformation (TDD) after excavation. The forces generated by TDD are significant and to combat this Technicore developed a unique proprietary compressible grout to fill the annular space of the outside tunnel lining.  Deep valley material with overburden and high pressurized water along the alignment of tunnel was also present during excavation, therefore Technicore utilized two EPB TBM to deal with hard ground (shale) as well as soft ground.