The work and the technique: Ashbridges Bay Treatment Plant Outfall, Canada

Operations for the Ashbridges Bay Treatment Plant Outfall project primarily involve constructing the massive tunnel excavated at the bottom of the shaft and through the rock beneath the lakebed; in addition, a significant number of operations will be carried out both onshore and offshore in the lake itself. 

There was also a significant amount of work on land and offshore. Besides the vast discharge point adjacent to the coast, the installation included 50 risers, vertical stainless steel pipes along the last kilometre of the tunnel that released the water.

After assembly operations, the Tunnel Boring Machine (TBM) started its journey from the shaft dug beside the lake's shoreline. The TBM installs and completes the tunnel lining in precast concrete, working two shifts of up to 10 hours each. Thirty ribs per day were installed during its run. A Robbins continuous conveyor system allowed the constant removal of cut rock from the tunnel.

The rock's poor condition and the possible infiltration of high water or gas pockets was the challenge for this project. For this reason, it was necessary to interrupt the excavation operations every 25 cycles of the TBM to probe the hole in front of the cutting head to check for water.