The C$9m (US$6.6m) contract, awarded to Clearwater Constuction Ltd, is “a matter of extreme urgency, as there was a significant health and safety hazard to the public”, the city says.

The TBM, installing a 900mm diameter storm sewer for the Basement Flooding Protection Program Phase 4 Contract on Old Mill Drive, was launched in March last year. It became stuck in June after being ensnared in steel tie-backs used for brace shoring for two mid-rise developments in the area.

Mika Raisanen, director of design and construction, linear underground infrastructure for the City of Toronto division of Engineering & Construction Services, said the tie-backs did not show up on as-built drawings before work began. Processes for tie-back mapping and shoring approvals are now being reviewed.

The TBM had completed 95% – 275m – of its drive when it hit the obstacles. The remaining 7m section of the tunnel to the shaft at Old Mill Drive and Bloor Street was hand mined to rescue the machine and complete the storm sewer work for project.

As well as retrieving the TBM, the remedial works include emergency repairs to the integrity of the subsurface soil to protect the road and surrounding infrastructure, repairing two sink holes, and removing four tie-backs. Weather permitting, the operation should be completed by early April.

The TBM was working at a depth of 6-18m to avoid conflict with the nearby Old Mill TTC Subway tunnel. The C$3m (US$2.2m) machine is 1.2m in diameter and 5m long.