The TBM, which was installing a 900mm diameter storm sewer, was launched in March last year. It became stuck in June 2022 after being ensnared in steel tie-backs used for brace shoring for two mid-rise developments in the area.

In March this year the City of Toronto issued an emergency contract to Clearwater Construction Ltd to retrieve the TBM and it was hoped the operation would be completed by early April.

The TBM being launched in March 2022

A spokesperson for the city’s Engineering and Construction Services division said the retrieval was a complex operation, requiring more time and cost than initially anticipated.

“During initial retrieval efforts, a significant amount of groundwater entered the tunnel, which needed to be removed and the ground needed to be stabilised in order to safely continue work and remove the machine. The ground stabilisation work was more complex than originally anticipated, however, the area always remained safe,” the spokesperson said.

The first purchase order for the retrieval was for C$8.99m (£5.4m). As efforts to retrieve the machine progressed, the city amended the purchase order for an additional allocation of up to C$16m (£9.6m).

“This represents the upper limit of the cost estimate, and it's possible that the cost won't require the entire amount. The final costs and actual payments will be based on substantiated and certified invoices,” the spokesperson said.

Work will now resume on the new sewer and work on the Old Mill Drive section is expected to be complete by the end of the year. The TBM had completed 95% – 275m – of its drive when it hit the tiebacks. The 1.2m diameter, 5m-long machine was working at a depth of 6-18m to avoid conflict with the nearby Old Mill TTC Subway tunnel.

The City of Toronto decided to retrieve the machine because leaving it in the ground would have required the sewer to be completely redesigned and reconstructed which would have been cost-prohibitive and extended the project’s duration.