By Ashley Karen
The cost to free a micro TBM (tunnel boring machine) that was trapped last year while building a storm sewer in Toronto, Canada, has increased to £14.6M (C$25M) due to ground stabilization challenges.
The rescue mission has become progressively expensive since February 2023, when city officials granted a contract worth £5.25M (C$9M) to free the TBM trapped under a street in Toronto’s west end.
Now, the estimated cost has nearly tripled to £14.6M (C$25M) after Toronto’s General Government committee authorized £9.3M (C$16M) of supplementary funds for the rescue at a meeting on 10 July.
City officials described to the committee that the new funds were required because emergency work to release the TBM had become more complex and was taking longer than initially expected.
When it became stuck in March 2022, the tunnel boring machine was constructing a 900mm diameter storm sewer along Old Mill Drive as part of the City of Toronto’s basement flooding protection program.
An unsuccessful hand-mining operation to rescue it discovered that the TBM was trapped in a steel tieback that is part of a deep foundation shoring system for the mid-rise buildings’ construction on the street.
The leading contractor, Canadian construction company, Clearway Construction, was set to provide the basement flood protection program for Toronto City Council. At the same time, Earth Boring CO was subcontracted to provide the micro-tunneling work.
Initial rescue operations were deterred by a considerable rise in groundwater infiltration into the recovery tunnel at the face of the micro TBM, which caused increased ground movement around the TBM that undermined the integrity of the roadway, nearby subway tunnel, and surrounding infrastructure. In addition, it resulted in a “sinkhole” in the work zone.
Then, Keller was selected to help execute a strategy to stabilize the soil near the machine, and WSP Canada has also been instructing the city on the rescue project.
The city council granted £5.25M (C$9M) to Clearway Construction in February 2023 for helping the machine rescue. Its contract contained reviewing the impacted area and safely proceeding with the rescue operations, such as de-stressing and removing steel tiebacks and jet and compaction grouting to stabilize the ground.
Nevertheless, since then, the contractor has faced several challenges during the ground stabilization works.
Officials involved in the rescue reported to the committee that challenges incorporated additional groundwater infiltration and the demand for further ground stabilization work.
Correspondingly, the TBM retrieval is anticipated to take six months longer than initially expected, which has also increased the project costs. The retrieval completion date was originally March 2023, but due to the increased complexity of the work, it has now been expanded to the end of August.
The report remarked that the continuation of the rescue work was “critically important” to enable the contractor to finish the remaining scope of the work under the contract.
The report noted: “The large shaft excavations on Old Mill Drive cannot be completed and restored until the micro-tunneling boring machine is retrieved and the remaining 5m of the tunnel is installed.”
