By Ashley Karen
It’s great to hear that crews have finally freed part of a micro tunnel boring machine that was trapped in underground infrastructure while constructing a storm sewer in Toronto.
After 18 months of setbacks, the head section of the machine was successfully extracted from a tunnel under a street in Toronto’s west end last Friday.
The City of Toronto has proved that the tail section will be removed in the coming weeks through a crane, which will be used to hoist the machine from the shaft.
When the micro-TBM became stuck in March 2022, it was constructing a 900mm diameter storm sewer along Old Mill Drive as part of the City of Toronto’s basement flooding protection schedule.
An unsuccessful hand-mining operation to retrieve it discovered that the TBM was trapped in a steel tieback that is part of a deep foundation shoring system for the construction of mid-rise buildings on the street.
The micro tunnel boring machine had excavated most of the length of the new sewer when it became stuck, with almost 7m to complete, and must be removed to complete the project.
The City of Toronto decided that the best option to finish the project as fast and efficiently as possible was to rescue the micro-TBM.
The authority stated that leaving the micro-TBM in the ground would need the sewer to be completely re-designed and re-constructed, which would be “cost-prohibitive and significantly extend the duration of the project”.
Because of several challenges during work to stabilize the ground near the area where it is trapped, the rescue of the TBM has already taken longer and cost more than expected.
The general contractor responsible for the project site is Clearway Construction, whereas the subcontractor working to retrieve the micro-TBM is Earth Boring.
In February, the city council awarded £5.25M (C$9M) to Clearway Construction for reviewing the impacted area and safely proceeding with the retrieval operations, containing de-stressing and removing the steel tiebacks and jet and compaction grouting to stabilize the ground.
The rescue mission cost had almost tripled to £14.6M (C$25M) by July after £9.3M (C$16M) of extra funds were authorized for the work to free the TBM due to challenges with the ground stabilization.
Therefore, the rescue completion date was initially March 2023, but due to the increased complexity of the work, that was postponed to the end of August 2023.
The City of Toronto claimed when the TBM is released, no further tunneling will be needed as crews have hand-dug a tunnel from the receiving shaft towards the stuck machine.
