The City of Toronto says an Old Mill Drive tunnelling job was 95-per-cent complete with the tunnel only seven metres from its destination when the micro-tunnelling boring machine (MTBM) doing the work became tangled up in debris from a previous private project.
Mika Raisanen, a director with the City of Toronto’s division of Engineering and Construction Services, recently offered new insights into why the city had to come up with a $9 million emergency contract to extricate the $3 million MTBM, which is ensnarled in old steel tiebacks.
According to Raisanen, the city started work in March 2022 on a Basement Flooding Protection Program contract on Old Mill Drive, installing a 900-millimetre-diameter storm sewer.
The work is managed by the city’s Engineering and Construction Services on behalf of Toronto Water.
The 282-metre-long underground section between two maintenance holes was assigned by the contractor, Clearway, to Earth Boring Co. Limited with tunnelling at a depth of six to 18 metres underground.
The sewer needed to be installed at that depth to avoid conflict with the nearby Old Mill TTC Subway tunnel. In order to complete the tunnelling work, two vertical shafts were constructed on Old Mill Drive: one at Riverside Drive and the other on Old Mill Drive north of Bloor Street West.
The MTBM became entangled in the spring. It was eventually determined there were up to 14 tie-backs in the vicinity.
“The unforeseen steel tie-backs were from previous brace shoring from the original construction of two private mid-rise developments and did not show up on as-built drawings before work began,” Raisanen states in his written comments.
Because the MTBM encountered the tie-backs and was no longer aligned, it couldn’t reach its destination and required rescue.
It was determined the remaining seven-metre portion of the tunnel to the shaft at Old Mill Drive and Bloor Street would be hand-mined to rescue the machine and complete the storm sewer work.
Keller Group reviewed the site conditions and proposed a two-phased approach using compaction grouting to stabilize the surrounding area and the upper soil strata followed by jet grouting to envelope the tunnel alignment.
Under the emergency contract awarded to Clearwater last summer, emergency repairs to the integrity of the sub-surface soil were undertaken to protect the roadway and surrounding infrastructure, fix two sinkholes, remove four tie-backs and rescue the tunnelling machine.
Over half a year after it became stuck, the MTBM remains trapped underground. The plan is to disassemble the machine and use the parts for future projects.
“This rescue operation of the micro-tunnel boring machine is planned to be completed by early April, weather permitting,” notes Raisanen.