CANADA | PROJECTS
At the small scale, a challenge was unexpectedly
met during the micro-tunneling work for a new local storm sewer in a residential part of the city. The cutterhead of micro-TBM got tangled up among some of the unmarked tensioned steel tie-backs of the deep foundations for buildings. The drive had been mostly complete but then work stopped for the MTBM to be rescued. At a bigger scale, and away from residential areas,
success was met for a tunneling operation to bore a large outfall tunnel as part of an effluent and environmental improvement program, at Ashbridges Bay. On transport, Eglinton is more than half way
excavated while tunneling is underway on the Scarborough subway.
TORONTO Tangle below The City of Toronto recently issued an emergency, non-competitive contract for retrieval of the MTBM that got stuck underground for a number of months after it became entangled among tensioned steel tie- back wires of long-standing building foundations during construction work for a new storm sewer. The local authority and project owner said the
tiebacks hadn’t shown up on infrastructure research carried out beforehand, such as on as-built drawings, during design or before construction began for the 900mm-diameter (3ft) sewer. The trenchless work was being performed on a 282m-long (308yd) section of sewer between maintenance holes OD5 and OD8.
The new contract – to the contractor, Clearway
Construction Ltd – was urged to be issued as a “matter of extreme urgency”, due to “significant health and safety hazard to the public” caused by the unforeseen conditions, according to the City’s Engineering and Construction Dept, in its summary of the advisory Report for Action (RFA), issued mid-February. The FRA was submitted to seek approval for the MTBM rescue contract and storm sewer completion works. The micro-TBM had been launched in March 2022 to
bore the sewer for the Basement Flooding Protection Program, Phase 4 Contract, on Old Mill Drive. However, it encountered the tensioned tiebacks well into the drive and then became ensnared. The steel tie-backs are used for deep foundation shoring of two mid-rise developments in the area. Wider complications for the local infrastructure arose
from the MTBM getting caught up in the unmapped foundation tie-backs. They complications, the RFA said, included: “The integrity of the roadway, nearby subway tunnel and surrounding infrastructure was undermined as a result of ground movement around the micro- tunnelling boring machine due to excessive ground water and poor soil conditions. This further led to the creation of a sinkhole in the work zone.” The RFA added that a non-competitive emergency
contract was needed to allow the contractor to “review the impacted area and safely proceed with the micro- tunnelling boring machine rescue operations, including destressing and removing steel tiebacks; and, jet and compaction grouting to stabilise the ground.”
Above left: Challenge of foundation steel tieback tie-up during Toronto sewer micro-bore PHOTO CREDIT: CITY OF TORONTO Above right: Emergency contract needed to disentangle MTBM from ties and fix area around sewer works
Spring 2023 | 15
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