Left:
MTBM prior to commencement
Encountering the tensioned wires was entirely
unexpected. The City of Toronto said details of the tiebacks were did not appear during prior research for the project, such as deep foundation shoring on as-built drawings for the mid-rise developments. The section of the Basement Flooding Protection
Program involved in the trenchless tunneling is Phase 4 Contract, on Old Mill Drive. It involved a 308yd- long (282m) stretch of new sewer tunnel between maintenance holes OD5 and OD8. To get over the problem, the City – as the local
authority and project owner – issued an emergency contract, to Clearwater Construction Ltd, aiming to retrieve the MTBM reasonably quickly. It also wanted to avoid wider difficulties for local infrastructure from excessive groundwater and poor soil conditions around the trapped machine near the unmapped foundations tie-backs. Risks were of concern such as creating difficulties for the integrity of the roadway, the nearby subway tunnel and surrounding infrastructure due ground movement. A sinkhole had appeared in the work zone. The recovery works would start with an examination
of the trap zone, then destressing and removing steel tiebacks, removing the MTBM, and undertaking jet and compaction grouting to stabilize the ground. It was hoped the works would be able to be executed quickly. However, they proved more of a challenge. The 3ft 11in.-diameter (1.2m), 16ft 4 in.-long (5m) MTBM was working at a depth of over 19ft to 59ft (6m-18m) to avoid conflict with the nearby Old Mill TTC
Subway tunnel. It had completed the majority of its drive when it hit the tiebacks. 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. A spokesperson for the City’s Engineering and
Construction Services division said that the retrieval was a complex operation, requiring more time and cost than initially anticipated. The spokesperson said: “During initial retrieval efforts,
a significant amount of groundwater entered the tunnel, which needed to be removed and the ground needed to be stabilized in order to safely continue work and remove the machine. The ground stabilization work was more complex than originally anticipated. However, the area always remained safe.” The first purchase order for the retrieval was for
Can$8.99 million (US$6.6 million). As efforts to retrieve the machine progressed, the City amended the purchase order for an additional allocation of up to Can$16 million (US$11.8 million). “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.” With the MTBM removed, work then restarted on the
new sewer line to be built on the Old Mill Drive section of the project, and completion is expected toward the end of the year.
Left:
Start of MTBM tunneling… before getting stuck, just before the finish
Winter 2023
| 27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57
