TECHNICAL | SOFT GROUND TUNNELLING
Highway surface ~3m C LTunnel 2
Pre-support pipes
C L Tunnel 1
Existing tunnel
Final lining 300mm
Tunnel Axis
Waterproofing system
Initial lining 300mm
Glacial till Sand and gravel fill, loose to compact Above: Selected option with twin mined tunnels TOR
Tunnel Axis
TOR 9.1m 10.6m
All options and consideration were shared with
Metrolinx during a five month design development phase, in four Design Presentation Meetings (DPMs) where TTP could present their technical concerns and solutions, while receiving feedback from the client and their technical team on the solutions presented.
DESIGN CHALLENGES AND CONSTRAINTS One of the project’s greatest challenges was managing the settlement criteria. The original settlement limit was set at zero for both the review and alert levels, and 1mm for the stop level, meaning that even the smallest movement had to be closely monitored and mitigated. With such stringent limitations, the tunnelling method had to be chosen carefully, with a strong focus on minimising ground disturbance. This became particularly complicated due to the highway’s heavy traffic load and the need to maintain uninterrupted service. The low overburden (2.2m between the tunnel
crown and the road surface, at minimum) posed further challenges, as did the curved alignment of the rail tunnel that had to tie into existing rail tracks. The project’s location between key infrastructure – existing
operational tunnel to the west and bridges to the east - further constrained the available space, complicating the alignment and tunnel geometry. Further, the top heading of the tunnel would be constructed within the embankment of the highway constructed from the 1960s onwards, where the unknown but expected obstructions, such as steel elements, etc., would pose more challenges.
INNOVATIVE TUNNELLING SOLUTIONS To overcome these challenges, the project team favoured the use of large (800mm-diameter, 12mm- thick) steel pipes for pre-support. These pipes were significantly larger than the originally proposed 138mm ones and were chosen to increase the stability of the pipe arch and to accommodate the risk of obstructions within the tunnel alignment. The larger pipes allowed the tunnelling crew to access the interior if needed to remove obstacles and continue pipe installation. This decision proved crucial as the project progressed, with many obstructions encountered during excavation, such as buried piles and debris from previous construction.
Jacking frame
Hollow stem auger
Steel pipes Cutting head Pilot rods
Above: Auger boring main elements 10 | November 2024
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
