SOFT GROUND TUNNELLING | TECHNICAL
LESSONS LEARNT The project provided valuable insights into the complexities of urban tunnelling, particularly in areas with very low overburden and high traffic volumes. One of the key project lessons learned was the importance
of flexibility in both design and execution. The ability to adapt the pre-support system to accommodate larger pipes and the decision to use open-face excavation methods were critical to the success of the project. Another important lesson was the value of collaboration
between the project team, client, and designers. Regular progress meetings and close communication allowed the team to refine the design as new challenges arose, ensuring that the project remained on schedule and within budget. The use of advanced numerical modelling also played a crucial role in managing risk, providing the team with the data needed to make informed decisions at each stage of the project. Cooperation between the contractor’s site team, the
designer and the owner’s team was essential for successful completion of the project. With this cooperation the project could be completed on time and budget, which was considered a great success for this project, being one of the most challenging projects executed by Metrolinx.
CONCLUSIONS The Highway 401/409 Rail Tunnel project in Toronto represents a significant engineering accomplishment. Through a combination of innovative design, rigorous risk management, and collaborative client-contractor engagement, the project successfully navigated the complex challenges of tunnelling beneath one of North America’s busiest highway intersections. The Design-Build-Finance (DBF) model, along with a
thorough comparative assessment of design alternatives, allowed the contractor to deliver the project on time and within budget. The lessons learned from this project, particularly in
terms of settlement control, innovative tunnelling methods, and advanced monitoring, will be invaluable for future infrastructure projects in similarly constrained environments.
QUESTIONS AND ANSWERS
Following the presentation, the speakers took questions from the floor in the BTS meeting. The questions and answers are summarised below, edited for clarity and space.
Q1: Was the road settlement limit 1mm and what was the maximum actual settlement on the highway? Did the road remain open? A1: The initial limit was set to 10mm but the design JV commented prior to the tender that this was not realistic and manged to get this limit modified. The maximum settlements were close to the retaining walls due to the vibration induced by the installation of the sheet pile walls for the shaft and were about 60mm-65mm before tunnelling commenced. A similar amount of settlement was observed in another location during the pre-support pipe installation due to an unknown obstruction. The road remained operational at all times. Even though the settlements were 60mm-65mm in local areas the slopes were very small as the troughs were rather smooth.
Q2: How was it possible to collect samples for geotechnical assessment during the tender process to provide confidence to the FE models, and how was it possible to build a shaft in the middle of an operational highway? A2: The highway was originally a six-lane highway and then additional lanes were added. The shaft was introduced in the part between the lanes on a grass median. There were samples taken during the construction of these embankments. The confidence in the numerical models arose from the extensive sensitivity assessments. Also the soil parameters taken into the models were taken conservatively.
REFERENCES ● Doherty, J. P. & Muir Wood, D. (2013). ‘An
extended Mohr-Coulomb (EMC) model for predicting the settlement of shallow foundations on sand.’ Geotechnique, 63:661–673.
● Ferraro, M.J., & Lahti, T. (2021). ‘Tunneling under highway 401: Construction of a large diameter pre-support pipe canopy.’ TAC Conference, Vision Underground.
● Urschitz, G.J. (2019). ‘At the Limit–Soft-ground tunnelling under the city highway of Toronto.’ Geomechanics and Tunnelling, 12(5), pp.440- 449
Q3: What are the considerations regarding the selection of mesh reinforcement for the initial lining and fibre reinforcement for the final linings, and what were the concrete parameters selected for the project? How was the simulation of shotcrete strength gain done in the numerical analysis? A3: Mesh and lattice girders were used for the initial lining as a conservative approach as a contractor’s preference. The shotcrete was regular 35MPa shotcrete mixed on site without any particular considerations. The strength gain of the shotcrete was not simulated explicitly as a time- dependent process, rather by applying a smaller
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