SOFT GROUND TUNNELLING | TECHNICAL
Highway 401/409
Above: Reference concept design – single box solution
Following an RFQ, contractors were prequalified
based on their technical expertise, financial capacity, and previous experience with similar infrastructure projects. This led to an approximately one year-long RFP process, where prequalified bidders were asked to submit detailed technical and financial proposals. Given the project’s high-risk profile, managing
financial risks became crucial. Contractors were responsible for securing financing and managing cash flows throughout the project, as payments were deferred until substantial completion. This financial arrangement required a robust contractual structure, similar to that used in full-scale PPP projects, to protect both the contractor and the client from potential financial exposure. From a technical aspect, the project was opened
to alternative solutions, allowing the contractor to explore more innovative tunnelling techniques. The evaluation criteria for the RFP phase were split equally between technical (50%) and financial (50%) considerations. This structure emphasised both innovative design solutions and cost-effectiveness. Metrolinx typically provides a stipend for the losing bidders, in this particular case Can$400,000, as the development cost for such a design and build project are significant.
At the start of the RFP process, Metrolinx provided a
Reference Concept Design (RCD) involving a single large box with piles drilled around the tunnel circumference forming the initial support (Figure 2) followed by a box being pushed through. The new structure was to be built next to the existing underpass which was established during highway construction in an open trench. During the RFP process, the client opened up to other construction methodologies than the method suggested in the RCD. The major challenges to overcome were the low
overburden, disturbance to the existing tracks and track alignment requirements to join the new tracks into the existing infrastructure, the tight settlement criteria on the road and existing structures, and very specific criteria imposed on the construction methodology for pre-support installation. TTP initiated a comparative assessment process to
evaluate various alternatives. Several tunnelling methods and designs were assessed based on their technical feasibility, cost, risk profile, and schedule (Figure 3). The outcome of this process favoured the use of twin Sequential Excavation Method (SEM) tunnels (Figure 4), which offered better control over ground behaviour, minimised the risk of settlement and allowed for easier construction in the curved alignment.
Above: Scoring of alternative options considered by TTP November 2024 | 9
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