CASE STUDY: With EPB technology in its early development, and with compressed-air, hand-excavation of the first rail tunnel under the St Clair River one hundred years earlier as their guide, engineers were convinced that pressurised mechanised TBM technology with a modern bolted and gasketed precast concrete segmental lining was the choice to be made in the early 1900s for the new larger diameter replacement subaqueous rail link.
SITE VISIT REPORT: MAY 1994
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n 1 March 1993, a joint venture led by Traylor Bros of the USA started work officially on the railway project under the St Clair River between Sarnia in Ontario, Canada, and Port Huron, Michigan, in the USA (Figs 1 and 2). On 1 November 1993, the JV launched a 31ft 2in (9.52m) diameter EPB TBM manufactured by Lovat Tunnelling Equipment of Canada from the Sarnia portal. The schedule for excavating the 6,000ft (1.8km) long x 27ft 6in (8.4m) internal diameter tunnel was 8.5 months and at planned rates of about 240m/month or 10m/day. “Experience to date [March 1994] has proven that an EPB TBM is the right tool for the job,” said Neville Harrison, Construction Manager for the Design and Project Management team, Hatch-Mott MacDonald, a joint venture between Hatch Associates of Mississauga, Canada, and Mott MacDonald of the UK, which also completed detail design for the scheme. “Through careful operation of the TBM, it is possible to limit ground movement to extremely fine increments with the relatively impermeable clay till responding to changes in ground pressures within one or two hours. Face loss has been limited typically to less than 1%.” The 100m3 of unbulked material for each 1.5m wide ring of precast concrete lining is excavated in about 30-35min and transported by three trains of 4 x 18yd3 muck cars. Only about 5% water needs to be added to the plenum for effective EPB extrusion of the clay till through the 1.2m diameter x 12.6m long screw conveyor of the EPBM. Once through the initial learning curve, progress reached a best to date of 12 rings (18m) in a day of three 8h production shifts. “We hope to improve this to a steady 14 rings (21m) a day,” said Michael Roach, Project Engineer for Traylor Bros. With Traylor Bros as lead (40%), the JV includes Foundation Co, Canada; Frontier-Kemper, USA; and Wayss & Freytag, Germany (20% each). Figs 1 and 2. The new St Clair River rail tunnel across the Canada/ USA border (below), is aligned in a narrow band of soft material (right)
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St Clair River
TBM purchase
In a departure from more normal practice, the TBM and the tunnel’s precast concrete segmental lining were purchased by the client, the Canadian National (CN) North America railway company, owner and operator of the existing 100-year old railway tunnel between its Canadian network and the network operated by Grand Trunk Western, its USA subsidiary. Pre-purchase of the TBM and the lining through what is known as a Negotiated Compressed Schedule (NCS) contract, was adopted by CN to help ensure completion of the Can$200 million facility within a tight planning, design and construction schedule of just 33 months (in March 1994 Canadian dollar values Can$1 = US$1.35). Delivery of a new TBM within the construction contract would have required at least a further 12 months in the contract period. Preorder of the lining reduced the risk of segment production holding up tunnel progress. As part of the NCS procedure, a short list of seven prequalified construction groups was required to join round table discussions about the special demands of the project and choice of machine and the tunnelling method. Only those groups participating in this Technical Committee and accepting Committee proposals could bid the project. Among the most significant developments, these discussions led to adoption of the EPB technique for tunnel excavation and provision of a backup compressed air facility in the tunnel to avoid potential inundation. Following the bid process, the Traylor & Associates joint venture of Traylor Bros (USA), Foundation Co (Canada), Frontier Kemper (USA) and Wayss & Freytag (Germany) was awarded the contract in October 1992 after submitting a price of Can$52.8 million, the lowest of three received. The owner also agreed to partnering as a feature of the contract management, following a proposal from Traylor. After an international tender, CN placed a base price EPB TBM order of about Can$17 million with Lovat. This is machine number 125 manufactured by Lovat during its 20year history and is the largest TBM built by the company to date. The 9.5m diameter x 10.25m long tunnelling shield weighs about 524 tonne. The 72m long trailing backup adds a further 200 tonne. The cutterhead is fitted with 6 pairs of flood control doors and is dressed with 200 spade type teeth and 53 backloading ripper teeth or 53
