Because of complex geologies like faults, fractured zones, karst cavities, unexpected loose ground, squeezing ground as well as water-bearing formations, the stability and safety of tunnels are challenging topics in the construction process. Three examples in Canada have recently illustrated these issues and recalled that only when the location and the size of the problematic geological
formations are accurately predicted in advance can relevant countermeasures be applied during the construction of the tunnels for the purpose of prevention of most geological disasters such as cave-in, water inflow or expensive downtime.
Difficult ground at Niagara Falls
At Niagara Falls, Ontario Power Generation (OPG) is digging a third tunnel to carry water from the Falls, under the town and to the Sir Adam Beck generating station in Queenston. The 10.4 km tunnel is supposed to be completed by 2009, but it looks as though it will be delayed by at least a year because of unexpectedly harsh conditions.
In the summer of 2007, OPG reported that they were encountering unstable rock and soils under the Saint David's aquifer. By September 2007, crews had excavated 1,350 metres of tunnel, less than half of what was hoped. Then this March, OPG reported there were further delays. At 31st March, 2008, the TBM had advanced 1,848 metres. In May, digging has passed the two-kilometre point, but work is slower than planned and so far behind that officials are reviewing its cost and budget. The TBM was 2,046 metres into the tunnel as of 19th May. That marks approximately one-fifth of the way into the 10.4 km tunnel.
The rock conditions under the Saint David’s gorge continue to be the problem that makes progress difficult. Loose rock from the tunnel ceiling falls in behind the machine. That overbreak rock has to be removed. The cavities in the rock will be filled in when the tunnel’s concrete lining is poured. Once they get beyond the Saint David’s gorge, crews expect better performance. Experts predict the loose rock conditions will last until the 2,300 m mark, a point the machine could pass in July.
Strabag, the contractor, has a CAD600 million fixed price contract to complete the work and wants to realign the tunnel. Strabag is using "Big Becky”, the world's largest hard rock tunnel boring machine made by Robbins, to dig the tunnel, which is 14.4 metres in diameter and constructed with a 600 mm-thick unreinforced prestressed concrete lining. It is 140 metres deep, parallel to the existing twin tunnels built in 1955, but deeper. The intake is under the existing barrage dam, and will send an additional 500 cubic metres per second of water to power the Beck generating station.
Hatch Mott MacDonald and Hatch Acres are owner's representatives on the project. Designers are Morrison Hershfield and ILF of Austria. Click here, here and ca/13. Visit www.therobbinscompany.com, www.hatchmott.com, www.hatch.ca, www.morrisonhershfield.com and www.ilf.com
Considerable uncertainty remains with respect to the schedule until the TBM has advanced beyond the Saint David's gorge and establishes consistent tunnelling performance. The project cost estimate will be reviewed in conjunction with any changes to the project completion schedule, and a dispute resolution process to review, among other things, the actual subsurface rock conditions compared to those that were anticipated as part of the design-build contract.
Safety concerns at Capilano-Seymour
Across the other side of the country in Vancouver, work on the Seymour-Capilano twin tunnels being dug by Bilfinger Berger below Grouse Mountain has been halted about half way. The 2 x 7.1 km tunnels, built by two 3.8 m-diameter Robbins TBMs, are to carry water between the Capilano reservoir to a water treatment plant in Seymour, as part of a massive water supply project by the Greater Vancouver Regional District. Click here, here and ca/25.
Bilfinger Berger, which has a CAD100 million fixed price contract, halted the work in January after falling rock prompted safety concerns. The firm encountered increasingly hazardous rock behaviour and conditions like falling and bursting rock in 2007 leading to unsafe working conditions. A new safety plan was prepared by Metro Vancouver, but the contractor refused to recommence work, after estimating its concerns around worker safety had not all been met to satisfaction. In late May, Metro Vancouver terminated its contract with Bilfinger Berger and is said to be in discussions with the two contractors who were also in the original bidding. It is estimated that the work will now cost twice as much to complete.
Golder Associates and Hatch Mott MacDonald were the tunnel engineers, and the overall project Seymour-Capilano filtration project is being managed by Pacific Liaicon, a subsidiary of SNC-Lavalin. Visit www.golder.com and www.pla.bc.ca
Langstaff trunk sewer in York region
Lastly, a Lovat EPB tunnel boring machine digging part of the York-Durham sewer system north of Toronto is stuck underground in mud. On 2nd May, the machine became stuck and six workers were forced to abandon it and its trailing equipment stranded underground at a point east of Dufferin Street, north of Highway 407 and to run 1.8 kilometres to the entry point when 1,800 cubic metres of waterlogged soil suddenly came pouring into the tunnel they were boring below the local aquifer. They were working on a section of the Bathurst and Langstaff trunk sewer project, one of many projects that comprise big pipe projects across the region. This section of the sewer project was expected to be completed by November of this year. While the project is delayed as a result, a one-year delay is a conservative estimate. Visit www.lovat.com
What exactly caused the soil to leak through a seal between rings of precast concrete that are laid behind the machine as it moves forward is still being investigated. Work on a bulkhead to seal off the pipe started right after the incident but it took until 12th May to make it airtight. The York region has also hired a firm to look at ways to mitigate the impact. As of 5th June, no final cost of the incident had been determined. Who's going to pay for rescuing the boring machine and unclogging the pipeline will only be determined after the outcome of an insurance probe that will consider whether human error was involved.
The rescue operation will involve building a 5 x 30 m shaft down through the aquifer to pull out the 10 m-long machine, plus trailing gear (hydraulic, electrical and ventilation equipment) that stretches about 50 metres behind, as well as suck out the mud that now fills 300 metres of pipeline laid deep under the neighbourhood. In the meantime, another new Lovat TBM will start excavating from the opposite end from Bathurst Street. The machine will dig its way to where the original TBM is, which should be removed by then.
McNally and Aecon are the contractors for the 9 km Bathurst and Langstaff trunk sewer, which they were excavating 22 metres below ground. The Langstaff sewer is part of a massive sewage system being constructed to carry waste from York region to a treatment plant on Lake Ontario in Pickering. Click ca/26. 26/08.
