With a healthy budget and years of experience, the City of Edmonton isn’t short on tunnelling projects. In fact its drainage department has had an in-house construction and design section for the past 50 or so years to make sure resources are available to meet the city’s tunnel needs.
“Even way back in the ‘50s the City of Edmonton was leading in this kind of construction,” says Siri Fernando, engineering manager for the city’s asset management and public works department. “The story behind it was at that time there wasn’t anyone else stepping forward to do the construction. So the City took it upon itself to build up a crew, and start developing equipment and expertise to do the work and we’ve continued doing that.”
Fernando attributes the increased amount of tunnelling in the past few years to two main drivers: growth and required upgrades to existing assets. Though the 1970s had been a busy time for tunneling in Edmonton, it had died down in the late 1980s, but picked up some 10 years later.
Located in the province of Alberta, the city of 730,000 people is Canada’s fifth largest, according to 2006 census data, which found the city had grown by more than 18 per cent in the past 10 years. Part of the growth is due to the Oil Sands further north in Alberta, for which industrial services have moved in to Edmonton.
Separately, a number of city-wide improvements have also been lined-up for Edmonton’s bid for the World Expo in 2017. And the drainage services department’s tunnelling teams also work on the city’s LRT projects, rerouting various utilities, and for other municipalities, like Calgary.
“We have continued to build up our expertise, from large diameter tunnels to now we’re doing trenchless work in small diameters, and we have expertise in the sequential excavation method,” Fernando says.
“The government has support behind us, and we continue to work right throughout the year with the current workload. And for the next five years it looks very promising. Provided the funding is still there, all the other projects will go ahead.”
This fall the drainage department saw one TBM finish on the North Edmonton Sanitary Tunnel (NEST), and the launch of the first of two TBMs that will be boring at the Mill Woods Double Barrel Replacement Project. At anytime there could be three to five TBMs working across the city. Plus, Fernando says he has another 17 sites for trenchless work at the moment.
Transitioning to trenchless
About two years ago the drainage services department decided to start doing trenchless work for projects where it had traditionally been doing open-cut work and began training staff and looking into boring machines.
Ray Davies, the department’s general supervisor for open-cut, has numerous projects on the go at the moment across the city using various trenchless methods including pipe ramming, pilot tube and pipe bursting. “We’ll never drop open work,” he says, “in general, hopefully, it’s cheaper, and it’s less disruptive. I think it’s the way of the future.”
To deal with the learning curve, he often takes equipment from one project to another as necessary and when it facilitates training oppurtunities.
For example, Davies is doing a 600m pilot tube for a sanitary sewer in Edmonton’s Sherwood Neighborhood. Both a Akkerman machine and a Bohrtec machine (distributed in the US by Icon) are being used to install a 21″ (53cm) No-Dig clay pipe to replace an 8in (20cm) line. “It was a good job to try trenchless,” he says. “If it did go bad, we’d just dig it up.”
Ground conditions aren’t bad, he says, tunnelling into typical clay till of the area. The difficulty of the project and training is dealing with the region’s cold winters. “We had to shut down last winter.” He explains. “We had difficulties heating the drilling fluid.”
And as November moves to December, the project, halfway finished, faces the same situation. Without a tent to keep equipment and supplies warm—one that can stand up to the cold weather and that can be easily moved as construction progresses—the project may stop and start up again in the spring, when it is scheduled to finish.
Necessity driven
The department is very unusual in Canada, says Arbind Mainali, a program manager with the City of Edmonton’s drainage department’s design and construction section. “We’re probably the only entity that has that kind of arrangement. Everyone, including the provincial government, is getting away from having to do their own construction.”
He describes the City of Edmonton as having a mixed approach. The city does design in-house, but will go outside if it needs help. And while it has a large construction workforce, it still uses outside contractors when needed.
“We have a pretty good balance, and so as a result it allows us to never be caught in any up or down market situation,” says Mainali. “It allows us to really put our costs down, and that’s the biggest advantage.”
In 2005 and 2006, an area in the north of the city was experiencing rapid development, and to start tunnelling on the 3.7km NEST project was urgently needed. Then, like much of the world as the recession dawned, demand died down.
September 27 saw the final breakthrough on NEST, though construction on the entry shaft started in January 2007, and the TBM had been lowered down later in July that year. Tunnelling had stopped and started during the past three years as resources where taken elsewhere for higher priority projects, explains Mainali.
The TBM was one of several owned by the city. The 2.5m diameter Lovat REM100 SE with segment erector and a mixed face cutterhead, had been acquired by the Edmonton in the mid-90s and went through a full refurbishment in 2000 at the Lovat factory. It’s now undergoing maintenance at the city’s machine shop.
For constructing the 2.4m i.d. NEST, the alignment was fixed just north along 153rd avenue in order to connect to an existing earlier phase of the NEST line. It’s sandwiched by a yet-to-be-developed residential area in the north and 153rd Avenue to the south.
Water flows from east to west, so the preference was to tunnel upstream. A large, rectangular entry shaft was dug at 153rd avenue and Manning Drive, about 100ft by 21ft and 52ft deep (30.5m x 6.4m x 15.8m).
“The TBM was assembled in our machine shop and because the pit was reasonably large that allowed us to take an assembled machine and then lower it into the pit with a crane,” says Mainali. “The cutterhead, forward shell and stationery shell were fully attached and once in the shaft then the tail shield and back-up gear were added.”
Three other 10ft (3m) diameter access points are located along the alignment, which curves for 1200ft (366m) and then travels straight for about 11 000ft (3.35km) to the 14ft-diameter exit shaft at 76th street (see Figure 1).
With depths ranging between 13m and 36m, the tunnel was constructed entirely in bedrock, with random layers of medium siltstone and sandstone. In general the tunnelling went well, Mainali reports. However, around the 700m and 1100m points the machine encountered wet clay.
“Managing the water was not a problem, but managing the face – it was very sticky -that was the problem,” says Mainali. “They found that just using water was sufficient in the face, making it slurrylike rather than sticky.
“The rest was just right through as expected in the bedrock, and there was only one other spot where they were anticipating the sandstone and the difficulty that posed was more to do with the fact that it produced lots of dust.”
The project had been doing approximately 25ft (7.6m) for a 10-hour shift, usually running one shift with weekly maintenance after a shift or during lunch breaks. “This is interesting because with the tunnel being so long, and having the muck cars travel quite a distance. Productivity pretty much stayed in there in that range and we were quite fortunate in that respect,” says Mainali.
NEST has a four-piece precast concrete segmental liner about 4.3in (110mm) thick, transported to the site by a local concrete supplier. A train would carry segments in the front cars to be erected, and the other cars would collect muck from the conveyor belt to be taken out. Using two sets of trains allowed for continuous operation.
Although 2-way tunnelling was considered to reduce cost, tunnelling was done in a single push, with no need for tunnel site changes, explains Mainali. As the tunnel excavation got longer, travel time for the muck cars increased and the need for a switch half way in the tunnel was considered to improve the schedule. “But that was unnecessary. There was not a lot of pressure on the schedule because the development in the north still hasn’t taken yet.”
There is the possibility of extending the tunnel, but that won’t happen for decades, says Mainali. “There is plenty of capacity for storage in this trunk. From a design perspective, this is much larger than what’s actually required.”
And that is just one of the many on-going projects going in Edmonton, he explains. “Just because of the system we have, tunneling is one of the main ways of constructing for it.”
Figure 1: Map of NEST project TBM-driven sewer and shaft positions Commencing a drive with a Lovat soft-ground TBM with conveyor spoil discharge and the first precast concrete segmental rings. Note expansion erector