The Mill Woods area of southeast Edmonton was developed in the 1970s and its existing storm water system is prone to flooding during large rainstorms. The system’s outlet to the North Saskatchewan River has a 5m diameter, but there is a 1.7km long section of pipe with a 2.5m diameter, which has also been constructed with an internal wall dividing it into a storm component and a sanitary component.
“Analysis showed this outlet from Mill Woods, the double barrel, was actually a choke point,” says Wayne Pelz, a program manager with the City of Edmonton’s Drainage Services Branch. “Hydraulically it was restricting the capacity of the sewer, and the ability to drain water out of the community.”
Tunnelling started in October for the Mill Woods Double Barrel Replacement Project, worth CAD 36M (USD 35M), which will alleviate the problem by making a new sewer and converting the double barrel to only convey sanitary sewage. It also includes a second tunnel to replace a second section of double barrel. For the City of Edmonton’s Drainage Services Branch, this one tunnelling project entails two different depths, different sizes and different linings.
The first tunnel will consist of a larger 3.5m i.d. replacement storm sewer constructed parallel to the existing 2.5m diameter double barrel outlet. The new sewer will connect to the existing 5mdiameter pipe, for storm water, and then the city will convert the double barrel, as previously mentioned.
Tunnelling will start on a second, shallower tunnel next year to replace the second section of double barrel pipe, about 1.6km long. This is smaller than the deeper double barrel, as it has an existing 1.8m diameter to be replaced by a 2.9m i.d. sewer.
The tunnels are to join at a 90-degree angle. The shallower tunnel, 14-15m deep, runs north to south, and will drop down to the bigger, deeper tunnel, which is about 35m deep, and runs east to west (Figure 1). This junction is where Pelz and the drainage tunnelling team are working right now, and where the project started.
“The one that we’ve begun is the deeper tunnel. It’s into what we call Edmonton bedrock, classified as a soft ground for tunneling,” he says. “It’s a very competent, very hard clay shale; a mixture sometimes of sandstone, siltstones, but generally clay shale. It’s very good tunnelling material; stands up very well. There is no water, and it’s below all of the glacial till that’s at the upper elevation.”
While the deeper tunnel is in more predictable and competent clay, boreholes are indicating that the 14m deep shallower tunnel is on the border of glacial till and Edmonton bedrock, which changes at around 12 to 13m, explains Pelz. “I anticipate we will be getting into clay shale but we’ll probably hit some clay itself, maybe some rocks and boulders. It will be softer ground than the bedrock and there’s a chance of hitting wetter material as well.”
Even so, the tunnels will be in such competent and predictable ground, he doesn’t expect any improvement measures will be necessary, before or during excavation though, he notes that the project will have the standby capacity for should wet conditions be encountered on the shallower tunnel.
Two Lovat soft ground shield machines are being used fortunnelling. One, the M126, is currently being used on another job. Pelz expects the Mill Woods project will have it for the smaller, shallower tunnel during summer 2011.
The other machine, a new 4m diameter Lovat MP159SE series 25500, has so far bored 100m of the 1700m deeper tunnel. This alignment is more or less straight with one 300m-radius curve to start and another 300-m curve at the end as the TBM comes into the receiving shaft. Since its start in October the team is averaging 5m a day (working one eight-hour shift).
“We’re still just starting out,” says Pelz. “It’s a new machine and we’re learning on it. And we’re on the curve—we’ve got about 120m to go. Once we’re out of that and running straight for a couple hundred meters I’m hoping we can do about 10m a day, maybe more.”
Both tunnels will have a precast concrete expanded lining consisting of four 1.2m-long segments, but the size and reinforcement is different due to the varying depth. “They have a joint in each of the panels,” Pelz says. “They are allowed to move somewhat and flex due to some settlement but they’re not bolted in any direction, in the circumferential direction or longitudinal direction.”
Once the liner is assembled and the TBM pushes forward, the liner is expanded slightly to contact the ground and the expansion joint is grouted. For the 2.9m i.d. tunnel, the segments will be about 180mm thick and reinforced with rebar mesh. For the deeper 3.5m i.d. tunnel the segments are 250mm thick and will be reinforced with steel fibres.
“It was one of first times we’re using these, and we worked with the manufacturers and designers who did some tests and calculations,” Pelz explains. “There is a little bit of rebar in there to hold up some lifting pockets, but as a whole, the reinforcing of the precast concrete is steel fibres.”
Deciding on the reinforcement took about two months, and the drainage department worked with an expert to go through theoretical tests before carrying physical testing. “We actually built a few and we tested them until they failed in various configurations of lifting, moving and handling them. We wanted to see that they didn’t crack or break no matter how we used them. So we ran some tests with the manufacturers, and we had approved that they would work,” says Pelz.
Construction started in mid-2009 by excavating a round shaft about 35m deep and of 7.6m diameter, as well as digging the undercut and assembling the TBM, which had been lowered down in three sections by crane.
A similar size shaft will be drilled at the end for the TBM recovery, and a 6.5m diameter shaft in the middle at the same 35m depth for a connection point. The department owns its own shaft drilling rig, which will end up being very beneficial for the Mill Woods project, as Pelz expects it will need 22 shafts in total by the time the project finishes in late 2013.
He estimates they’ll drill the installation shaft early next year for the shallower tunnel, which, with two right angle bends in its alignment, will need two additional shafts so a mobile crane can access the TBM to turn it, plus the recovery shaft. The first three will be of 6m diameter and about 14m deep, and the final shaft of 5m diameter and 12m deep.
“And there are going to be lots of shallower minor shafts to make all of these connections because we’re paralleling this double barrel sewer,” Pelz says. “A lot of them are 8ft (2.4m) diameter, 10m to 12m deep.”
For the deeper tunnel spoil is being removed from the TBM’s cutterhead via a conveyor belt, then into muck cars, which are then transported by a small electric loco running the length of tunnel. With the 35m depth of the shaft, a mobile crane is needed to bring the muck cars to the surface, and the material is then taken to a landfill or recycled when possible. Spoil removal will be more or less the same procedure for the second machine.
Launch chamber for the first TBM drive of the Mill Woods project The Edmonton team assembling the new Lovat TBM for launching Figure 1 – The larger and deeper tunnel runs east to west, and the smaller, shallower tunnel, north to south
