FAULT ZONES, active subway lines, and a long-disused quarry were just a few of the obstacles facing a recently completed water tunnel in Montreal, Quebec. Although the 4km-long tunnel through the limestone bedrock of the city seemed simple enough, multiple challenges presented themselves on the road to success. Local contractor Foraction Inc. tackled the project using a 3m diameter Robbins double shield TBM.
FROM PLANS TO REALITY
In eastern Montreal, home to more than one-third of the city’s 1.7 million inhabitants, water in many areas is supplied by only one primary reservoir.
Addition of a buffer into the system for peak seasons was needed, and this combined with a growing population meant that the project was designated of primary importance by Montreal’s Water Department in 2010. Luckily, the city already had a healthy start to the project. The Rosemont Reservoir, a large structure for potable water built in 1962 but decommissioned 20 years later, could be modernized and brought online. According to the Water Department, the project has five phases: renovation of the reservoir; construction of the 4kmlong new water main; rebuild of the new pumping station; construction of several connecting water mains in the existing network; and further improvements of the reservoir site including waterproofing the roof.
Investigations of the tunnel site prior to construction were originally conducted in the 1970s. "Over the course of two years from 1976 to 1977 they drilled about 38 vertical boreholes directly over the planned tunnel route," explains project geologist Brigitte Gagné for Exp Inc.
"Due to the presence of buildings and existing underground infrastructure, there were big gaps between the boreholes with no information. In addition the physical data including the rock cores wasn’t kept."
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By GlobalDataExp was hired by the City of Montreal to conduct a new geotechnical study in 2012 and fill in those gaps. Geologists conducted 27 diamond-drilled borehole tests to a depths ranging from 21m to 65m below residential and commercial neighborhoods along the tunnel alignment. Special areas of interest included an abandoned quarry directly above the tunnel path and the area surrounding the city’s subway system.
To avoid surface structures, the new boreholes were drilled at inclined angles so that they would terminate below, and just next to, the tunnel route — a safer option compared to the boreholes conducted in the 1970s." Gagné explains, in the original study the vertical boreholes were left open and equipped with standpipes to monitor water levels. When the TBM hit these holes, groundwater would flow into the tunnel, sometimes up to 5 gallons per minute. At the time of the second study, Exp anticipated that this could create a problem and directed their boreholes to end within meters of the future tunnel. They were grouted and several were equipped with vibrating-wire piezometers for groundwater level monitoring, The new core sampling program indicated the presence of medium to thinly bedded limestone, with some shale and intrusive rocks, mainly dykes and sills. While the limestone averaged anywhere from 50 to 300 MPa (7,200 to 43,500 psi) UCS, rock in the intrusives ranged from 100 to 430 MPa (14,500 to 62,400 psi). The dykes and sills were as small as a few centimeters wide and as large as 8m to 10m wide. In addition the company identified three areas with potential fault zones that were not indicated on published geological maps. Due to the harder intrusives, it was decided to lower the tunnel alignment by a few meters, making the rock mass more than 90 per cent limestone for easier TBM excavation.
Even with the lower tunnel alignment, contractor Foraction, took precautions prior to the start of excavation with the TBM. "Some zones during drilling showed fractures, one with water. More borings were conducted in these areas and no more water surfaced," says Roger Lepinay, equipment manager for Foraction. "We went to these fracture zones, drilled 50m deep from the surface, and cement grouted with four grouting holes in two areas. Two holes took a lot of cement to improve those areas.
"Of course we didn’t know everything about the rock there, but we reduced potential impacts where we knew there were some issues that could arise. Like any tunnel there could be surprises, but we did what we could to prevent any damage."
Machine Mobilized
The TBM was rebuilt in Robbins’ Solon, Ohio, shop after being used on tunnels in Melbourne, Australia, and Kansas City, Missouri. The machine was delivered to the jobsite in Canada in 2014 following assembly. The double shield machine was not equipped to install segment rings, but rather was designed to provide shielded protection to workers in the tunnel. Ground support included rock bolts and wire mesh. A back-loading cutterhead with 17-inch disc cutters provided long cutter life and safe, efficient cutter changes during excavation.
The machine was set up at the 33m deep access shaft as the Montreal winter set in. Lepinay says, "we launched the machine and did the first few meters before Christmas 2014, and worked on improving advance throughout the winter months."
Even with a drill-and-blasted back gallery 30m in length and a front gallery of 59m, the TBM required an abbreviated startup arrangement. Boring began with just three back-up decks and two muck cars, and was converted to a full setup in February 2015. The full setup included a California switch and two trains to take an estimated 6,300 muck loads–about 73,000 metric tons of rock–out of the tunnel.
The muck train could accommodate two pushes worth of excavated material to minimize downtime in the long, small diameter tunnel.
Ventilation logistics were also of concern due to the small tunnel size. The first kilometer was ventilated from the launch shaft, while three surfacedriven 800mm diameter surveying wells at the 1, 2, and 3km marks ventilated the rest of the tunnel as the TBM progressed. The surveying wells also provided monitoring of vibration, groundwater, and other parameters as the TBM passed.
Below City Structures
The City of Montreal anticipated several challenges including excavation directly below the Montreal Subway. The tunnel section crossed 15m below the subway and was the subject of intensive study. Prior to the passage of the TBM below the subway line, the city ordered 3D modelling and measured vibrations in the subway tunnel, and conducted a thorough examination of rock cores between the tunnels using angled boreholes. "All our studies indicated the crossing would not impact existing structures and the rock was competent.
We deemed the installation of seismographs unnecessary. Everything turned out just fine," Gagné says. The TBM then passed below an old quarry that had been filled with industrial and domestic waste. "The quarry utilized dynamite so there were a lot of fractured walls. Luckily the fracture network did not reach the rock mass at the level of the tunnel. The tunnel ran about 10m below the fractures. However, four old boreholes that had not been grouted over the quarry site leaked soiled groundwater (leachate) into the bored tunnel. This caused a depression of the groundwater table by 10m to 12m. No other adverse effects were encountered and the city is still monitoring the area for ground subsidence," she explains.
Gagné continues that even with the tunnel being 40m below ground surface, construction could still be heard as the TBM passed below homes and businesses.
"We received some calls from residents above the tunnel that could hear it. They could hear the cutterhead grinding, starting, and stopping, and we correlated this with the TBM. We found that homes with foundations directly on rock could clearly hear it, whereas if the home was built on thick overburden on top of the bedrock they could not. Installation of seismographs inside residences confirmed that no structural vibration accompanied the sounds."
The City of Montreal notified residents along the rest of the tunnel path after initial sounds were reported that they could expect to hear the TBM during its construction. Overall residents were very tolerant of the extra, low-level noise.
Breaking through the Bedrock
Ground conditions on the project were described as "nearly ideal", with the exception of the aforementioned fault zones. Excavation proceeded at a slight upward grade of 0.5 per cent in the first 1.8km of tunnel, while the following 2.2km had an upgrade of 1.18 per cent.
The contractor was able to successfully navigate these zones despite the varying rock strengths. Even with geologic challenges including some water inflows and over-break in small sections, the contractor was able to achieve advance rates of up to 38m per day in two shifts of nine and a half hours each. Most of the ground was self-supporting, though the contractor installed rock bolts every 2.5m into portions of the tunnel crown, while mesh, rock bolts, and steel sheets were used in short sections of unstable rock.
In one of the fault zones, Lepinay describes difficult overbreak conditions: "We were stuck for some time in one spot. The machine could not grip and we had a cavern of over-break 5ft high and 10ft long. We also had some water coming in that had to be pumped out of the tunnel."
The machine was pushed through the over-break section and was able to successfully navigate the rest of the fault zones by reducing speed and doing half or quarter pushes rather than a full TBM stroke. "They went through very successfully in the other fault zones by regulating the controls of the TBM," Gagné observes.
Towards the end of the drive the TBM achieved its best advance rates, up to 600m in one month. "By the end of the drive we had improved any weak points and minimized downtime. The distance of the tunnel did not penalize us, and the further we got the greater the production we achieved," Lepinay says.
Breakthrough into a 45m-deep exit shaft occurred on November 12, 2015, and was celebrated with a ceremony presided over by the mayor of Montreal, Denis Coderre. For Lepinay, the ceremony held special significance: "For me personally, this is my last contract before retiring. I started in 1987 and have worked on many construction projects. I’m also proud of Foraction, as this is the first job of this type that they have worked on."
With tunnelling complete, the contractor cleaned the tunnel and as of February was installing the carrier pipe, in 6m sections, a total of 650 pipes. The 2.13m I.D. pre-stressed concrete cylinder pipes (PCCP) are 15.8cm (6.23 inch) thick and weigh 19,550 kg each. After the pipe is installed the annulus will be grouted with cellular concrete grout. There will be several more work stages to be carried out before the Rosemont Reservoir is finally reconnected to the water main network in 2019.
