For the engineers extending Line 2 of Montreal’s Metro northwest to the suburb of Laval, one of the trickiest sections was crossing under Rivire des Prairies, the large waterway that separates Laval from the Island of Montreal.
Charles Chebl, P. Eng. was the project manager for SNC-Lavalin, which led the consortium Groupement SGTM on the project. The consortium did the studies, engineering, procurement and construction management on behalf of Agence mtropolitaine de transport (AMT), the authority that oversees public transit throughout the region of Montreal. The new line was officially opened last April.
Chebl explains that the Laval subway extension stretches 5.2 kilometres. It starts from a new platform built at the existing Henri Bourassa station on the Montreal side of the river. From there it tunnels for 400 metres below the river to arrive at Laval where there are three new underground sta- tions. First is Cartier, then De la Concorde, and finally Montmorency station at the line’s end.
The geological conditions along the route are mainly limestone, a hard rock that is good for tunnelling. However, under the river the rock needed reinforcing before excavations could begin. Working from floating platforms on the river, crews inserted 35-mm steel bolts measuring up to 6 metres long into the bedrock to strengthen it.
While conventional drill and blast techniques could be used to excavate most of the tunnel, in the sections below the river and some other areas, mechanical excavation equipment had to be used. A roadheader excavator was imported from Austria, which ground away the rock using a rotating wheel with large teeth. Two roadheaders and two crews worked simultaneously from opposite ends of the tunnel, each advancing approximately 6 metres every day.
The disadvantages of mechanical excavation are that it produces lots of dust and requires lots of water, but it poses a reduced risk of “overbreak,” eliminates vibrations, and is preferred for environmentally sensitive situations.
Once excavated, depending on the location and the geotechnical properties of the rock, the tunnel structure was reinforced by anchor bolts tied into the surrounding rock, and by wire mesh and shotcrete on the interior. A lining of 350-450-mm concrete was then poured in place.
Auxiliary structures provide dewatering and ventilation
The tunnel opening is 7.5 metres wide in section, with a rectangular profile and low arched roof.
Every 763 metres are auxiliary structures. These are emergency exits with stairs up to the ground level. The structures also house the ventilation equipment, each with two fans — one for back-up — of between 100,000 and 130,000 cfm capacity each. The ventilators provide air to the tunnels and station, but their main function is to evacuate smoke through ventilation shafts in the case of a fire. Hoses and other fire fighting equipment are located in niches cut out from the sides of the tunnel every 100 metres.
The auxiliary structures are located at low points of the tunnel since they also contain pumping pits for evacuating water that seeps down from the rock face. This water drains around the outside of the tunnel casing and is collected below the concrete floor slab. From there it is drained into the pumping pits and out to the city sewers.
Besides the three train stations and eight auxiliary structures, the project involved constructing a garage, a workshop, a reversing station, underground fire safety training centres, parking lots and bus terminals along the line.
Two dedicated 25 kV lines bring in power via the Montmorency station to run the trains, which require about 4.5 MW each. There are also two 1.5 MW emergency generators that keep the lighting, ventilation and other emergency systems operable. About 67,000 kilometres of 25 kV cable is installed to connect different equipment to a central control station. For security there are 100 surveillance cameras and 900 loudspeakers.
Station construction involved contaminated soils and steep gradients
Each subway station was designed by a different architect, and each presented its own challenges. Chebl explains that at the Cartier Station, for example, contaminated soils had to be removed and treated. As well, the Cartier station sits below the water table, so they had to install a watertight bentonite wall all around the site before they could start the excavation. The 700-metre long wall varies from 10 to 14 metres high and is 1 metre thick.
The De la Concorde station is the only one that is built partly within the tunnel. The other two were carved out by cut and cover methods. While the engineering design of the De la Concorde station was “not that complicated,” says Chebl, he goes on to explain that the centre core is “quite impressive, with big exposed columns 14 metres high and 1.2 metres wide.”
At Montmorency, the tunnel had to pass below a college building close to the station. Since the gradient for the track is limited to 6.5 per cent, the station had to be very deep. Its slab level is at 21 metres below ground. Consequently, the poured-in-place station structure has to be robust in order to withstand very heavy loads.
No general contractor
“The managing of the project was a major challenge,” says Chebl, explaining that one reason was the number of different stakeholders involved. Besides the client, Agence Mtropolitain du Transport, the project was done with the collaboration of the Montreal city transit authority, Socit de Transport de Montral (STM), which operates the metro. The city of Laval and the city of Montreal were involved, as were the Quebec Ministry of Environment, Ministry of Transport, local firefighter authority, civil emergency authority and the neighbouring residents and building owners. The SGTM team had daily meetings with these different parties.
There was no general contractor, which meant dealing with many different contractors and suppliers — 100 packages. Weekly coordination and safety meetings were held. Despite over five million man-hours of labour, there were no major accidents. “We are very proud of that,” says Chebl.
Environmental issues were relatively straightforward, and after preliminary studies were done in 2001, excavation began in March 2002 and the project was finished five years later in April 2007 — three months ahead of schedule. The project was done for $748 million. Of that amount, $412 was for the infrastructure, the rest being for fixed equipment, financing and professional fees.
Client: Agence Mtropolitaine de Transport (AMT)
Engineering, procurement, construction management: Groupement SGTM. SNC-Lavalin (Charles Chebl, ing.) procurement, planning, cost, construction management); Tecsult (Francois Labelle, ing.) engineering
Urban planning, architecture: MBGF Consortium. Municonsult; Bisson, Fortin; Giasson, Farregut; Martin, Morris, Marcotte
Major contractors: Pomerleau, Simard-Beaudry, Neilson, EBC, Opron, Kiewit, Garnier, Alstom, US &S