The Louis-Hippolyte La Fontaine bridge-tunnel

Narrator [00:00] The tunnel wasn’t bored directly under the St. Lawrence riverbed. Engineers chose to prefabricate seven components that would form the central part of the structure, and then immerse them in a trench dug at the bottom of the river. Before work could start, a dry dock of gigantic proportions had to be created: 2,000 feet by 1,000, 600 metres by 300, at the spot where the tunnel now crosses the river. This dry-dock was used to prefabricate the components and build part of the tunnel’s south shore section. A dry-dock of this size required the construction of several dikes and the excavation of the riverbed to a useful depth of 90 feet, or 27 metres. By the end, over two million square metres of earth and rock had been hauled out. Those were replaced with only around 250,000 square metres of concrete. Such a quantity of concrete required the construction of a plant nearby. From production to casting, concrete was subject to rigorous checks. During the hot summer months, ice was put in the concrete to cool it down.

Only a comparison can give people an idea of the scope of this project. Each of the seven components forming the immersed portion of the tunnel weighs 32,000 tons, which is equivalent to the largest boats able to dock in the Port of Montreal, and the approximate size of a football field. Since each section of the tunnel is 360 feet long by 120 feet wide, it cannot be cast all at once. Rather it is divided into sections of about 50 feet, or 15 metres, in length. Verification of tension for the prestressing cables as well as the homogeneity of the concrete mix is carried out at each step of construction. When the water rises to the right level, the components start to float. Once the dry dock is flooded, the components can be moved. Hauling around these 32,000-ton behemoths to the spot where they will be lowered into the river and them joining together under nearly 80 feet (25 metres) of water, and all this without disrupting maritime traffic: that is the challenge the project’s engineers are facing. Towing the components in place, lowering them, and connecting them in their definitive position; these operations form the most crucial part of the project.

To move such a frightening mass and assemble it in such a precise way requires both enormous strength and uncanny precision. As each tunnel section is lowered to its final position and connected, the watertight joints are closed up, joining all the components. These joints consist of metal plates and rubber collars that fit together tightly. A 250-ton strength tightening screw is used to temporarily seal the assembly. The natural 10,000-ton hydraulic pressure of the river then compresses the rubber collar. These collars ensure that the tunnel remains watertight until the reinforced concrete interior can be completed to make the entire structure homogeneous. Machines are used to pump sand at tremendous pressures in the space between the base of the components and the bottom of the trench, so as to ensure a very compact foundation. With an average depth of 80 feet below low-tide level, at the lowest point of the roadway, the tunnel is literally resting on sand cushions. Afterwards the trench is filled with crushed stone. Maritime traffic is in no way disturbed by the tunnel. It was actually designed to double the current width of the channel, and increase the minimum draft to 40 feet or 13 metres. Thanks to the success of this Quebec-led initiative, everyone can now accomplish without even realizing it a once insurmountable challenge: crossing the St. Lawrence River. [06:58]