A crucial piece of the new Port Mann water supply tunnel will be the concrete tunnel liners. See how these custom-designed sections are made, and find out how they will be installed in the 1 km tunnel being bored beneath the Fraser River.
Row upon row, semi-circular pieces of pre-cast concrete lie stacked in a Richmond storage yard. There’s an interesting story behind these unusual shapes. Like a puzzle, they combine to become something more than a collection of individual parts.
But that will happen far underground, beneath the bottom of the Fraser River. In their finished form, they will never be seen by the public. Nonetheless, these sections of precast concrete are destined to become an important piece of Metro Vancouver’s water supply system – as part of the Port Mann Water Supply Tunnel.
The existing Port Mann water main is one of several key water supply links to municipalities south of the Fraser River. When complete, the new, larger water main will help ensure the continued, reliable delivery of clean, safe drinking water to these municipalities, and will double the capacity of the existing main.
The water supply tunnel is being constructed underground, beneath the Fraser River. A ‘T-B-M’, or tunnel boring machine, will dig its way north beneath the river, creating the tunnel. Along the way the tunnel liner sections are installed, to protect and support the steel pipe that will eventually carry the water. But first, those concrete tunnel liners must be manufactured.
The liners are made in Richmond, at the Armtec precast concrete plant. A dedicated crew has been working for about a year, manufacturing the six thousand individual sections required to create the one thousand ring segments that will line the one kilometer tunnel.
“To produce the one thousand ring segments, which are six thousand units, is going to take approximately 14 months,” says Alan Maurer, Armtec’s plant manager. “Generally we produce about 18 sections in a day, which comprises 3 rings. We do double cycle if the schedule warrants and we can turn over the forms twice in a day, to produce 36 units if necessary. Each segment is one metre long. Put together, the one thousand rings make up the one kilometer tunnel.”
It takes 6 specially shaped pieces to form each ring of the tunnel liner. Once underground, sections of steel sleeve are inserted inside the liner. The thick liners are built to withstand the extreme pressure far below ground, because the water supply pipe cannot be pressurized — due to variability in the water flow… and the weight of the earth and the river above the tunnel is considerable.
Producing the six segments that make up one section of tunnel liner takes approximately 3 hours. Alan Maurer explains how they do it.
“Form preparation is the first step. The forms are open and they are oiled with a bond-breaking material to keep the concrete from sticking. From there we make sure they are closed up properly, with all the inserts in place. We check the final dimensions, do a final quality check and pre-pour sign-off and after all this is done and documented we are ready to start pouring concrete.”
A warning bell rings and an enormous yellow bucket, transported on an overhead track, is positioned over one of the forms. A worker pulls a large lever and the form starts to fill. Each batch of concrete is tested before it is poured into the form, to ensure it meets specifications. The concrete is a relatively dry mixture. A unique feature is the use of steel reinforcing fiber, rather than the traditional rebar cage. These tiny slivers of metal do the same job as traditional reinforcing steel, giving each segment additional strength that can’t be achieved with just concrete.
A crucial step is demolding, when the concrete segments are removed from the forms where they cured. A vacuum lift is used, to make it possible to hoist the concrete liner, with its smooth, curved surfaces. Each piece must be flipped 180 degrees using a special machine designed for this express purpose. Then they are stacked on a pallet for storage, until they are needed at the job site.
A total of 6000 individual sections will be produced, creating the 1000 – 1m wide rings, which will line the 1km length of the water supply tunnel. The final manufacturing step is to glue on the gaskets which will provide a seal between the sections of ring. Once installed the space between the rings and the inner steel pipe will be filled with cellular concrete.
According to Maurer, cooperation and collaboration are important aspects of this project.
“McNally/Aecon (project general contractor) and Armtec have worked together in the past and on this project, right from the outset we got together and partnered on it. In pooling our expertise, it really helped in making this a successful project.”
The project is expected to be completed at the end of 2015.