Digging It: How to bore a tunnel, in pictures

Construction of the Viaduct replacement tunnel is about to begin. The project's workhorse is a state-of-the-art boring machine that will generate as much thrust as 13 space shuttles.
Construction of the Viaduct replacement tunnel is about to begin. The project's workhorse is a state-of-the-art boring machine that will generate as much thrust as 13 space shuttles.

The tunnel boring machine is anything but boring. Its guts, including the control cab, cutterhead, 56 thrust jacks, 24 engines and bundles of tubing that, like a circulatory system, deliver hydraulic fluid to its many moving parts, are all housed within this clean white shield. The most sophisticated TBM in the world, it's poised to bore a two-mile-long tube under the city.

Greg Hauser, Deputy Project Manager, is a veteran of many tunnel works. He's standing at the steel bulkhead which forms the internal rear wall of his soon-to-be-rotating cutterhead.

And speaking of the cutterhead, its green face, which won't be green much longer, comes equipped with 20 of these 14-inch, carbide-tipped Robbins' disc cutters, which can spall (meaning smash) any large boulders the machine may encounter. 

As the cutterhead bores ahead, these tunnel liner segments are fixed and bolted into place behind it. The pre-cast concrete liners form the inside wall of the tunnel. These three segments are ready to be hoisted down into the boring machine by the overhead gantry crane in the background.

This set of 10, six-foot-long, two-foot thick liner segments, when assembled inside the machine, will form one of the tunnel's 1,450 liner rings.

The bright red erector arm sets each 16-ton liner segment in place. The plug-shaped black thrust jacks brace themselves against the liner segment so they can push the cutterhead forward another six feet. All this takes place  out of sight, inside the machine's white shield.

Closeup of the segment erector arm and three of the machine's 56 thrust jacks.

The white tailgate by the construction workers's leg is the muck hopper. Muck is the term used to describe anything and everything the machine digs up. The tunnel muck will fall from the tailgate onto a conveyor belt that will transport it to a waiting barge. You can see the belt's framework is in place. The belt itself has yet to be installed.

This control cab, onboard the TBM, is where two operators and two engineers will monitor and control every single mining minute.

All photos courtesy of Doug MacDonald.

 

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About the Authors & Contributors

Doug MacDonald

Douglas MacDonald

Doug MacDonald is a pedestrian activist who once served as the Secretary of Transportation for Washington state.