Seattle digs deep for light rail

Written by Mischa Wanek-Libman, editor
image description

Three machines the length of a football field tunnel a three-mile extension for Sound Transit's light-rail system.

 {besps}December13_tunnels{/besps} {besps_c}0|1tunnels.jpg| The massive head of one of the TBMs.{/besps_c} {besps_c}0|2tunnels.jpg| Sound Transit’s TBMs named Togo and Balto at the Capitol Hill station box being lifted out.{/besps_c} {besps_c}0|3tunnels.jpg| A closer look at two of U-Link’s TBMs, Togo and Balto, which were named after famous Huskies who were canine heroes of a grueling sled dog relay to deliver medicine 674 miles from Anchorage to Nome, Alaska, during a diphtheria outbreak in 1925.{/besps_c}


Three machines the length of a football field tunnel a three-mile extension for Sound Transit’s light-rail system.

All photos courtesy of Sound Transit.

In 2009, Sound Transit began construction on a 3.15-mile extension of its light-rail system. The University Link Extension (U-Link) is expected to add 71,000 daily boardings by 2030 and will connect a trio of Seattle’s urban centers including downtown Seattle, Capitol Hill and the University District.

The $1.9-billion project consists of two bored tunnels between Westlake Station and the University of Washington, with two new stations to be constructed at Capitol Hill and the University campus. Boring for the project began in 2011 with three tunnel boring machines (TBMs) constructing the subterranean cavity that will host future light-rail service. Two machines worked in tandem from the University of Washington to Capitol Hill and the third machine traveled from Capitol Hill to downtown and then was dismantled and moved back to Capitol Hill to launch again for the second tunnel.

Tunneling operations were performed 24-hours a day and on an average day, the TBMs could construct 50 to 75 feet of tunnel. Bruce Gray, spokesperson for Sound Transit, says the TBMs productivity was based on maintaining good pressure on the face of the machine and not encountering anything on the face that wasn’t expected.

“There’s extensive monitoring that goes on to make sure you’ve got the right pressure on the face of the machine and not allowing any opportunity for stuff to come in from above that you don’t want to,” said Gray. “We were able to maintain good pressure on the face of the machines throughout the mining and the contractors had a good system set up to keep the segments and the supplies coming into the machines and haul out the muck on the other, so we didn’t end up being muck-bound and having to slow our process.”

The I-5 challenge

Prior to the TBMs launch, Sound Transit had to deal with a tricky situation concerning the tunnel route and its planned course under Interstate 5. According to Gray, boring under I-5 was one of the biggest risks of the entire U-Link project. The portion of the project that traveled under I-5 is in a deep cut through a hillside, which has retaining walls on either side that go down 180-feet deep. In addition to the challenge of boring through the retaining walls, there was the added concern of what the walls held back: A dense Seattle neighborhood on one side and downtown on the other.

“We needed to be 100-percent certain those walls were not going to move when our TBMs came through,” said Gray.

The solution involved the contractor digging down to the depth the TBMs would be passing, which was only about 17 feet from the roadway to the crown of the tunnel, and cutting four windows within the retaining walls, which were then backfilled with control density fill.

“We had all sorts of monitoring equipment on the walls to watch for any kind of movement as the TBMs got close and as they passed underneath,” said Gray. “Everything went off without a hitch.”

Additional hurdles

While Gray says the mining portion of the U-Link project couldn’t have gone any better, expected and unexpected challenges presented themselves.

The U-Link extension also travels under the Lake Washington Ship Canal, which is where Lake Washington drains into Portage Bay, Union Bay and, ultimately, Puget Sound. The TBMs were constructing tunnels 20 feet below the man-made, concrete canal and Gray says a lot of work went into making sure the TBMs could traverse without water coming down and the section was also carefully monitored during construction.

One unplanned problem concerned a shallow area near the University of Washington, where the project ran into some noise issues from the supply trains that were running tunnel segments and other materials to and from the TBM. Gray says noise complaints were coming in from as far as 300 feet away from the tunnel construction.

“The more we looked at it, it had to do with the type of rails and some of the spacing on the rails. We worked with our contractor to put in special padding beneath the rails, vibration dampening, because it’s really the thump from the supply train’s wheels going over the gap in the rails that was causing the sound. We went in and welded the rails together better and put in these vibration-absorbing pads beneath the rails and ties and that helped a lot,” said Gray. “Now, we know how far that noise can travel and how sensitive people can be, especially when you have mining operations going on at night and someone hears a thump under their house.”

Next steps

“The project is going fantastic,” said Gray.

He attributes the well-executed mining with the project currently being more than $100 million under budget and six months ahead of schedule.

According to Gray, the project overall is 80-percent complete with U-Link’s rail systems still being installed and its two stations still under construction.

“One [station] is about 80-percent complete and the other is about 25 percent. The one behind is the one where we had all the TBMs converging, so they couldn’t get started on their station finishes as early as the other one,” said Gray.

He credits the success of the tunneling operation to good planning, a little bit of luck and excellent partners.

“We’re really happy with how it’s going. We did extensive soil testing throughout the entire route to give us as good an idea as we could possibly have of the geology we were going to be mining through and we had great contractors,” said Gray.

Contractors who will again be participating in a Sound Transit tunnel project as the agency recently awarded its largest contract ever for mining four miles of twin tunnels from the north end of the U-Link extension to Northgate.

“One of our contractors on U-Link was the low bidder for that work, so we have a lot of confidence going into Northgate based on how well things went with U-Link. We’re doing as much soil testing and getting as good a picture of the underground conditions as we can for this next section of tunnel based on how well things went with U-Link,” said Gray.

He continued, “To me, we talk about all this technology and amazing machines, but the coolest part [of this project] is being down there and seeing the people who are actually putting these tunnels together. They are literally bolting the segments together and operating these underground cranes and sliding the pieces into place. At the end of the day, it’s people working with theirhands and we couldn’t do it without them.”