March 23, 2006

Sound Transit digs a cutting-edge tunnel

  • Crews are using state-of-the-art construction techniques to build a light-rail line beneath Beacon Hill.
    Sound Transit

    Photos courtesy Sound Transit
    Beacon Hill station's tunnels are being excavated in sections. Temporary structures built in the middle help support the ground during the process and will ultimately be removed.

    Sound Transit is in the midst of building one of the most exciting parts of the Central Link light-rail project: the almost mile-long underground segment beneath Beacon Hill.

    Our Central Puget Sound region's hills, valleys and waterways pose significant geographic and geotechnical challenges for building a mass transit system. Light-rail technology solves them by operating effectively in all three possible configurations: elevated, surface-level and underground.

    Going underground through Beacon Hill enables connecting Seattle's Sodo and Rainier Valley areas while also providing service to Beacon Hill. Effectively serving all three of these important communities wouldn't be possible with a surface-level or elevated alignment. To make these connections happen, Sound Transit is using state-of-the-art tunneling methods as we build a fast and reliable mass transit system for the region.

    The Beacon Hill segment's underground work consists of two main tasks that are happening concurrently:

  • Working from the top of Beacon Hill downward to build the 160-foot-deep Beacon Hill station, using techniques known as slurry wall construction and sequential excavation mining (SEM).

  • Using a tunnel boring machine (TBM) to build two parallel tunnels.

    In both cases, Beacon Hill's glacial soils — typical of the Central Puget Sound area, with strata ranging from sand to hard clays with large boulders — offer technical challenges. But we've come at the job with experienced engineers, geologists and a respected contractor, Obayashi Corp., armed with seasoned staff and state-of-the-art equipment.

    Building under Beacon Hill

    Riders will access the Beacon Hill station via high-speed elevators that transport them 160 feet down to the underground platforms. The station is being built on a one-square-block site located at the intersection of Beacon Avenue South and McClellan Street South.

    The project's designers grappled with how to dig 160-foot-deep shafts, including a 50-foot-wide main shaft and a 30-foot-wide ancillary shaft, without their vertical walls becoming unstable during the process. To meet this challenge, the team adopted a slurry wall design.

    Working in small sections or panels, special equipment including a hydrofraise was used to dig approximately 3-foot-wide holes to full depth around the shaft perimeters that would be filled with concrete to form vertical walls. To keep these narrow and very deep panels from collapsing inward during the excavation phase, they were filled with thick bentonite slurry — thus the name slurry wall. Once a panel was excavated, reinforcing steel was installed and then concrete was pumped in, displacing the slurry.

    When all the panels were done, the finished product was a 3-foot-thick concrete wall. Today people are amazed to look into the shafts and find out the walls were built before any of the ground inside was removed.

    Meanwhile, using jet grouting techniques from the surface, we worked to improve the stability of underground areas where we would later mine. This involved using a tall-mast jet grout drill boom to pump cement grout into the ground at high pressure. This work, to reduce mining risks, targeted key underground areas where geotechnical tests showed particularly crumbly soils.

    Once the main vertical shaft was excavated, Obayashi began horizontal sequential excavation mining with specialized articulating-boom excavators and tunnel-mining drill rigs. The SEM work uses a predetermined, highly disciplined sequence for excavating horizontal passages in sections and temporarily supporting the ground using steel girders and shotcrete. The continual cycle of excavating and reinforcing requires 24-hour work shifts.

    SEM techniques have been used extensively in Europe and Asia, and to a lesser extent in North America. The Beacon Hill project is the deepest North American application of the techniques in glacial soils.

    Boring the twin tunnels

    The Emerald Mole is boring the first of two tunnels through Beacon Hill. Excavated material passes through an area behind the machine’s cutter head to a conveyor system.

    With the station under construction atop Beacon Hill, Obayashi crews working on the west flank of the hill began preparing the west portal of the Beacon Hill tunnel for the launch of the tunnel boring machine, called the Emerald Mole. The machine was launched in January and is currently excavating the southbound tunnel. Once the southbound tunnel is complete this fall, the machine will be disassembled and transported back to the west portal where it will be re-launched near the end of the year to construct the northbound tunnel.

    The machine, a type known as an earth pressure balance TBM, is ideal for glacial soils. It is a truly impressive piece of equipment: stretching longer than a football field and weighing about 642 tons with all its trailing gear.

    The business end of the TBM is a 21-foot-diameter cutter head that revolves up to 2.5 times per minute. The cutter head is equipped with disks and other cutting tools, as well as openings for excavated soil and rocks to pass into the front of the TBM. The excavated earth is mixed with biodegradable conditioning agents sprayed through nozzles in the cutter head. Depending upon soil types, the conditioning agents include water, bentonite or polymers. The resulting paste is transported by a corkscrew-like conveyor located behind the cutter head, which deposits it on a conveyor belt that carries the spoils to a staging area outside the portal.

    As the tunnel is constructed, it will house a temporary rail system used for moving in sections of the precast concrete segments that form the permanent lining of the tunnel. Each section, or ring, is made up of seven segments that when installed form 5 linear feet of completed tunnel. A hydraulic arm inside the TBM positions each piece and then the completed ring is secured into place with steel bolts and cement grout.

    This permanent tunnel lining is what enables the TBM to move forward by providing a fixed surface to push against. The front section of the TBM is equipped with 16 hydraulic jacks spaced around its perimeter. Pushing against the liner, these jacks are used to propel and steer the machine. They are guided by sophisticated technology that controls the machine's position to an accuracy of within an inch.

    January's launch of the TBM was timed for the machine to reach Beacon Hill station once SEM excavation has hollowed out the southbound passenger platform. When the TBM arrives, Obayashi will have a chance to perform maintenance on the cutter head before the machine continues onward. The TBM work will intensify after the machine leaves the station, moving from one eight-hour shift each weekday to around the clock five days a week.

    Looking to the future

    Tunneling will play an important role as we work to expand the Link system northward. Given Capitol Hill's steepness and development density, and the need to get across the Montlake canal, tunneling is the only effective way for us to provide service between downtown Seattle, Capitol Hill and the University of Washington.

    A major priority for Sound Transit is securing the $700 million federal grant we need to move forward with construction on the university Link extension. The Federal Transit Administration recently awarded Sound Transit's application its highest possible rating based on the project's benefits, including adding more than 66,000 daily riders to the system.

    The university Link project includes 3.1 miles of tunnel-boring and the Capitol Hill and UW stations. These two stations are shallower than Beacon Hill's, so they can be built from the surface downward using less costly cut-and-cover methods. Our goal is to start university Link construction in 2008 — right after we finish the Beacon Hill project.

    The Sound Transit board is also working with the public to identify the projects that should be part of a future Sound Transit 2 ballot measure. Among the priorities is extending light rail farther north from the university to Northgate via the Roosevelt community, including 3.9 miles of tunneling with two additional cut-and-cover stations.

    Our Beacon Hill work offers an exciting glimpse of the future.

    Joe Gildner is deputy director for Sound Transit's Link Light Rail Department. Dick Sage, deputy construction manager, is overseeing construction of the Beacon Hill segment.

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