Shani Wallis, TunnelTalk
Two running tunnel contracts and contracts to build two underground stations have mobilized for construction of the new University Link extension to Seattle's light rail transit system. Sound Transit has applied the experience of the Beacon Hill underground section on the 14-mile (22.5km) Central Link system that opened last year to the specification of the TBMs and their operation. TunnelTalk visited the tunneling contractors in June to know of their plans of attack.
Fig 1. Seattle's U-Link tunnel contracts
- Three EPBMs will be used to complete the 3.15 miles (5km) of twin running tunnels for the Seattle's University extension (U-Link) north from the Pine Street stub tunnel (PSST) of the downtown LRT/electric bus combination tunnels, through an underground station at Capital Hill and on to a U-Link terminus station adjacent to Husky Stadium at the University of Washington campus (Fig 1).
- Traylor Bros/Frontier-Kemper has ordered two Herrenknecht EPBMs for its twin 2-mile (3km) tunnels for Contract U220 while on Contract U230, the Jay Dee/Coluccio/Michels JV has ordered an Hitachi Zosen EPBM to drive both 3,800ft (1.1km) running tunnels.
- The two Herrenknecht machines for Traylor/Frontier Kemper will drive from the University of Washington (UW) station to the Capital Hill station box and the Hitachi Zosen machine for Jay Dee/Coluccio/Michels will work south also from a working shaft in the Capital Hill station to a reception shaft adjacent to the PSST, transferring back to Capital Hill to drive the second tunnel.
- The U-Link running tunnels pass a maximum cover of 310ft (94.5m) on the Traylor/Frontier Kemper U220 contract.
- Extensive geotechnical investigation completed by the project's NTP design JV, led by Jacobs Associates, resulted in a GBR (geotechnical baseline report) that describes geological conditions as comprising plastic and non-plastic fine-grained clays; sand; gravel; and manmade fill. The consolidated 'lake bottom' clays are relatively stiff and impermeable and are described as not very abrasive and it is in this geology that the tunnels are aligned as far as possible (light blue on the geological section Fig 2). The sand and silts that infill the clay valleys in the strata are described as abrasive and highly permeable, resulting in compressed air pressures of up to 6 bar if man-entry interventions are needed under the high hydrostatic groundwater head while tunnelling through these reaches.
Fig 2. Geological section of investigation bore holes along the 3.15-mile U-Link extension
- "This makes for a technically interesting project," said Mike Krulc, Traylor Bros' Project Manager for the JV. "As well as the high cover and high water table, we launch into a steep down gradient to get under the Mortlake Cut and must be prepare for abrasive wear on the tools and the body of the machines." Traylor Bros has accumulated a lot of experience from its recent JV projects and this is being applied to the design and operation of the two 6.59m o.d. U-Link EPBMs.
- The machines for Seattle are follow-on orders by Traylor Bros and its JV partners with Herrenknecht. Their first experience with the German technology was on the Los Angeles Metro Gold Line LRT tunnels. In addition the company has ordered two Herrenknecht slurry machines for its East Side Access JV contract for the Queens soft ground tunnels in New York.
- Working with Herrenknecht design engineers, special attention is being given to the design of the U-Link TBMs to cope with the potential for high pressure interventions and to guard against excessive wear of the machines in the abrasive sands and silts. To the project Herrenknecht brings its experience of designing machines also for other JV customers and projects in the USA. These include the slurry machines supplied to the Central Tunnel contract for the Brightwater project in King County just north of the U-Link project in Seattle that have suffered extensive wear in the abrasive glacial deposits, and the machine supplied to the Lake Mead project where hydrostatic pressures could be as high as 17 bar as the intake tunnel advance out under the bed of the lake.
- To address the potential need for man-interventions under high pressures, the two U-Link machines are designed with two dual-compartment air locks each, which can accommodate three men in the inner compartment into the excavation chamber, and two in the outer compartment from the free air environment of the heading. This is a belt and braces safety measure in case of an emergency. "The two locks provide for efficient use of time, with one crew in decompression while another is heading into the air," said Krulc. "The dual compartment design allows a medical team to reach the working crew almost immediately if necessary, instead of waiting for the working crew to pass through perhaps hours of decompression to reach attention on the free air side."
- The two U-Link TBMs are dressed with double disc cutters to attack the hard over-consolidated clay and foam soil conditioning will work to minimise wear of machine body parts. The cutterheads are fitted also with a wear detection bar that is filled with hydraulic fluid. Any leak of the hydraulic fluid will warn of excessive wear.
Cutterhead rim wear protection on the U-Link EPBMs will be similar to the Herrenknecht LA Metro
- Another wear resistant feature is applied to the rim of the cutterhead. "On the LA Metro machines we experimented with different designs for rim wear resistance," said Krulc, "and we found that attaching short wear resistance bars laterally to the rim of the cutterhead performed better than the resisters that wound around the rim like a band." The two new U-Link machines have lateral wear protector bars around the rim of the cutterhead and have tungsten carbide bit inserts for extra protection."
- Another opportunity to check and perhaps repair wear damage is at the16 cross-passages that are to be excavated. The cross passages are set at about 800ft distances or less and the plan is to excavate these SEM shotcrete supported excavations as the lead TBM advances.
- "Most will be in good clay," said Krulc, "and we are preparing pre-support methods for excavation of those in less stable ground." Concurrent excavation of cross passages will slow up advance of the TBM somewhat but the chance to inspect, and repair if necessary, the parallel TBM when it intersects each completed cross passage is a considerable advantage and it also provides overall program advantages.
- The U-Link machines have long shafted screw conveyors, similar to those on the LA Metro machines, but are even longer at 65m. They comprise three separate sections, each with its own set of drive motors. "On the LA machines we needed to get the discharge point back beyond the ring build area and the drive motors and to a point where ventilation would be installed to dissipate expected gas contamination of the ground," said Krulc. "On these machines, we need to get the discharge point back to a transfer onto the continuous conveyors that are specified for the job."
- With regard to using conveyor belt weighing systems to guard against over-excavation, Krulc said that Traylor Bros had never used belt weighers in the past. "Operating the EPB system properly by making sure for example that the excavation chamber is full of material at all times for example, is the way to avoid over excavation. Weighers however are specified for these tunnel drives and we have two on the belts behind each TBM. We have no scanners on the belts."
- Within 300ft (less than 100m) of launch the TBM passes under the Montlake Cut with a clearance of just 8ft (2.5m) beneath the 35ft (10.5m) deep shipping channel between Lake Washington and Lake Union. "This makes for a 4.5% down gradient and a 4.1% up gradient the other side," explained Jim Salley, Field Engineer for the Sound Transit CM team on contract U220. "These are too steep for rail bound muck haulage." The specification also prevents a repeat of a steep down gradient derailment accident of a full muck train that cost one worker his life on the Beacon Hill contract.
- Similarly to the LA Metro project, the JV will use a two-part annular grouting system and pump both the mortar and the accelerator from plants on the surface directly to the TBM heading. Conditioning agents will also be batched on the surface and pumped directly to the heading.
UW station TBM working shaft will be at the south end at the left of this aerial view
- Traylor/Frontier Kemper ordered the two U-Link EPBMs on the same day that it ordered the two slurry Mixshield for the Granite/Traylor/Frontier JV's soft ground Queens tunnels in New York City. The U-Link machines are expected to be ready for delivery at the end of the year (2010) and launched by May and June of 2011. Each drive is expected to take about a year before breakthrough and recovery at the Capitol Hill station box in about June and July 2012.
- Working from the University of Washington station and with muck removal from site not possible via barges loaded at wharfs in the Montlake Cut, all tunnel and station excavation muck is trucked away from site on the main roads, including the draw-bridge road across the Cut. To avoid adding to rush hour traffic congestion in the University district, muck removal is not allowed between 7-9am and 3-7pm daily.
- While installation of the slurry walls around the full 800ft long (including the terminus turnback crossover) x 100ft wide at the widest x 120ft deep station box is part of the JV's contract, excavation of the core is not. A separate General Contractor/Construction Management U250 contract has been awarded for core excavation of the station box together with construction of the interior civil structures and installation of the M&E. Traylor/Frontier Kemper however will excavate its working access shaft for driving the two running tunnels at the south end of the station. When TunnelTalk visited the site in June installation of the slurry walls by JV subcontractor Nicholson/Condon Johnson was progressing and the job site precinct was being prepared for excavation of the TBM working shaft and laydown areas for segment storage and muck discharge. All is programmed to be ready for assembly and launch of the TBM as soon as they arrive on site before summer next year.
- Contract U230 – Capitol Hill to Pine Street
Maximum cover on the 1.1km long twin tubes of the U230 contract is about 130ft (40m), which reduces the maximum hydrostatic pressures at tunnel horizon to about 1.7 bar. There is a standard type of air lock on the machine but its use is not expected to be high. There are five cross passages on the alignment that are designed as open SEM excavations in locations of good ground. "Interventions for cutterhead inspection are prescribed by the contract in these locations for both drives and compressed air intervention is not anticipated," said Mike DiPonio, Project Manager for the JCM U-Link JV. Interventions are also prescribed at the location of cross-passages on contract U220 where man-entry under free air is also expected although entries at up to 4.7 bar under the high cover and more is to anticipated.
Capitol Hill station box site looking south
- Wear is also not anticipated as a major concern. EPB excavation with foam soil conditioning is expected to limit more aggressive wear. The Beacon Hill experience is also that the total 8,000ft (2.5km) of EPB machine tunnelling was completed with low tool consumption and little wear. "On our JV contract for the Brightwater project, it was only towards the end of the drive, where we encountered more boulders, that interventions to replace damaged rather than worn cutters become more frequent," explained DiPonio.
- Settlement however is a concern on U230. "The depth to diameter ratio of cover on the alignment is shallow," said DiPonio, "and the insurance policy for the project leaves the contractor more exposed to any damage of the older, multi-storey buildings that we pass under along the route. We also pass with little cover under the I-5 freeway at the end of each drive, so we are taking extra care about settlement control. The machine has a double length screw conveyor, which provides better control of EPB excavation. The twin screw takes the discharge point beyond the segment handling zone as well."
- As specified, we have volume weighers on the transfer belt conveyor and also specified by contract is continuous conveyor muck hauling. "We are on a maximum down gradient of almost 5% for the two drives," said DiPonio, "and that is too steep for haulage of a train of full muck cars up the gradient."
- The 21ft 2in (6.5m) o.d. Hitachi Zosen EPBM ordered by the JV for contract U230 will be shipped from Kobe in Japan in early 2011 to be launched in July next year. This joins a list of Hitachi machines used previously in the United States and the two Hitachi Zosen slurry TBMs ordered recently for the first underground contract on the Bangalore Metro in India among others. Mike Shank has a long manufacturing association with Hitachi Zosen on the machines that he used on the Narragansett CSO project in Rhode Island, the Arrowhead and Badlands tunnel machines for the Inland Feeder Project in California, and the recently completed New Crystal Springs water tunnel in Northern California.
- Station construction
Ahead of the machine arriving, the JV is excavating the Capitol Hill station box to base slab level. The station contractor will build the interior and install the M&E works. When TunnelTalk visited the site in June, installation of the last soldier piles around the perimeter of the 560ft long x 60-80ft wide station box were waiting for diversion of an old brick storm drain to be completed. The drain cuts across the station box had to be rebuilt as a steel pipeline capable of being suspended across the opening until buried again with the station backfill.
- Soldier piles of 100ft deep support the 80ft deep station box. Tie-backs will anchor the long walls while jet-grouting pre-treatment and soil nails will support the tunnel eyes on the end walls. Dewatering is limited to only eight or ten 3ft diameter local vacuum well points. "The watertable is high but the ground is mostly impermeable with very hard clay about 20-30ft down," explained DiPonio.
- During the visit, jet grouting at the tunnel eyes was on-going at the north end after completing the south end. The pre-grouted ground comprises two rows of vertical jet piles, plus one slightly inclined and three steeply inclined rows. The new Super Jet grouting method was used on the contract. The system, operated by sub-contractor Malcolm Drilling and with new jet grout drilling and batching equipment from Germany, injects grout at 7,000psi delivering grout down the middle part of the drill string and compressed air in the annulus.
- "Work is progressing largely to schedule," said Angus Speirs, Construction Supervision Manager for Sound Transit. "The jet grouting is going well although they did encounter erratic wood and peat in the ground as mentioned in the GBR which clogged and interrupted the injection process. The soldier piling is almost complete and core excavation of the box is set to start soon." A high sound wall around the station box site has also been installed to allow for 24h tunnelling in the built-up residential area.
- Segmental lining production
Tunnel segments for both contracts will be produced at the one casting yard set up by a joint venture of Traylor Bros and Technopref of Quebec, Canada. The plant will use the same CBE moulds that were used for production of the five segments and a key in each 1.5m (5ft) wide bolted and gasketed ring of precast lining for the LA Metro contract. Dan Henderson of Traylor Bros will manage the precasting factory after relocating from California at the end of a successful job casting the PVC lined segments for the Upper Northwest Interceptor 1&2 (UNWI)) Project in Sacramento. The project was featured in a video site visit report by TunnelCast in October 2009. Set up of the U-Link project's facility in Tacoma has started and the first segments are expected to be produced in mid-August ahead of launch of the three TBMs in mid-2011.
- Owner's approach to Seattle's U-Link extension - TunnelTalk, July 2010
Lake Mead TBM designed for the extreme - TunnelTalk, Nov 2009
Brightwater TBMs in trouble - TunnelTalk, June 2009
New start for LA Metro contracting - TunnelTalk, Sept 2005
Brightwater plan and rescue strategy - TunnelTalk, May 2010
East Side Access soft ground tunnels awarded - TunnelTalk, Oct 2009
Hole through for Crystal Springs drive - TunnelTalk, March 2010
Underground advance for Bangalore Metro - TunnelTalk, Jul 2010
First use of embedded line in tunnel segments - TunnelCast, Oct 2009
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