From world-record-breaking tunnel bores to world-class transit and treatment facilities, the Seattle area has been home to many tunneling projects over the last few decades. In this issue of TBM: Tunnel Business Magazine we take an in-depth look at the SR 99 Tunnel Project, subway tunnels for Sound Transit, and the Brightwater conveyance tunnels that were recently completed for King County Wastewater Treatment Division. Additionally, we take a retrospective look at the Mount Baker Ridge highway tunnel that was built in the 1980s and is still among the largest soft-ground bores ever completed. All of these projects have been completed through a mix of planning and teamwork, as well as the innovation that is a hallmark of the region.
As far as transit agencies go, Sound Transit is a relative newbie. But what it lacks in years, it is making up for in experience. Since it was created by voter approval in 1996, the Seattle-area agency has embarked on the construction of an ambitious light rail transit system designed to improve transportation in Washington’s busiest metropolis.
On July 18, 2009, Sound Transit opened its Central Link project for revenue service. The project spanned from Tukwila to downtown Seattle. Then in December of that same year, an extension from Tukwila to SeaTac International Airport was opened, bringing the first phase of the system to 15.6 miles of track with 13 stations.
Currently, the agency is overseeing the construction of its University Link extension project, which extends the light rail line northward from downtown to the University of Washington. The extension spans 3.1 miles, all underground, and includes the construction of two stations, one at Capitol Hill and one at the University of Washington near Husky Stadium. TBM excavation has been recently completed and the project is on schedule for a 2016 opening.
Sound Transit officials are already busy planning further extensions to the north (Northgate Link Extension), east (East Link) and south (S. 200th Link Extension) to be completed by 2023. In fact, the agency in August hosted a ground-breaking ceremony for the Northgate Link Extension – a 4.3-mile extension from the University of Washington terminating at Northgate Mall with intermediate stations serving the University District/Brooklyn and Roosevelt.
Sound Transit was created in 1996 after voters approved a tax measure that would pay for light rail in Seattle and Tacoma. The voter-approved plans called for the construction of Tacoma Link, an independent, at-grade system running in downtown Tacoma that opened in 2003, and Central Link, a line to connect the airport to downtown and the University of Washington. In 2000, Sound Transit solicited bids for tunnel construction, but had to reconsider plans when bids for the design-build contract exceeded the project budget.
As part of the new planning effort and reorganized management, Sound Transit decided to end the Central Link line downtown and build the extension to the University of Washington in a second phase (University Link).
“Ridership is growing every month,” said Sound Transit Public Information Specialist Bruce Gray, who says that the system accommodates an average of 26,000 riders each weekday. “We are seeing double digit growth annually, so people are latching onto it. Overall, the region really supports building out our light rail system.”
While the majority of the light rail system is at-grade, several tunneling projects are integral to the efficient operation of the new system. In downtown Seattle, much of the alignment is underground, as well as the extension to the University of Washington. Additionally, a tunnel was constructed under Beacon Hill south of downtown.
The first major tunnel contract for Sound Transit was the Beacon Hill station and tunnels as part of Central Link. The $280 million station and tunnels contract was awarded to Obayashi, which constructed the project from 2005 and 2009. The project included twin 4,200 lf tunnels driven using an EPB TBM, a 380-lf station platform and cross passages constructed by sequential excavation methods, and a 181-ft deep, 46-ft diameter slurry wall shaft.
Because of the challenging and variable soils in the Seattle area, Sound Transit took an innovative approach by constructing a test shaft in the area of the Beacon Hill shaft and station. The agency spent about $2.5 million on the test shaft, which gave engineers and potential contractors a glimpse of the ground and helped identify the optimum depth and construction methodology for the station and running tunnels.
“The test shaft was a home run for us,” said Dick Sage, Director of Construction Management for Sound Transit. “Because of the variable geology that we have here in western Washington, we wanted to verify the construction methodology that was being proposed for the station and shaft was appropriate for the work; we found that it wasn’t. As a result, we changed the support system from a shaft support of SEM to a slurry wall to accommodate for the groundwater. By building the test shaft we probably saved $25 million because of the lack of problems that we had in developing the shaft with the slurry wall.”
Other challenges associated with the Beacon Hill tunnels and stations included a crossing under Interstate 5 and TBM mining in the variable soils. These provided lessons learned that were later used on the University Link project.
“One of the things that we learned on Beacon Hill was the need to monitor and control face pressures on the TBMs at all times – to actually observe the face pressure in relation to the ambient pressure in the ground,” Sage said. “With the water bearing sands we have, if you don’t maintain a face pressure that is above the ambient pressure in the ground, you will get flowing conditions and potentially create voids.”
In the downtown area, the $2.6 billion Central Link light rail was routed through the existing Downtown Seattle Transit Tunnel, a bus-only tunnel completed in 1990, that was retrofitted to accommodate both bus and light rail transit. Central Link currently terminates at Westlake Station at the northern end of the Downtown Seattle Transit Tunnel, where it will connect to the University Link project.
“Comingling of the bus traffic and light rail is a unique aspect of our system, although there are challenges associated with combining the two technologies – particularly in the a.m. and p.m. rush when there are very tight headways,” said Sound Transit’s Joe Gildner, Executive Project Director/University Link. “In addition, we needed to build an 800-ft stub tunnel by cut-and-cover to put our double crossover track in and give us the capability to turn trains back to the airport.”
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The main tunneling contracts for the University Link in include U220, which runs from the University of Washington to the new Capitol Hill Station, and U230, which runs from Capitol Hill to downtown.
U220 was awarded to the joint venture team of Traylor Brothers and Frontier-Kemper in June 2009. It consists of twin 11,400-ft, segmentally lined, bored tunnels with a finished diameter of 18-ft, 10-in. Additionally, the contractor is to construct 16 cross passages between the twin bores. All TBM excavation was completed using twin Herrenknecht TBMs, and 15 of the 16 cross passages have been excavated.
U230 was awarded to the joint venture team of Jay Dee, Coluccio and Michels (JCM). It consists of twin 3,800-ft, segmentally lined, bored tunnels, also constructed by EPB TBM. Because of the shorter length of the tunnel, the JV elected to use a single Hitachi Zosen TBM to complete both tunnel bores. Also included is five cross passages.
As tunnel activities wind down on the University Link, Sound Transit will be gearing up for the 4.3-mile Northgate Link Extension, which includes 3.7 miles of tunnel. Sound Transit expects to tender a single contract for the tunnels and station in 2013.
“Based on our experience with previous tunnel contracts, we are planning to bid the Northgate tunnels and station under a single contract to minimize contract interfaces,” Sage said. “We’ve learned that it is better to eliminate as much as possible the interface dependencies between contracts.”
Sage also points to Sound Transit’s construction management approach to a key factor in managing a successful tunneling program. “We take a very hands-on approach with our construction managers and share the same office with them,” he said. “We strive to be fair and equitable, and get the contractor paid as quickly as possible for money that is due. If there is a dispute, we’ll get them paid the money that is owed while we talk about the amount in dispute. We also set up a dispute resolution board in which all three members were selected jointly by us and the contractor, thus avoiding the ‘yours, mine and ours’ syndrome.”
The completion of the Northgate Link Extension will mark an end to the major underground work associated with the Sound Transit light rail system, although plans for continued development of the overall system continue.
“We have gone through quite a lot as an agency and it is gratifying to be able to ride on the trains,” Gildner said. “We have learned a lot in managing projects, particularly project controls and procedures, and I hope that we are looked at as an agency that contractors want to do business with. It is also gratifying to bring some of the information we have learned to the table and share it with our peers.”
Jim Rush is editor of TBM: Tunnel Business Magazine.
HC Global, a Pittsburgh, Pa.-based tunnel communications firm, was chosen to provide underground communications for the Sound Transit University Link tunnel project in Seattle. A considerable amount of pre-planning went into this project as it had to be carefully coordinated between the contractors Traylor Bros./Frontier-Kemper (U220 Tunnel) and Jay Dee/Collucio/Michels (U230 Tunnel). Sound Transit, the project owner, provided oversight to the project and worked closely with the Seattle Fire Department as the project unfolded. All parties involved worked well together and were committed to ensuring a proper communications vehicle for both those working on the project above and below the ground.
The design and engineering phase of the project took several months to complete with HC Global staff traveling to Seattle in addition to weekly phone sessions to refine and coordinate the implementation plan.
Seattle Fire Department also was deeply involved as communications coverage needed to be developed within the tunnel in case of an emergency. The fire department utilizes an 800 Mhz system that initially proved somewhat problematic since the leaky feeder was a VHF-oriented system. HC Global designed an interface so that the public safety providers would be directly linked to underground communications with ease. After completion of the engineering and design, HC Global dispatched an installation crew led by Jack Jones, Senior Technician at HC Global, to install the system and conduct detailed testing with all involved parties. Overall, the project took several months to implement.
At present, the system is working well with all three shifts utilizing the system as work continues. Overall the communications modality has increased efficiency and created real-time transfer of time-sensitive information. Robert Taaffe, Construction Safety Manager for Sound Transit, noted that the system has been virtually maintenance-free and the communications is crystal clear and in some cases even better than the above-ground communications.
|Seattle’s Big Bore
A Look at the Design Elements of the Alaskan Way Viaduct Replacement Tunnel
By Richard Johnson, P.E.
Seattle is no stranger to innovative tunnel construction. More than 100 tunnels have been constructed in and around the city over the last century, including the Mount Baker Ridge Tunnel, which at 68 ft in diameter became the world’s largest mined tunnel in 1989.
Soon, Seattle will be home to another world-record holder, the largest diameter soft-ground bored tunnel. In December 2015, the Washington State Department of Transportation and its design-build team, Seattle Tunnel Partners, will open the mammoth, two-level, 57.5-ft diameter State Route 99 Tunnel. This tunnel will surpass current title holders in Spain and China by 7 ft in diameter/
WSDOT is undertaking this massive project in partnership with King County, the Port of Seattle, the City of Seattle and the Federal Highway Administration. The bored tunnel will convey traffic under downtown Seattle, replacing the SR 99 Alaskan Way Viaduct, a double-deck highway built in the 1950s that runs along Seattle’s downtown waterfront. The viaduct was damaged in the 2001 Nisqually earthquake, and is vulnerable to future earthquake damage.
A bored tunnel was selected to minimize disruption to the city and open up 9 acres of new public open space on the waterfront, The project also includes north and south cut-and-cover approaches, two tunnel operations buildings and tunnel life safety and operations systems.
The project’s design-builder team, Seattle Tunnel Partners (STP), is a joint venture of Dragados USA and Tutor Perini Corp. HNTB Corp., which also designed the Mount Baker Ridge Tunnel, is the lead designer. WSDOT implemented a best-value procurement process for the design-build project and awarded approximately 9 percent of the project value to technical credits. Of $100 million in potential technical credits, Seattle Tunnel Partners won approximately $71 million, nearly twice that of its competitor.
Technical innovations, including schedule acceleration, risk reduction and effective use of project funds put the STP team over the top. The technical elements are outlined below:
The team’s innovative acceleration of the project resulted in the start of utility relocation in November 2011, three months after the start of final design, and installing 5-ft diameter drilled shafts and walls to support excavation of the south cut-and-cover tunnel in early March 2012. This enables completion of the tunnel boring machine (TBM) launch pit in March 2013 and start of mining in June 2013. The tunnel completion in December 2015, a year ahead of the original date, will allow WSDOT to demolish the viaduct just that much sooner.
Protecting the Viaduct: The first 1,500 ft of the bored tunnel runs parallel to and within 60 ft of the Alaskan Way Viaduct, which will remain open to traffic. STP has designed and will construct a barrier wall of 5-ft-diameter drilled shafts between the east side of the tunnel and the viaduct. These shafts will shield the viaduct foundations from ground movement.
Protecting the TBM: The first 1,500 ft of the bored tunnel will be shallow and in extremely soft urban fill. A second row of piles will be installed on the west side of the tunnel and a lid will be placed on top of the piles, creating a tunnel start-up box. This innovative “box” creates a highly controlled, confined environment for TBM machine operators and maintenance crews to become familiar with and test-drive the machine. It incorporates safe havens (TBM maintenance stops) to facilitate cutterhead maintenance and inspection.
Protecting Buildings: There are 158 buildings identified to be within the zone of influence of the TBM in the high-density urban downtown. Crews will take a number of steps to protect buildings above or near the tunnel route. The team used FLAC3D to model the TBM performance features and predict the tunneling settlements. This limits the required physical building protection measures to a few buildings at each end of the project, saving both time and money.
Protecting the Traveling Public: The RFP required a 54-ft minimum outside diameter tunnel, which accommodates 30-ft roadways and a 15-ft vertical clearance. Seattle Tunnel Partner’s 57.5-ft outside diameter tunnel will have 32-ft roadways including a wider 8-ft shoulder and 15.5-ft vertical clearance to further improve the motoring public’s safety.
Effective Use of Project Funds
At the south end of the project, the team rearranged the roadways to stack 450 ft to the south. This significantly reduced the footprint of the southern approach, thus saving schedule time and a sizable amount of construction dollars, which Seattle Tunnel Partners reinvested into the south end settlement mitigation plan. It also moved the TBM launch pit.
The project has several noteworthy design features, the most remarkable of which is the fact that the team is constructing a $1.1 billion tunnel in downtown Seattle with relatively little impact to the general public. Because it is a highway transportation project, American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design (LRFD) is the standard for design, which is something less than common for tunnels.
Other design features include:
Two levels: The roadways inside the bored tunnel will be stacked, with the southbound lanes on top and the northbound on the bottom. Despite the smaller footprints they create, stacked roadways are not common in tunnels. The author knows of only three tunnels with similar configurations: A-86 motorway Tunnel outside Paris, Malaysia’s Smart Tunnel and Seattle’s own Mount Baker Ridge Tunnel.
High ground motion values: The tunnel will be located in AASHTO’s Seismic Zone 4, the most demanding, which corresponds to a ground motion with a 2,500-year average return period.
Fire resistance: Fire protection material as well as polypropylene fibers will be applied at strategic locations to enhance the passive structural fire resistance.
Single-point extraction emergency ventilation system: This system will efficiently siphon the pollutants, heat and smoke away from a fire incident location, leaving the other portions of the tunnel tenable for evacuation and for the fire brigade.
Dedicated emergency egress passage way in the single bored tunnel, rather than use of a parallel tube is another uncommon feature.
Fire detection/suppression. Linear heat detectors, traditional smoke detectors, closed-circuit TV for human detection, and fix fire suppression, which includes a deluge system over roadways and a sprinkler system in egress areas, also are included in the design.
Paired operations buildings. Two buildings, one at each end of the tunnel, will provide the redundancy and capacity required for the single-point extraction emergency ventilation system. They also are designed in substantial conformance to Leadership in Energy and Environmental Design (LEED) standards.
Context-sensitive design. WSDOT made a conscious effort to blend the structures and architecture of the tunnel approaches with their urban surroundings to enhance and complement the cityscape of Seattle.
Strict environmental compliance. The WSDOT and the City of Seattle are known for their strides in environmental protection and preservation. This communal value is evident in the SR 99 Tunnel as it exceeds environmental regulation compliance.
Longer design life. This tunnel will have a 100-year design life, which is somewhat uncharacteristic in the highway transportation industry.
Depth. The tunnel’s depth is 270 ft from surface to invert, the equivalent of burying nearly half of the 604-ft Space Needle below ground.
Evolution of Bored Tunnels
In the past 30 years, the U.S. tunneling industry has accumulated significant expertise in the design, construction and excavation of bored tunnels. That experience and knowledge has ushered in a golden age of tunneling, allowing us to construct longer, larger and less disruptive tunnels in less time and with less risk. In short, tunnels have become realistic solutions, especially in areas where real estate is at a premium. That is why transportation agencies, such as WSDOT, are not only willing to consider a bored tunnel option but to select it as a preferred alternative.
Execution of these tunnels depends on an aggressive, innovative team approach with the owner, construction team and the designers pulling together. The SR 99 Tunnel is a high-aspiration project both in regard to budget and time. The features and technical innovations the design-build team has incorporated into the plans make the subsurface structure a highly desirable and practical solution that has the potential to further advance the tunneling industry.
Rich Johnson, P.E., is vice president and the director of the bridge and tunnel group at HNTB’s Seattle office. He currently serves as design manager for Seattle Tunnel Partners. Johnson has 30 years of experience in structural design and project management. He can be reached at (425) 450-2510 or firstname.lastname@example.org.
The earth pressure boring machine is being manufactured by Hitachi Zosen Corp. in Japan, where it will be fully assembled and tested later this year, then disassembled and shipped to the Port of Seattle in early 2013. It will arrive in pieces of up to 900 metric tons.
Hitachi Zosen, which will have representatives on-site to reassemble and demonstrate the machine, will own the TBM during a warranty period that covers the first 1,600 ft of the dig.
On average, the TBM will put in six rings – or 39 ft – per day in a double shift, remarkable for a machine of this size. Each shove will remove 600 yards of earth, equal to the payloads of 60 single-axle dump trucks.
Design features include:
Completion of Brightwater Tunneling Sets Stage for Improved Services in Seattle’s Northern Suburbs
By Gunars Sreibers
This fall, the Brightwater Treatment System’s 13-mile conveyance tunnel will begin operation, carrying highly treated effluent to Puget Sound through one of the world’s deepest municipal outfalls.
The King County (Wash.) Wastewater Treatment Division began construction on the Brightwater Project in 2006 to add needed treatment and conveyance capacity in the suburbs north of Seattle. In addition to the tunnel, Brightwater facilities include a treatment plant north of Woodinville designed to treat 36 million gallons of wastewater a day, an influent pump station in Bothell, and a 600-ft deep marine outfall a mile off of Point Wells in Puget Sound.
Engineering firm HDR led the pre-design on the tunnels, and design was carried out by a joint venture of MWH/Jacobs Associates, with CDM providing geotechnical services. Jacobs Civil oversaw construction management on the conveyance project.
King County divided the conveyance tunneling into three separate contracts that were bid out by design-bid-build in 2005 and 2006.
Design and construction of the tunnels addressed challenges posed by the Seattle area’s glaciated soils and complex geotechnical conditions. Additionally, the tunnel alignment crossed five jurisdictional boundaries and traversed beneath both public rights of way and private property, adding technical, regulatory and political complexity to the project.
Tunnel construction success was a result of collaboration between King County and the contractors and consultants, with a goal of keeping the project on schedule and budget, and also to maintain the high level of public trust the project earned during a three-year siting process to determine the facilities’ locations.
The first contract, valued at $131 million, was awarded in early 2006 to Kenny/Shea/ Traylor JV to build the 2.7-mile East Tunnel. Construction began in Bothell’s North Creek area with the 74-ft deep, 80-ft diameter shaft that served as a launch portal for “Luminita,” the 19-ft, 3-in. earth-pressure-balance tunnel-boring machine manufactured by Lovat.
The contractor also built a 2,400-ft long microtunnel from the North Creek tunnel shaft to the existing North Creek Pump Station, and excavated an 83-ft deep double shaft for a new pump station, built in a follow-on contract by Kiewit Pacific.
Tunnel mining began in October 2007 and was completed in December 2008 when “Luminita” emerged at the Brightwater Treatment Plant Site. The 16-ft, 8-in. ID tunnel is 80 to 260 ft below ground surface and lined with gasketed, pre-cast concrete segments. The contractor later installed four pipes inside the tunnel to carry treated and untreated wastewater, and backfilled the space outside the pipes with cellular concrete. One of the pipes is a “purple pipe” to deliver reclaimed water from Brightwater to customers in Bothell and the Sammamish Valley, including the Willows Run Golf Course.
Vinci/Parsons/Frontier-Kemper JV (VPFK) was awarded the $212 million Central Tunnel contract in August 2006 to build two 14-ft, 4-in. diameter tunnels. Both tunnels were driven from the 90-ft deep, 54-ft diameter launch shaft in Kenmore. The eastbound machine “Helene” was launched in September 2007 to complete a 2.2-mile drive and westbound machine “Rainier” was launched in February 2008 to mine a 1.4-mile drive to Ballinger Way in Shoreline. Both tunnels were mined by slurry tunnel boring machines manufactured by Herrenknecht.
VPFK’s contract also included construction of a 21-ft diameter, 200-ft deep receiving portal near Ballinger Way in Shoreline. The initial support of excavation was provided by ground freezing, performed by subcontractor Moretrench.
King County awarded the $102.1 million West Tunnel contract to Jay Dee/Coluccio/Taisei JV in 2007. This contract included construction of a 50-ft deep rectangular shaft at Point Wells to launch “Elizabeth,” an earth pressure balance TBM manufactured by Lovat. The machine began mining in 2008 and completed its 21,000-ft drive extending from Point Wells in unincorporated Snohomish County to Ballinger Way in Shoreline in 2010.
In addition, the contractor microtunneled a 540-lf, 60-in. diameter tunnel to connect to the marine outfall. After all tunneling was complete, the contractor built a sampling facility in the portal at Point Wells to monitor treated wastewater just prior to discharge to Puget Sound.
The path to completion of the tunnels took an unexpected detour in spring 2009, when inspections of the Central Tunnel TBM cutterheads revealed significant damage. The contractor developed and implemented a plan to repair the eastbound “Helene” underground, by dewatering to reduce groundwater pressures. The TBM was at a depth of about 300 ft.
“Helene” was repaired and resumed mining in February 2010, with about 4,000 ft remaining to be completed. The TBM’s condition was closely monitored to ensure that no further damage occurred, and the tunnel was successfully completed when “Helene” holed-through at the North Creek Portal in June 2010.
The westbound “Rainier” TBM was stalled about half-way through its 4-mile drive, 300 ft below a residential area in the City of Lake Forest Park. The contractor submitted a schedule to repair the TBM and complete the remaining two miles of tunnel, but King County found that schedule unacceptable. King County Executive Dow Constantine issued a declaration of emergency to waive procurement requirements so that the County could hire the West
Tunnel contractor team to complete the remaining 1.9 miles of tunnel. Jay Dee/Coluccio/Taisei’s machine, “Elizabeth” was approaching the Ballinger Way portal, and could be upgraded to continue mining eastbound toward the idled “Rainier.” The BT-3 Completion Contract with Jay Dee/Coluccio (JDC) was signed in April 2010. Taisei had other commitments and was not part of this joint venture.
Before it could start mining the BT-3 tunnel, JDC modified the TBM to prepare for the ground conditions anticipated on this drive. The alignment was on a downhill grade, so JDC added pumps with redundant power supplies to ensure against flooding of the TBM.
King County worked with both contractors to determine the best method to stabilize the ground at the connection point between the two tunnels. VPFK needed stable ground to remove the TBM, bulkheads and cutterhead, so that the JDC TBM could enter the empty shield. JDC needed stable ground to mine into the shield without any ground losses. The team decided to use ground freezing. SoilFreeze designed a system of freeze pipes installed from the surface, 300 ft above the tunnel elevation. VPFK was able to remove the cutterhead in July 2011 and “Elizabeth” broke through into the empty shield in mid-August, completing its 1.9 mile drive from Ballinger Way.
In the following months, JDC dismantled its TBM and constructed a permanent concrete transition between the two tunnels. By July 2012, the entire tunnel system – including all the pipes installed inside the various tunnels – was complete from the treatment plant to the outfall, and ready for planned start-up in fall 2012.
Successful completion of these tunnels is a result of the hard work of many engineers, contractors, suppliers and others involved in the design and construction of this project. When challenges and obstacles were encountered on the project, the participants found collaborative solutions that moved the project toward completion.
Gunars Sreibers is the Brightwater Project Manager for King County Wastewater Treatment Division.
|Monitoring the Big Bore
By Jim Rush
Tunneling in an urban environment comes with myriad challenges, not the least of which is dealing with the maze of existing buildings, structures and utilities – both public and private – that are potentially at risk due to tunneling operations. For the SR 99 Tunnel Project in Seattle, those concerns are magnified considering the scope of the project – a 1.8-mile bore under the heart of downtown using a 57.4 ft diameter tunnel boring machine (see p. 18-19 for further details). As such, extra caution is needed to ensure that structures are not affected or, if they are, crews know as soon as possible to be able to take corrective action.
SolData Inc., the U.S.-based subsidiary of Soldata Group and sister company to Nicholson Construction, was hired by the contractor – Seattle Tunnel Partners (a Dragados/TutorPerini JV) – to monitor tunneling along the alignment. SolData has been involved with monitoring for nearly 20 years, beginning with the Jubilee Line project in London. Other high-profile urban rail and metro projects in its portfolio include High-Speed Rail Line 9 in Barcelona, North/South Line in Amsterdam, King’s Cross and CrossRail in London, KCRC in Hong Kong and M4 in Budapest.
The primary components of the monitoring program in Seattle consist of instruments that measure the impacts of construction activities on structures, including the tilt and settlement, as well as below-grade soil movements. The SR 99 tunnel goes under Seattle landmarks including Pioneer Square and Pike Place Market Historic District, necessitating a robust monitoring and risk mitigation program.
The multiyear instrumentation contract, still in the preparation phase, will include about 35 CYCLOPS automated monitoring total stations (AMTS) in conjunction with more than 600 target prisms to monitor the 3D movement of buildings, structures, ground surface and rail lines during construction. The AMTS will also be used to survey roadways without the use of the reflector prisms, one of the first such applications in the United States, according to SolData. Additionally, about 120 extensometers will be used to monitor ground movement between the tunnel crown and the ground surface. All monitoring data will be managed with the company’s proprietary Geoscope package. SolData is also using satellite interferometry to complement the monitoring data. Satellite interferometry uses satellite imagery to provide settlement information with accuracy up to 3 mm over a large area, according to SolData general manager Boris Caro Vargas.
In setting up a monitoring program, the owner needs to determine what the risks are and consequently what needs to be measured and where, according to Caro Vargas. “Once you determine what your risks are, you can then begin to design a monitoring program to help mitigate the risks,” he said. “One important element beyond the instrumentation, is defining your threshold limits so that you receive alerts in time so that corrective action can be taken.”
Caro Vargas said that typical actions include increasing the frequency of monitoring, installing additional instruments, adjusting TBM operating parameters and implementing specific features of this unique tunneling machine, such as shield injection and post-grouting of the liner segments.
Given the scope of the project and its location downtown, the monitoring program is one of the largest implemented in the United States to date. “This is a very large project with potential risk to existing structures, so it requires a state-of-the-art monitoring program,” Caro Vargas said. “The challenge associated with these types of projects is that they have never been done before. You are dealing with unknowns, and trying to manage that is something that is very exciting.”
Jim Rush is editor of TBM: Tunnel Business Magazine.
|Clearing the Path
J. Harper Contractors Turned to Atlas Copco for Viaduct Demolition
By Joe Bradfield
An urban tunneling project isn’t just all about the underground. In advance of the boring operations, preparatory work, including traffic diversions and utility relocations, is needed to clear the way for the tunnel operations.
In Seattle, J. Harper Contractors of Maple Valley, Wash., handled the concrete structure demolition for the Alaskan Way Viaduct replacement project. Demolishing and removing 10,000 tons of concrete of a 1,300-lf span on the existing viaduct made way for crews who would immediately begin building the traffic diversion.
J. Harper vice president Jeff Slotta said he knew what he wanted for this job right away: an Atlas Copco Combi Cutter CC 6000 U. Slotta was aware of the CC 6000’s use in Europe and knew it would handle the job, so he arranged with Atlas Copco and distributor Modern Machinery to use the large Combi Cutter for the first time in the United States.
J. Harper completed the job in 30 days. Slotta said the heavy structural concrete of the bridge would have taken a lot longer without a larger cutter. And time was a factor: other contractors were waiting on demolition before they could complete their work.
The Combi Cutter has an option that features steel-shearing blades in the throat of two concrete cracker jaws. The jaws are driven by separate pistons and operate, by Slotta’s description, “like alligator jaws, with teeth in front to pulverize, crushing and swallowing concrete down its throat to get to the No. 18 rebar.”
J. Harper recovered more than 1,000 tons of the 2.25-in. diameter rebar during this section’s demolition. The viaduct’s steel-reinforced 60-ft towers were 4 by 4 ft wide. In addition to the concrete bents, the supporting system that elevated and supported the roadway, its concrete beams were 2 to 3 ft thick. “The CC 6000 didn’t ever hesitate. It just squished them, munching down on those columns, slicing through rebar all day long,” said Slotta.
J. Harper Contractors allowed about three months of lead time before the unit would see action on the project. The CC 6000 required a larger carrier than the usual 300 and 400 series excavators that the company uses for its demolition fleet. The CC 6000 has a service weight of over 7 tons. So after the Combi Cutter arrived at Modern Machinery, the dealer mounted it to a Komatsu 800-series excavator. Slotta said that they made additional modifications on the Komatsu such as installing extra hydraulics and adding extra guards to protect the operators.
Slotta was pleased with his team. “I’d rate this as one of the smoothest wrecking jobs we’ve ever done. Mardy Olson, our project superintendent, just did a fantastic job, the best demolition management I’ve ever seen.” The task included coordination of transporting concrete rubble to the company’s portable crusher at a staging area about a mile offsite.
While completing the first phase of demolition, J. Harper Contractors was awarded additional portions of the project, including crushing 30,000 tons of concrete for reuse at this job. The company was also asked to be involved in demolition of other sections of the viaduct.
Joe Bradfield is a senior writer at Ellenbecker Communications.
|Mt. Baker Ridge
Still the World’s Largest Soil Tunnel
By Robert A. Robinson, Director of Underground Services, Shannon & Wilson, Inc.
Most of the tunneling industry knows that Seattle will soon be the home of the largest shield-driven tunnel in the world with startup of the 57.5 ft diameter earth pressure balance machine on the Alaskan Way Viaduct (SR 99) Tunnel in 2013. However, this will be only the second largest soil tunnel in Seattle. The largest soil tunnel in Seattle, and reportedly in the world, was built in the mid 1980s as a double-deck, five-lane highway tunnel on Interstate 90. The tunnel passes beneath a 300-ft high north-south ridge along the east side of downtown Seattle, and portals to the east onto one of the three floating bridges that cross Lake Washington.
The need for eight to 12 lanes of freeway capacity across Mercer Island, Lake Washington and into downtown Seattle and merging with Interstate 5 was conceived by the Washington State Department of Transportation (WSDOT) in the 1960s. Initially the alignment across the 300-ft high residential area, known as Mt. Baker Ridge for its stunning eastward view of the Cascades and Mt. Baker, was as either a large lidded cut, or a series of two-lane tunnels, paralleling two double-lane highway tunnels built by conventional stacked-drift method in the 1940s. However, the construction of these twin 29-ft wide tunnels, initially supported with timber sets, resulted in up to 1 ft of surface settlement and severe damage to overlying houses, utilities and streets. Consequently, local residents lobbied heavily for a single large 1,332-ft long tunnel that would limit the width of the settlement trough, and limit the number of houses that might be impacted by tunneling.
Through the combined efforts of the project designer, Howard Needles Tammen and Bergendoff, the geotechnical firm of Shannon & Wilson Inc., and the Design Review Board of Ralph Peck, Al Mathews and Chuck Metcalf, a unique version of the stacked-drift method with a final inside diameter of 63.5 ft and an outside excavated diameter of 82 ft was developed. With this concept, the tunnel liner was to be constructed first, consisting of 24 to 32 largely unreinforced concrete-filled drifts that formed a horizontal cylinder, much like a staved barrel, in which the overconsolidated glacial clay, silt and till that formed most of the ridge would act as the steel hoops of the barrel, holding the staves together to form a horizontal, flexible, compression ring liner. Once the liner was in place, then the 63-ft diameter soil core would be excavated in five equal-height benches.
Not only was this a unique engineering achievement, but it was also the first application of the combined geotechnical design summary report (GDSR, which was later to become the modern geotechnical baseline report or GBR), escrowed bid documents and a disputes review board (DRB) in Washington. The GDSR and DRB had been utilized successfully on the Eisenhower Tunnel and was introduced to WSDOT by the Design Review Board.
Several other unique contracting features, included: 1) risk sharing provisions to cover the changes in the cost of labor and materials (concrete, steel, fuel, etc.) due to high rates of inflation at the time; 2) a performance specification that provided considerable latitude in the configuration, size and number of individual drifts (liner elements) ranging from 24 to 32 openings of any uniform shape; and 3) a comprehensive instrumentation program to assess ground and liner behavior.
Nearly 20 years after the conceptual design of the tunnel was developed, after several legal challenges, uncertain funding sources, and a final design that involved one of the first uses of finite element method for a tunnel as well as a quarter scale model test of the interaction and deformation limits for two adjacent drifts, the project was awarded in January 1983. Potential bidders were encouraged to visit a test shaft and 3 test adits near the west portal. A total of 17 bids were submitted ranging from $38.3 million to $61 million, with the low bid tendered by the Guy F. Atkinson Construction Co.
Atkinson chose to construct the minimum of 24 overlapping drifts using a pair of oval-shaped Milwaukee Boiler digger shields, with a concave bottom. The oblong shields could be oriented with the long axis anywhere from vertical to horizontal, with the operator’s seat and console, and the digger assembly rotated into an upright orientation. Each drift took about 4 to 8 weeks to excavate from west to east through Mt. Baker Ridge, support with “junk” concrete segments and backfill with concrete. Pressure grout was used for joints between adjacent drifts and then the shield was relaunched from the west portal. The 24 drifts were constructed over a period of about 18 months. Another 6 months was required to excavate out the 158,625 cu yd soil core in five lifts. The tunnel project was completed in May 1986.
Any project as unique at this tunnel is likely to encounter some challenges. At startup of the tunneling, ground deformations measured 3 ft above invert Drift No. 1 were within the specified 1-in. limit. However, deformations measured over the adjacent Drift No. 2 exceeded 3 in. Consequently, construction was halted for 11 days, and the contractor, WSDOT and their consultants evaluated and discussed the sources of excessive lost ground and remedial measures for reducing ground losses and future surface settlements. Remedial construction measures derived from interpretation of the instrumentation data included: a reduced shield overcut thickness, improved expansion of the five-piece lightly reinforced segmental concrete liner, replacement of the three PVC grout pipes with 13 steel grout pipes as well as additional injection ports between drifts to attain higher injection pressures and greater grout takes, eliminating excavation ahead of the leading edge of the shield and “cookie-cutting” through soil, and several other minor refinements. Subsequent drifts were excavated with ground movements within the 1-in. limit.
Ultimately, excavation of all 24 drifts, and excavation of the 63.5-ft diameter soil core, resulted in average surface settlements of 2 to 3 in. These settlements relate well to the overall ground loss of 0.65 percent calculated for the entire tunnel excavation, although ground losses for individual drifts averaged 2 percent. No significant damage was experienced by the residences, streets or utilities over the tunnel, except for areas around the west portal where an old landslide was remobilized. Concrete stress meters, crack gauges and tape extensometer points indicated that total loads were roughly equivalent to full overburden and that horizontal loading was equal to vertical loading, suggesting a Ko of about 1.0. Total tunnel deformation during core removal averaged 0.1 in. or about 0.013 percent diameter change.
Nearly 15 years after completion, the tunnel experienced the magnitude 7.1 Nisqually Earthquake on Feb. 28, 2001, that nearly destroyed the nearby Alaskan Way Viaduct. The tunnel successfully rode out the earthquake with no apparent damage, although wear marks showed where utility lines had moved back and forth by several inches in their hangers.
The success of this unique, award winning tunneling project is due in part to:
The Mt. Baker Ridge Tunnel was completed ahead of schedule, and at a cost of $36.1 million, about $2 million under the bid price, due to the limited number of claims and the risk sharing elements implemented by WSDOT.