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Seattle’s SR 99 Alaskan Way Tunnel Progresses

Work continues steadily on the $2 billion project.

Thu February 06, 2014 - West Edition
Irwin Rapoport

Crews operating a Hitachi Zosen Corporation tunnel boring machine began boring operations on July 30 on the $2 billion, 1.7 mi. (2.7 km) tunnel to replace the SR 99 Alaskan Way Viaduct (SR 99 Tunnel Project) in Seattle, Wash., and have made substantial progress since.

The tunnel, a design-build project for the Washington State Department of Transportation (WSDOT), is being built by the Seattle Tunnel Partners (STP), a joint-venture of Dragados USA and the Tutor Perini Corp. When completed, the tunnel will replace a more than 60-year-old double-deck highway with a double-deck tunnel with two lanes in each direction. A replacement was necessary following a 2001 earthquake that damaged the structure.

The tunnel and a new Alaskan Way street are designed to work together to replace the functionality of the viaduct, according to the project’s Web site. “The tunnel will have the capacity to accommodate trips through downtown, while the rest of today’s viaduct users will access downtown using ramps at either end of the tunnel. Along the waterfront, a new Alaskan Way street will provide several east-west connections to downtown, replacing the function of today’s midtown viaduct on-ramp and off-ramp.”

In addition to the tunnel, the project — Alaskan Way Viaduct Replacement Program — also includes: a mi.-long stretch of new highway that connects to the south entrance of the tunnel, a new overpass at the south end of downtown Seattle that allows traffic to bypass train blockages near the city’s busiest port terminal, and the demolition of the viaduct’s downtown waterfront section in 2016. Additional improvements will be conducted by King County, the city of Seattle and the port of Seattle in terms of street upgrades, transit, a new seawall and a redesigned waterfront.

The tunnel will change the way current traffic uses the SR 99. Those wishing to bypass downtown, will continue through the tunnel, while those wishing to exit into downtown will use new on and off ramps at either end of the tunnel. At the south end, SR 99 will connect to a new Alaskan Way providing easy access into downtown.

The tunnel is expected to be completed in late 2015 or early 2016, with all the various projects costing more then $3.1 billion, with funding coming from a variety of sources including federal, state, local, tolls and the port of Seattle.

The Hitachi Zosen machine — named Bertha, is 57.5 ft. (17.5 m) in diameter, 326 ft. (99 m) long and weighs nearly 7,000 tons (6,350 t). Prior to the start of operations, STP crews were trained to operate the boring machine. It is part of a complete system — a cutter head bores into the soil and the debris is then transported out of the tunnel. Simultaneously, pre-cast concrete sections are installed to form the tunnel liner.

Tested in Japan, Bertha —nearly as tall as a five-story building, arrived in Seattle on April 2 and was reassembled in a launch pit to the west of Seattle’s sports stadiums. Bertha will cross under 154 buildings, numerous utilities and other infrastructure assets in downtown Seattle through various soft ground conditions along the route.

It is expected that the boring operations will remove more than 850,000 cu. yd. (649,871 cu m) of soil. Tunnel spoils will be barged to CalPortland’s Mats Mats reclamation facility at Port Ludlow where they will help fill a gravel quarry. The machine’s cutterhead chips away the ground as it rotates and carry excavated soil back through the machine using a spiral screw conveyor. From here, the spoils are placed on a conveyor belt which will eventually reach 9,000 ft. (2,743 m) in length.

The spoils are then placed on barges based at nearby Terminal 46 in the port. The Foss Maritime Company is responsible for the barging operation.

Tunneling under a major city is no easy task.

“There’s a lot to keep track of,” said the project’s Web site. “Steering, for instance. And of course the soil in front of the machine. The important thing to know about soil is that not all of it is the same. There are eight different types of soils along our tunnel route. In general, the looser the soil, the more likely it is to move as you tunnel through it. Sand, for example, is harder to control than clay. Other things workers might encounter underground: boulders, gravel, logs and various man-made objects.

“[The] machine can handle almost anything, but that doesn’t mean our crews won’t use extreme caution,” the Web site continued. “They will constantly monitor ground conditions as they drive the machine forward. Safety measures began before tunneling even started, when some 200 buildings above the tunnel alignment were examined and fitted with monitoring equipment that allows crews to detect even the slightest movement. Buildings and other structures that are thought to be sensitive are stabilized prior to tunneling. There are a number of ways to do this, including ground improvements and construction of angled walls below the ground that hold the earth in place above the tunnel.”

The boring operation has been divided into 10 zones and currently crews are in zone 1. On average, Bertha can cover approximately 35 ft. (10.7 m) per-day. Studies were completed on each section to determine soil types and the location of buildings and underground infrastructure. Bertha requires a crew of 25 people, including 2 operators.

After the soil is removed, curved concrete segments are installed behind the machine’s front end to form rings that serve as the tunnel’s exterior walls.

The tunneling machine uses a laser as a reference as it moves forward through the earth. Projected from a fixed point behind the machine, the laser is received by a guidance system at the front of the machine that is precisely calibrated to the tunnel’s predetermined path. The guidance system is referenced by the machine’s operator to ensure the machine remains on course. The operator steers the machine by making slight adjustments with each push forward.

Having started near the city’s sports stadiums, Bertha is expected to emerge near Sixth Avenue North and Harrison Street (near Seattle Center) in the late half of 2014.

Bringing in and installing the precast concrete panels to line the tunnel is the shared responsibility of several managers. A full ring panel weighs 360,000 lbs. (163,293 kg); although some vary in weight. Two erector arms lift the segment using a vacuum and set them in place.

Seismic activity still poses a danger for the remaining viaduct and this is not taken lightly but routine safety inspections and maintenance keep the viaduct safe for public use.

“In 2008, crews strengthened four column footings where the viaduct had settled approximately five-and-a-half inches into the ground since the 2001 Nisqually earthquake. The column safety project limits settlement in this area of the viaduct and prevents further damage to the structure.

“We also installed a system designed to close the viaduct automatically in the event of a moderate to severe earthquake in the greater Seattle area,” said the Web site. “The new automated closure system consists of traffic gates at all viaduct access points controlled by an earthquake detection system. If the earthquake monitoring system detects significant ground movement, it will simultaneously lower all nine traffic gates and safely close the viaduct in two minutes.

STP will take ownership of Bertha after it has completed 1,000 ft. (305 m) of operations without any problems. Maintenance and repairs of Bertha is a major task and this requires a sizable crew of mechanics, electricians, and related personnel to do scheduled maintenance and effect repairs on unexpected breakdowns.

Bertha operates 20 hours per-day. Information technology systems allows operators and repair crews to monitor systems to determine when parts, oil and various fluids, batteries and other elements need to be replaced and repaired.

Some of the major equipment employed by STP includes an American HC230 230-ton (208.6 t) crawler crane, a Grove RT9100 100-ton (90.7 t) rough terrain crane, a P & H CN-150 50-ton (45 t) crane, a Bauer BG 40 rotary drilling rig, a Bauer BG 50 rotary drilling rig, a Caterpillar 330CL excavator, a Caterpillar 345CL excavator, a Caterpillar 365BL excavator, a Caterpillar 385BL excavator, a Komatsu PC750LC-6 excavator, a Komatsu PC1100LC-6 excavator, a Volvo A40 articulated rock truck, a Caterpillar 140H VHP motorgrader, a Caterpillar D7 dozer, a Caterpillar 973C track loader and a Caterpillar 980H rubber-tired loader.

Major subcontractors include HNTB (for design), the Frank Coluccio Construction Company (for utility relocation), J.H. Kelly (for mechanical and electrical), Malcolm Drilling (for secant piles and jet grouting), Harris Rebar (for furnishing and installing reinforcing steel), J. Harper Contractors (for demolition), Ballard Diving & Salvage (for hyperbaric services), and Barnhart Crane (for tunnel boring machine assembly).

About 75 percent of the subcontractors, including contractors, consultants and suppliers, are with firms within the state.

The lead designer for the new tunnel is HNTB Corporation.

“This unique design-build project will be delivered nearly a year ahead of the initial estimated schedule,” said Phil Petrocelli, HNTB president west division. “The alternative technical concepts developed by HNTB, and vetted within the design-build team, represent cutting-edge thinking. These industry-first efforts drew from staff and projects from every division within HNTB.”

HNTB is responsible for the design of the tunnel structure, which includes cut-and-cover sections, open cut sections and the bored tunnel structure — mechanical and electrical components of the tunnel structures, ventilation buildings, fire life safety, civil work, roadway design and building settlement mitigation measures for structures along the tunnel alignment.

With more than 45 years of experience in the design, construction and restoration of tunnels and underground structures, including soft ground, rock and underwater crossing tunnels, HNTB engineers take into account the needs of construction companies to do their work more efficiently and rapidly in terms of materials used, construction processes, and safety for crews and equipment.

“Collaboration between the design team and the contractor (joint-venture) was of paramount importance during the initial stages of the design,” said Rich Johnson, HNTB vice president and design manager of the SR 99 Alaskan Way Viaduct. “There was a synergistic relationship achieved by combining the expertise, experience and know-how between the contractor and the designer. This yielded several noteworthy innovative alternative technical concepts, including the reconfiguration of the south roadway approach to the tunnel to reduce the footprint, and the incorporation of a portion of the final in-place structure as part of the temporary support of excavation.“

The experience of the companies in the joint-venture on other tunnel projects influenced the HNTB design team, resulting in the South End Settlement Mitigation (SESMP) being incorporated into the proposal and construction plan.

“Protection of the existing Alaskan Way Viaduct (AWV) from tunnel induced ground movement for the first 1,500 feet of mining was of pronounced concern for WSDOT, as well as the contractor,” said Johnson. “To address the concern, geo-structural solutions were developed and referred to as the SESMP. They consist of a collection of five-foot diameter drilled shafts used to isolate the foundations of the AWV from potential tunnel induced ground movement in the first 1,500 feet of mining. Beyond the first 1,500 feet, double rows of eight-inch diameter micro-piles are used to isolate the AWV foundation from potential tunnel induced ground movement.”

He added that consultation with the contractor (joint-venture) was daily and sometimes hourly from the first kick-off meeting at the start of the tender period, through the submittal of the proposal, and the ongoing construction. The join-venture and HNTB have offices adjacent to the construction site to coordinate the construction and trouble-shoot problems that arise.

“The onsite personnel include two geo-structural engineers, a civil/utility engineer and me,” said Johnson. “Our role is to address questions from the contractor in the form of requests for information; review the construction as it progresses at the request of the contractor for conformance with the construction documents and liaison between the contractor and the balance of the offsite design.

“As the world’s largest soft-ground bored tunnel,” he added, “the project is providing profound experience in incrementally expanding the feasibility of increasing the diameter of machine mined tunnels.”

Future seismic activity that could impact the new tunnel is critical to the HNTB design.

“The bored tunnel was designed to a dual-level seismic criteria with an upper level 2,500-year average return period (rare earthquake) and lower level 108-year average return period (expected earthquake). The deformation due to seismic events controlled the design of the gaskets, bolts and shear bicones located between the concrete segments of the tunnel liner. Additionally, approach structures were designed to withstand liquefaction of the upper 60 feet or so of soft soils,” said Johnson.

HNTB brought together 65 engineers and various professionals to downtown Seattle and another 40 professionals working offsite during the project’s design phase, which took place between 2010 and 2011.

The design team did not provide a specific maintenance plan for the tunnel, however Johnson said that “operations and maintenance manuals are provided through the equipment manufacturers’ for the various mechanical and electrical components of the tunnel. The owner, such as WSDOT, are typically very experienced and have the appropriate equipment for the maintenance of the more traditional heavy civil and building aspects of the project. Considered as a collective whole, WSDOT has the information necessary to synthesis a formal and automated maintenance plan, which I believe they will do.

“WSDOT and its peer agencies are very knowledgeable owners,” he added, regarding the importance of feedback between HNTB and DOTs. “The standards they set as necessary for a project to achieve a 75- to 100-year life and to protect the value of their assets. The standards they set are based on known standards, AASHTO or the International Building Code for example. If each party knows and understands contractual relationships and requirements, the collaboration is organic and beneficial to all parties involved.”

HNTB was presented with the Tunneling Advisor/Program Manager of the Year Award at the 2012 International Tunneling Awards for the Devil’s Slide Tunnel project in San Mateo County, Calif.

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