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Transformative Tunnel


The design of the new State Route 99 tunnel improves safety and mobility along Seattle's waterfront. Its design overcomes settlement and seismic challenges, incorporates a one-pass segmental liner, and includes state-of-the-art safety and ventilation features.

Completion of the State Route 99 (SR 99) tunnel earlier this year was a transfor-mative event in Seattle's storied transportation history. It was also a breakthrough for the worldwide tunneling industry, expanding the possibilities of construction size and safety. The approximately 2 mi long tunnel opened to motorists on February 4, 2019, creating a fast, convenient travel option for motorists.

The new tunnel replaces the six-decadeold Alaskan Way Viaduct and Battery Street tunnel. It features a stacked roadway configuration with two southbound lanes on top of two northbound lanes. The tunnel also decreases aboveground congestion along SR 99's former route from South Atlantic Street in the SoDo district (shorthand for south of downtown) to Mercer Street in the South Lake Union district. Vehicles traveling north to the famed Space Needle from the south end of the city, for example, now enter a 1,500 ft long cut-and-cover south portal next to the Port of Seattle, travel through a bored tunnel under Seattle's downtown core for 9,300 ft, and surface via a 460 ft long cut-and-cover north portal, just east of the attraction.

The project is unique for three reasons: the size, the variety of soil conditions, and the separate and completely pressurized evacuation area with its own ventilation system—the first of its kind.

The project is one of several designed to improve safety and mobility along SR 99 and Seattle's waterfront. Collectively, these projects are referred to as the Alaskan Way Viaduct Program, a $3.3 billion infrastructure program that is one of the largest in the country.

In 2001, the Nisqually earthquake damaged the Alaskan Way Viaduct, causing the deteriorating double-decked elevated freeway to sink, which damaged the viaduct's joints and columns, further weakening the structure and threatening public safety. The Washington State Department of Transportation (WSDOT) made emergency repairs to the viaduct, reinforcing the damaged concrete columns and beams that supported the viaduct roadway. However, these repairs were only stopgaps. WSDOT knew that the viaduct needed to be replaced with a new highway designed to meet current seismic standards and updated highway design criteria.

While considering how to replace the viaduct, WSDOT learned a city-owned seawall, which protected the waterfront from seismic events and tsunamis, was also failing; timber and concrete sections were deteriorating and needed to be replaced. To create a solution that would fix both issues, WSDOT and the city worked together to replace their aging infrastructure in a coordinated effort.

After the Nisqually earthquake, WSDOT and local agencies studied more than 90 options for replacing the waterfront section of the viaduct. Four alternatives were assessed in the final environmental impact statement, submitted in January 2009: a bored tunnel, a cut-and-cover tunnel, an elevated structure, and a no-build alternative that would have demolished the viaduct without replacing it.

The preferred alternative, a bored tunnel, promised the least disruption to businesses and traffic during construction. It also meant a repaired and reinforced viaduct could be maintained as a vital downtown route until the tunnel opened. After it opened to traffic, the viaduct could be demolished to make way for the city's large-scale waterfront renewal effort. Plans included public space and a new Alaskan Way surface street, named after the viaduct.

In 2003, a multijurisdictional partnership between WSDOT, the City of Seattle, and King County was formed to deliver the project. After the National Environmental Policy Act record of decision was approved by the Federal Highway Administration (FHWA) in August 2011, the following occurred:

  • The FHWA provided federal funding and ensured the viaduct replacement projects met federal regulations.
  • WSDOT led the efforts to build the new SR 99 tunnel and its connecting roadways and demolish the existing viaduct after the opening of the SR 99 tunnel to traffic.
  • King County implemented transit changes and improvements associated with the program.
  • The city completed the seawall repairs concurrent with the tunneling efforts and is leading the restoration efforts for the street area.
  • Infrastructure to connect the Port of Seattle to the highway and city streets was incorporated into the project with funding from the port.

WSDOT chose to procure the tunnel via a design/build contract to attract the best technical solutions, secure a price based on a conceptual-level design, and achieve the shortest overall design and construction schedule.

In December 2010, WSDOT awarded a $1.4-billion contract to Seattle Tunnel Partners (STP), a joint venture of Dragados USA Inc., of New York City, and Tutor Perini Corp., of Sylmar, California. The Bellevue, Washington, office of HNTB Corp. was the lead designer and engineer of record for the tunnel. STP was responsible for the design, construction, testing, and commissioning of the bored tunnel, the north and south access structures, the tunnel systems, and all permanent structures and facilities. HNTB was responsible for design of the tunnel liner and interior tunnel structures, the two operations and maintenance buildings, and all the associated facility systems, including mechanical and electrical systems and architectural finishes.

The north approach project was designed by WSDOT and constructed by Guy F. Atkinson Construction, of Golden, Colorado. The south approaches were designed by WSDOT and the Seattle office of WSP. They were constructed by Gary Merlino Construction of Seattle; Skanska of New York City; Guy F. Atkinson Construction; and Interwest Construction Inc., of Burlington, Washington.

STP incorporated two innovative concepts: building the roadway with more shoulder width and greater clearance than was initially thought possible and including an alternative technical concept (ATC) that optimized the roadway at the south portal. The ATC lengthened the bored tunnel by moving the south portal 450 ft beyond the point of its preliminary design location, enabling engineers to stack the roadways much earlier than in WSDOT's concept. The modified STP design accommodated all required traffic movements at the tunnel's south portal and shrank the footprint of the south cut-and-cover tunnel and associated roadways by 50 percent in surface area.

WSDOT's preliminary design concept featured a single-bore tunnel with a stacked roadway configuration—a 54 ft overall tunnel diameter to include 30 ft wide roadways with 11 ft wide lanes, a 2 ft wide east shoulder, a 6 ft wide west shoulder, and a 15 ft vertical clearance. STP's design improved on this by proposing a larger tunnel to enhance vehicular safety and traffic operations. It would be excavated with a 57.3 ft diameter tunnel boring machine (TBM) to provide larger horizontal roadway clearances.

With a larger machine, STP's design was able to accommodate a 32 ft wide roadway, which includes two 11 ft wide travel lanes, a 2 ft wide shoulder on the east, and an 8 ft wide shoulder on the west. Vertical clearance was increased to 15 ft, 5 in.

The tunnel was excavated by an earth pressure balance (EPB) TBM, which was built to control the rate at which the excavated ground was removed through the pressurized face at the front of the machine. The pressurized, or closed, face maintained ground stability and allowed operators to work in safe, controlled conditions within the tunnel. With a 57.3 ft diameter cutterhead, it was the world's largest-diameter EPB TBM, which makes the tunnel the largest soft-ground bored tunnel in North America. The epm TBM was manufactured by Hitachi Zosen Sakai Works in Osaka, Japan, designed specifically to the requirements of the project, and shipped to Seattle in spring 2013. On its arrival, the TBM was dubbed "Bertha" after Seattle's first female mayor, Bertha Knight Landes, who served in the early 20th century.

Designing and constructing the massive tunnel presented countless challenges, but three of the biggest were controlling ground deformations, designing the tunnel to meet stringent safety requirements and seismic criteria, and protecting everything above and adjacent to the tunnel while the TBM dug its way through Seattle's core. The team identified and managed four high-risk zones during tunnel boring.

The first high-risk zone was the launch pit. Alignment of the tunnel's south portal would take the TBM through extremely soft, unconsolidated soils. Excavating through that zone without creating settlement and subsequent damage to structures and utilities was the first real test of the TBM's effect on aboveground structures.

To mitigate the risk, the HNTB design moved the launch pit south, starting the tunnel sooner. Crews could then install pilings on either side of the tunnel, fashioning a startup box with a buoyancy slab. This box was designed to overcome the shallow depth at this location, which facilitated the TBM's launch, its descent underground, and the start of excavation without damaging adjacent structures. (The 57.3 ft diameter tunnel is buoyant and could have pushed itself out of the ground at shallow depths.) The startup box was made of a 5 ft thick concrete slab on a foundation of 5 ft diameter, vertical, concrete tension piles to prevent the structure's buoyancy uplift and mitigate the risk of surface settlement during mining. Once the depth reached about 120 ft, the weight of the soil above the tunnel resisted the uplift of the water-saturated soils and there was no additional need for the startup box.

After leaving the zone next to the waterfront and passing under the existing viaduct, the soil conditions were more conducive to tunneling. There was variability, depending on the location and depth, but in general, the soils were stiffer. However, the tunnel had to travel more than 1,500 ft through soft soils and cross under the viaduct before reaching those conditions.

Boring through this section and crossing under the viaduct was another deformation risk and the second high-risk zone. Where the tunnel's alignment and the viaduct were parallel, a continuous row of vertical reinforced-concrete pilings between the tunnel alignment and the viaduct were drilled in advance of tunneling. The piles' sizes—3 and 5 ft in diameter and 80-100 ft in depth—ensured retention of the soils supporting the viaduct and isolated ground loss from tunneling.

As the viaduct veers west, the tunnel crosses underneath it, which placed the crown of the TBM only 14 ft below the then-active viaduct's foundations. Before tunneling under the viaduct, vertical and battered steel micropiles were drilled to form a canopy over the tunnel. These piles were installed between the tunnel and the viaduct's existing pile foundations. In addition to the subsurface work, the viaduct beams were reinforced with carbon fiber wrap for added moment and shear capacity.

Additional Insight: Tunnel Safety

Before mining, differential settlement of the viaduct was estimated to be roughly 3/4 in. During the actual mining, the TBM was operated with such precision that ground movement was limited to 1/10 in., causing no damage to overhead and adjacent structures.

The third high-risk area was under Seattle's dense urban environment. The tunnel's alignment runs under 157 buildings, including single-story structures and high-rises as well as at-grade and elevated roadways, active bridges, an active railroad tunnel, several large sewers, and public and private utilities. Structure foundations varied from spread footings and mat foundations to deep shafts and piles, ranging from 8 ft to 63 ft long and reaching as close as 16 ft above the tunnel crown. Building basements reached as deep as 87 ft.

The tunnel's depth, 215 ft below grade at its deepest point, was influenced by the minimum depth required for tunneling and the need to avoid multiple foundations. HNTB analyzed the potential of tunnel-induced ground deformations and the damage to numerous structures. Several structures were deemed to be at risk for damage and were evaluated further for structural reinforcement before tunneling. Throughout the majority of the alignment, however, no advance work was deemed necessary.

Proper operation of the TBM, placement of the precast rings, and subsequent grouting efforts to fill voids as the tunnel advanced were sufficient to control the ground settlement. An extensive monitoring program was used to detect ground movements during tunneling. STP carefully advanced the machine as the team monitored progress and watched for any indication of settlement or movement. Progress through this zone was steady, and because the tunneling efforts went as planned, very little, if any, settlement or ground movement was detected within this core of downtown Seattle.

While a one-pass tunnel liner is not an unusual design for a tunnel liner system, in this case it was an essential design component dictated by the tunnel's size and location in an active seismic zone that also had high groundwater. The tunnel's required depth made exits to other downtown roadways impractical. As such, exits to downtown and entry into the tunnel are made at each portal via new ramps constructed as part of the project.

The fourth high-risk zone was the entry into the receiving pit at the tunnel's north end. At the end of the 2 mi drive, the TBM had to be perfectly aligned to drive through a limited-clearance opening in a heavily reinforced concrete wall. The TBM had to break through the prepared hole without damage to sensitive utilities located just above the tunnel crown in an adjacent city street.

Operation of the TBM and continual monitoring of the alignments by the contractor were keys to the successful completion of tunneling. The TBM emerged from underground on April 4, 2017, punching through the north receiving pit wall and hitting its target dead-on.

HNTB'S SEGMENTAL tunnel liner design enabled the contractor to make minor adjustments and better control the tunnel's geometry-its horizontal curvature, vertical profile, and any accidental deviations caused by the TBM. The design/build team chose a 6.5 ft wide and 2 ft thick universal ring composed of 10 adjoining tapered segments: 7 rectangular A segments, 2 right trapezoid B/C segments, and 1 isosceles trapezoid K segment. Segments A, B, and C look similar but have slightly different shapes, while the K segment is the key section.

The 10 precast segments form the one-pass lining ring and were assembled with watertight gaskets within the tail shield of the TBM. The segments that form the ring were designed so that the rotation of the ring would incrementally change the TBM's heading.

Additional Insight: Unexpected Delay

The tunnel's interior roadway deck structure rests on continuous corbels that are held in place by dowels drilled into the ring segments. The corbels were cast in place (CIP) behind the TBM. The formwork for the corbels was held in place on a rail-supported traveling gantry system just inside each corbel face. The corbels were cast in 50 ft lengths with end forms between castings. Before casting, the ring surface was cleaned, reinforcement cages were placed, and embedded conduits were installed. The traveling formwork was set in place, and concrete was pumped in from ready-mix trucks.

The corbels support a series of 650 ft long moment-frame systems comprising walls and slabs that were detailed to expand and contract longitudinally. The walls and slabs were constructed in a similar way to the corbels, with preassembled reinforcing cages and CIP concrete. The walls were skipcast sequentially using rail-supported traveling formwork.

To facilitate movement, the lower roadway walls were primarily pin-connected to the corbels below and fixed to the upper roadway slab and traffic barriers above. The tops of the upper roadway walls were pin-connected to the tunnel ring and were detailed to accommodate relative longitudinal and transverse deformation between the frame or tunnel ring. The electrical room and egress corridor slabs are cantilevered from the interior walls; the cantilevered slabs and upper roadway walls are sufficiently clear of the tunnel ring to accommodate the anticipated deformation of the ring into an oval shape that is expected during seismic activity.

Construction of the lower roadway slab was delayed until completion of tunneling operations because tunnel excavation needed to continue during construction. The lower portion of the tunnel had to be kept clear of anything that would block passage of personnel carriers and equipment that hauled segments and materials. The lower roadway was designed as a series of precast slabs that were posttensioned together, so it could be installed quickly after completion of the tunnel.

Because of Seattle's location in an active seismic region, the liner design met stringent operational and performance requirements. The largest expected earthquake during the design life of the tunnel is one with a 108-year return period. During such an event, operational performance objectives required the tunnel to sustain minimal damage to the liner segments and joints and remain watertight. The lining is designed to respond in an elastic manner; by limiting strain stresses under the design loads, the tunnel materials will quickly rebound to their original shapes after the earthquake.

When strain is high, the materials become inelastic. They may withstand the load, but they will not return to their original shapes. During a rare, large earthquake, which would be on the order of a moment magnitude 9 with a 2,500-year return period, life-safety performance objectives for the tunnel allow inelastic deformations. In this situation, watertightness is expected to be maintained through the gaskets, which are placed between all the contacting surfaces of the liner segments.

Construction of the tunnel spanned seven years, between August 2011 and October 2018. In the nine months since its opening, the SR 99 tunnel has transformed downtown Seattle's mobility and waterfront. This world-class tunnel sets a new standard in innovative and safe tunneling solutions under densely populated cities and will serve Seattle as a groundbreaking transportation and mobility solution for generations to come.

Brian Russell, P.E., is a project manager, Gerald Dorn, P.E., S.E., is a lead structural engineer, Sean Cassady, P.E., is the lead fire protection and ventilation engineer, Yang Jiang, Ph.D., P.E., S.E., is a tunnel liner structural engineer, Gordon Clark, P.E., M.ASCE, is a technical director-tunnels and complex underground structures, Andrew Herten, P.E., is a senior project engineer and structural construction engineer, and Thomas Cossette, P.E., is an interior structures structural engineer for HNTB Corp. All are in the Bellevue, Washington, office except for Clark, who is in Salt Lake City.

PROJECT CREDITS Owner Washington State Department of Transportation, Olympia Partners Federal Highway Administration, City of Seattle, King County, Port of Seattle Planning and preliminary engineering WSP, Seattle Design/build contractor Seattle Tunnel Partners, a joint venture of Dragados USA Inc., New York City, and Tutor Perini Corp., Sylmar, California Lead designer and engineer of record HNTB Corp., Bellevue, Washington Design subconsultants Intecsa-Inarsa, Madrid; Hart Crowser Inc., Seattle; Tres West Engineers Inc., Tacoma, Washington; Integrated Design Engineers LLC, Seattle; Earth Mechanics Inc., Fountain Valley, California; and CivilTech Engineering, Bellevue Construction manager WSP, Seattle, and McMillen Jacobs, Seattle, joint venture TBM manufacturer Hitachi Zosen Sakai Works, Osaka, Japan

Civil Engineering, December 2019, © American Society Of Civil Engineers. All Rights Reserved


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