In 2001, a 6.8-magnitude earthquake shook Seattle, damaging one of the city’s most important north-south traffic connections, the State Rte. 99 Alaskan Way Viaduct.
Built in the 1950s, the viaduct skirts Seattle’s downtown waterfront, carrying more than 100,000 vehicles a day.
Following the Nisqually earthquake, regular inspections documented continuous and severe settlement damage to the viaduct, which was already nearing the end of its expected life span. Geological scientists were predicting a high probability of renewed seismic action within the next decade, which could further compromise or collapse the structure. To ensure the safety of motorists, it was time to take action.
More than 70 replacement options for the viaduct were initially examined. A retrofit was also considered, but its cost would almost equal a replacement and it would not solve issues that make the current structure functionally obsolete (alignment, clearances and sight distances). After a 2007 public vote rejected two replacement options, the Washington State Department of Transportation (WSDOT), King County and the city of Seattle agreed to move forward with projects to replace the southern half of the viaduct, while the three transportation departments began a process to determine the replacement for the downtown waterfront section of the viaduct—the more controversial part of the project.
A 13-month stakeholder process culminated in January 2009 when the Washington state governor, King County executive, Seattle mayor and Port of Seattle CEO recommended replacing the downtown waterfront section of the Alaskan Way Viaduct with a bored tunnel beneath downtown, a new waterfront surface street, transit investments and downtown city street and waterfront improvements.
The 1.7-mile bored tunnel would be 56 ft wide and accommodate a stacked roadway with two southbound lanes sitting atop two northbound lanes. It would be the largest bored tunnel in North America and possibly in the world. The environmental review for the central waterfront viaduct replacement is expected to be complete in summer 2011. Tunnel construction is proposed to begin in late 2011 under a $1.1 billion design-build contract signed in January 2011.
An essential detour
Since the viaduct’s south end replacement proceeded first as a stand-alone project that could connect to any central waterfront replacement, a solution had to be found to maintain S.R. 99 traffic during construction, a solution that also was compatible with all central waterfront replacement options. Given the high use of this traffic artery, relying on existing surface roads to create a detour was not a viable option. Traversing an industrial district, which is home to the city’s baseball and football stadiums, the neighborhood already has high traffic density that simply could not be increased.
As a result, a decision was made to build a new S.R. 99 detour structure that would maintain through traffic during south-end construction. Given its short proposed life span, WSDOT stipulated that much of the structure be recyclable. This affected the design philosophy and the choice of building materials and made the already difficult project even more challenging. The first stage of the project involves the reconstruction of the south end of the Alaskan Way Viaduct, which accounts for almost half of the entire structure. This southernmost section, approximately 1 mile long, extending from South Holgate Street to South King Street, is being replaced with a new side-by-side roadway that meets current earthquake standards and has wider lanes that improve function and mobility. Construction commenced in mid-2010 and is slated to be completed by winter 2014.
The detour structure was planned to maintain traffic and allow demolition of part of the viaduct to facilitate construction of the replacement structure. Design of the detour structure, which features a ramp leading to a two-lane elevated bridge, was completed in 2010. Construction began in January 2011. To keep the project schedule on track, the detour structure needed to be completed by April 2011 in advance of the added traffic volumes generated by the start of baseball and soccer seasons and as part of the preparation for major demolition of the viaduct’s south end in 2012. The construction sequence demanded expedited construction of the detour to minimize closure of this vital route through downtown.
Quick but not easy
With the first priority being expedited construction, the design also had to accommodate other constraints. The detour structure is sandwiched by the existing southbound off-ramp and First Avenue. The utilities and existing foundation leave a very limited footprint for the detour structure. The numerous underground facilities that could interfere with the proposed driven-pile foundations include the overhead utilities that have been relocated underground; existing bridge foundations; drainage; combined sewer; transmission lines; communication; and illumination systems. Pothole locations were requested in line with each proposed bent. Potholes revealed actual locations of the underground utilities, and the contractor was advised to pre-excavate all driven-pile locations to ensure they missed all items that were not identified for relocation.
Driven by the time and space constraints, the design called for 12-in. precast prestressed slab girders with nominal span lengths of 22 and 29 ft. Manufactured by Concrete Technology, they were delivered to the project site as needed, with no on-site storage necessary. The qualified manufacturer selected for this project is located less than 30 miles from the jobsite, and it takes approximately half an hour for the one-way delivery trip. Quick and easy-to-install hot-mix asphalt was put down on the precast deck to protect the roadway. The overlay has 2.5-in. minimum and 4-in. maximum thickness to match the finish grade.
No cast-in-place slab was used. There are eight precast slabs across the roadway section. The keyways between the adjacent slabs were filled with nonshrink grout. After grouting in the keyways, transverse tie rods are tensioned and locked off to fasten the eight slabs across the roadway section. The concrete slabs are supported by a steel substructure that consists of twin W18 cap beams and 20-in.-diam. pipe piles.
The steel piles also extend above the grade as columns that are braced in the transverse direction and the longitudinal direction as necessary. Accordingly, pile alignment is critical to ensuring the cap beams and cast-in-place deck slabs fit as intended. All steel substructure elements utilized welded connections, which allowed for some alignment flexibility in the field and eliminated the time otherwise required by a conventional cast-in-place substructure.
The driving of the steel piles was carefully planned utilizing a single rig moving one way to avoid turning around, which is not possible in the work island located in the middle of operating roadways. Precast traffic barriers with built-in scuppers to allow for proper deck drainage are anchored to the concrete slab to further reduce the construction time.
In addition to speeding up the cast-in-place effort, the selection of these short-span modular units decreased the seismic demand on the detour structure. This is especially necessary for the north end of the detour structure, which is adjacent to the existing structure and uses the existing bent as a support. This particular span has to be short enough not to exceed the thermal movement that is allowed by the existing bent and the gravity and live loads that are allowed by the existing foundation. As a result, the proposed detour structure does not harm the existing remaining structure. To meet WSDOT’s demand for recyclable or reusable materials, the steel piles and steel beams and bracings can be salvaged after the structure is dismantled. Other materials that will likely be reused include the concrete slabs.
Holding it all together
To achieve integrity, seismic resistance and flexibility of construction, considerable thought had to be given to the connection details between the steel substructure and the concrete superstructure. Three types of connections were developed: fixed, expansion and integration with existing structure. The fixed type, which was utilized most, connects the concrete deck slabs with the closure pour reinforced with seismic stirrups. The closure pour has a typical width of 14 in. and serves as a buffer zone to provide the needed construction tolerance. The shear connectors welded to the steel cap beams are embedded in the closure pour along with the dowel bars and steel strands that extend from the ends of the precast slabs. This connection detail is intended to form a “fixed” connection between the superstructure and the substructure.
The expansion type features elastomeric bearings and longitudinal restrainers and allows the expansion of the superstructure subjected to the thermal movement or seismic loads. The third type, which utilizes a fixed connection, utilizes the existing structure’s split-column substructure to accommodate movements. The shear studs, bolts and welds that are applied to the connection details are designed for the seismic and thermal demand. The demand was derived from conducting a three-dimensional structural analysis. The functionality of the detour structure allows the use of 75% of the LRFD live load and a reduction of the seismic response spectrum by 2.5 per the owner’s design criteria.
The overall goal of the Alaskan Way program is to create a safe, seismically sound replacement for the viaduct while providing increased capacity and enhanced mobility to and through downtown Seattle. With the timely completion of the detour structure, this essential infrastructure effort is off to a good start.