By Paul Guenther, P.E., S.E., William Elkey, P.E., S.E., Eric Herzstein, P.E., S.E., M.ASCE, and William J. Perkins, P.E.
Constructed in the early 1900s, the Elliott Bay Seawall, in Seattle, protects the city's downtown waterfront from the erosive forces of Puget Sound and Elliott Bay. Because of its deteriorating condition and seismic vulnerability, it was replaced with a new seawall, one that makes extensive use of modular precast concrete components that sped construction, enhanced durability, reduced project costs, and minimized impacts to adjacent properties
THE WATERFRONT along Elliott Bay, in Seattle, once consisted of beaches, marshes, and mud flats that met the forested hills and bluffs that the city was built on. As Seattle grew in the late 1800s and early 1900s, the demands on its waterfront increased, and the city became a major shipping and industry hub for the Northwest region and a launch point for the growth and development of Alaska and other points north. Railroad lines were built along Railroad Avenue, now Alaskan Way, as well as numerous piers—each with a railroad spur and sheds Typical Cross Section of Existing Seawall from the 1930s to facilitate the storage and movement of cargo. As pier access developed, the shoreline moved 400 ft west into Elliott Bay. The Elliott Bay Seawall was constructed to protect this new shoreline and the growing Seattle downtown from the destructive waves and erosive tidal forces of Puget Sound.
In 2003, a report titled "Geotechnical Analyses for Existing Alaskan Way Seawall" was produced by Shannon & Wilson Inc. that identified the original seawall as being in poor condition in many locations and vulnerable to significant damage and potentially catastrophic failure during a major earthquake. The soil conditions along the waterfront are underlain by competent glacially overridden soils at depths of about 30 to 90 ft. These competent soils are overlain by relatively loose/soft, naturally occurring beach and estuarine deposits, which are overlain by fill placed in the late 1800s and early 1900s to extend the shoreline west into Elliott Bay. The fill includes hydraulically placed material from other city projects, debris from the Great Seattle Fire of 1889, and sawdust and mill ends from sawmills located on the waterfront. The largely nonengineered fill is highly susceptible to earthquake-induced liquefaction and movement.
The original central seawall, 3,730 ft long, consisted of several distinct structure types, including pile-supported gravity walls. The majority (2,455 ft) of the original seawall, however, consisted of a timber relieving platform (with concrete face panels), which resulted in a forest of timber piles spaced between 2 ft 8 in. and 5 ft 4 in. apart, shown in the drawing on the left.
This wall type was constructed in the 1930s and is of particular interest because it featured one of the earliest uses of large precast-concrete elements for a major public infrastructure project in the Northwest. The concrete portions performed well over the years, showing only limited deterioration; however, the timber relieving platform experienced significant damage in some areas from marine borers, and the steel sheet piles below the concrete panels deteriorated due to corrosion where they were exposed.
As part of the project, the new seawall uses a combined system for support: soil strengthening in the form of cellular jet-grout improved soil mass (ISM) and secondary structural elements (seawall superstructure). This combination provides the static and dynamic load-carrying capabilities the seawall requires.
The jet-grout ISM supporting the seawall superstructure consists of a zone of secant, jet-grouted soil-cement columns laid out to form a cellular subsurface structure that provides vertical support to the seawall superstructure and lateral restraint for the combined system under seismic loading, including seismically induced liquefaction. The design jet-grout ISM has a replacement ratio that varies between 50 and 64 percent and extends between 30 and 60 ft behind the new seawall. A cast-in-place (CIP) support slab is on top of the ISM to provide a solid foundation for the gravity seawall superstructure.
The superstructure makes extensive use of modular precast concrete components, which sped construction and reduced project costs. More than 1,200 individual precast pieces encompassing more than 12,000 cu yd of precast concrete were fabricated. The primary precast components included custom concrete face panels fitted with habitat shelves, zee-shaped superstructure segments, and light-penetrating sidewalk panels fitted with architectural glass blocks.
THE NEW SEAWALL was designed for load combinations and factors per American Association of State Highway and Transportation Officials Load and Resistance Factor Design Bridge Design Specifications (fifth edition). Load combinations evaluated included uplift and stability checks due to vertical wave pressures and tsunami loading.
The typical sections of the seawall were designed as gravity structures. The CIP support slab and soil above it were sized as counterbalance elements to resist overturning, sliding, and bearing forces. Live loads on the cantilever beams included consideration of vehicular loading and uniform pedestrian live load. Multiple configurations were analyzed for the typical seawall sections for variations in water levels, fill heights, and live-loading conditions.
The seismic design of the seawall system employed a performance-based approach, and criteria were established for three ground-motion levels. The ground-motion levels and performance criteria are shown in the table on page 62.
The new seawall achieves seismic stability from the ISM, which restrains the unimproved, liquefiable soil landward of the ISM and provides vertical support to the seawall superstructure.
The ISM is the primary element of the seismic lateral forceresisting system. The design criteria were developed to ensure that the ISM failure mode would slide along the base; toppling or shear through the ISM would be unacceptable.
Because the primary design criteria included seismic displacement and rotation requirements, the ISM design relied extensively on nonlinear time-history analyses in which the soil liquefaction behavior and its interaction with the ISM were modeled. Conditional mean spectra were developed for the various earthquake sources that are significant contributors to the ground-motion hazards at the site (including Cascadia Subduction Zone interplate and intraplate sources and local shallow crustal faults) to determine time-history target spectra for rare earthquake (RE) and maximum considered earthquake ground-motion time histories. Recorded ground motions representative of the different earthquake sources were then selected and modified to be compatible with their respective target spectra.
Two-dimensional nonlinear time-history analyses were then performed using finite difference computer models for the final design of the ISM's width, strength, and area replacement ratios to meet the performance criteria. Three-dimensional pushover-type analyses of individual cells were also used to evaluate the sensitivity of the cellular structure performance to imperfections. For example, an existing timber pile might block the jet-grout spray area, resulting in a zone of unimproved soil or "shadow zone" within a jet-grout column. These three-dimensional analyses ensured the cellular structure was sufficiently redundant to perform as intended.
The seawall superstructure elements were designed to deform in a way that is compatible with the ISM, to resist seismic loads imposed on the elements from inertia and adjacent soils, and to undergo inelastic behavior under RE ground-motion levels.
An additional challenge to the seawall project was improving the waterfront's adjacent marine environment to benefit juvenile salmon, forage fish, and crabs. To increase the overall health of the ecosystem, several habitat improvements were constructed. One addition was an uninterrupted intertidal migratory habitat bench that runs the length of the seawall. The bench was constructed using specific rock that young fish are attracted to and is protected by scour-resistant rock.
Other habitat improvements that were constructed were improved bottom substrates, extended habitat benches in open (unshaded) areas of the seawall, and textured precast concrete habitat shelves attached to precast concrete face panels. The textured shelves vary in length from 3 to 7 ft and are staggered vertically.
The canyons and shadows created by Seattle's urbanized shoreline drastically impacted native salmon that traveled their ancestral waterfront migration route. To encourage salmon migration, natural light was incorporated into the seawall via custom light-penetrating sidewalk panels constructed of precast concrete inset with glass pavers. The pavers provide an illuminated passageway under the 15 ft cantilevered sidewalk, which will attract juvenile fish to the area and help the salmon thrive during their annual migration. All the panels were designed to be installed as part of the overall modular structural system.
THE DESIGN AND CONSTRUCTION of the new seawall directly impacted utilities along Alaskan Way, which directly serve adjacent piers and properties and function as elements within larger utility systems that serve the city, the Puget Sound region, and the West Coast of the United States.
Additionally, along the length of the existing seawall, there were multiple electrical vaults and duct banks owned by Seattle City Light (SCL) that were required to be protected in place. Large notches were designed into the CIP support slab to accommodate these vaults and duct banks, which necessitated additional structural modeling to determine the overall stability and structural behavior of the wall in these areas. In some locations, pile and drilled supports were required where sufficient space was not available for a gravity solution.
The ISM was constructed using jet grouting because of that method's ability to create a large-diameter soil-cement column from a small-diameter hole. This method also offered the flexibility to design and construct around obstructions. Jet grouting uses high-pressure fluids, including cement grout, pumped through a horizontal nozzle at the end of a typically 4 in. diameter drill rod. The highpressure grout erodes, fluidizes, and mixes in situ soils with cement. The drill rods rotate as the grout is injected to form a circular column of soil-cement that can be several feet in diameter. Soil-cement columns can be constructed using other methods-paddles, augers, or mills that mix the soil with the cement grout. But these methods require that a tool the size of the soilcement column be inserted into the ground to mix the soil and cement.
The existing timber piles from both the original seawall timber relieving platform and the various trestles built out to the piers were obstructions that were worked around using jet-grouting techniques. In addition, jet grouting was more conducive than other methods to avoiding existing utilities The precast elements were laid out using a basic 8 ft module that was repeated to the maximum extent. This pattern was chosen because it minimized the number of unique pieces, which resulted in an economical precast fabrication process. The precast face panels were designed to allow small utility penetrations to be installed at virtually any panel location to accommodate the dozens of utilities required to provide services to the piers. In addition, the precast face panels were designed with aesthetic form-liner surface treatments unique to the project. In some locations, where existing major utilities needed to be protected in place, special panels were designed to accommodate the existing conditions.
Though not required by the project specifications, the precast concrete supplier proposed using self-consolidating concrete (SCC) to construct the precast elements. To validate the suitability of SCC for this application, a mockup was carried out to confirm that SCC would perform well for the various precast elements, given the complexity of the geometry, reinforcing levels, et cetera. A full-scale precast zee segment and precast face panel were constructed, then saw-cut at several locations to confirm that the mix performed well, with good flow characteristics, attaining adequate consolidation and aggregate distribution without segregation.
AFTER JET GROUTING and the initial excavation were complete, a CIP concrete support slab was constructed and placed on top of the ISM. The contractor elected to construct the CIP slab within dewatered excavation sites, which were isolated from the bay using a continuous cofferdam wall that also doubled as a containment wall during the jet-grouting process.
Precast concrete face panels were then set and temporarily supported from the support slab by corbel brackets attached to the back of the face panels. The precast concrete habitat shelves and in-water fish habitat corridor were then installed in front of the face panels. The precast zee segments were then set on the support slab on top of the face panels. Both elements were set on adjustable shims to enable the fine-tuning of their final positions for field fit-up.
Once the precast face panels and zee segments were set and aligned, a field-cast closure wall and shear block were installed to tie the system together into an integral composite unit. The zee segment base was then pressure grouted. The zee segments were designed to be independently stable under their own weight until the completion of the field-cast closures.
Next, a CIP edge beam was constructed at the cantilevered end of the precast segments, which serves as a diaphragm to tie the segments together and allows the distribution and sharing of concentrated loads applied to the cantilever. Finally, the precast sidewalk panels were set on bearing pads and seated into recesses formed into the precast and edge beams to create the finished sidewalk surface. A steel cover plate was used to span the gap between the new seawall and adjacent pier structures. After the completion of the seawall, a new waterfront promenade was constructed to complete the adjacent surface treatments and pedestrian amenities.
THE ELLIOTT BAY Seawall Project required a large, multidisciplinary design team, consisting of 29 consulting firms that coordinated with multiple stakeholders, including the Seattle Department of Transportation (SDOT), SCL, Seattle Public Utilities, and multiple private pier owners, among others. To progress and integrate the design, discipline-specific task forces were created so that members of the design team and the SDOT could meet to resolve design issues and document decisions. To streamline the organization and ensure that interdisciplinary issues were resolved at the right levels, related disciplines were grouped into "super" disciplines:
SEAWALL— structural, geotechnical, and coastal engineering
PUBLIC REALM— habitat design, art support, urban design, and public involvement
CIVIL— roadway, utilities, drainage, and civil engineering
These larger task forces increased efficiency in resolving most interdisciplinary issues. Issues that affected multiple super disciplines were addressed at the engineering management level.
This project had a long development history that predated the final design services contract. As an initial project task, the final design team, led by Parsons Corp., was asked to organize and facilitate a cost/risk assessment and value engineering workshop to review the current design for validation and improvement of the base concept. This workshop served to inform the final design team of the design history and development, ensuring continuity during the project handoff.
Because of the large number of stakeholders, the complexity of project interfaces, and the value of early contractor involvement to resolve constructability challenges, the general contractor/construction manager (GC/CM) method was selected as the preferred method of project delivery. In this method, the GC/CM contractor is allowed to perform up to 30 percent of the work itself and subcontract the remainder of the work as competitive bid packages. Additionally, the GC/CM contractor can bid for a maximum of 40 percent of the subcontracted work.
After a short list was created and interviews were conducted, the contracting team of M.A. Mortenson Co./Manson Construction Co. was selected as a joint venture. The team estimated the total cost of the project below the prescribed cap of $300 million. After fully executing the GC/CM contract, the final design team worked with the GC/CM to develop the final procurement packages for the self-performed work and the subcontracted packages.
The final design team served as the engineer of record for the project and delivered the environmental documentation and permits, coordinated public outreach, developed procurement documents for construction, supported right-ofway activities, and provided construction engineering and construction management support. The delivery of this project required extensive coordination with project stakeholders, city committees, elected officials, business owners, partner agencies, and regulatory agencies.
The new Elliott Bay Seawall serves as the structural backbone of the waterfront's redevelopment and will protect it for many years to come. The unique and innovative design solution and the GC/CM delivery approach were critical to the successful design and execution of this complex project. Mortenson/Manson JV was awarded the GC/CM contract in early 2013, and the project is scheduled to be substantially complete this year.
Paul Guenther, P.E., S.E., is the Pacific Northwest marine practice lead in COWI North America Inc.'s Seattle office. William Elkey, P.E., S.E., is a vice president and Eric Herzstein, P.E., S.E., M.ASCE, is the regional bridge practice lead, in Parsons Corp.'s Seattle office. William J. Perkins, P.E., is a vice president at Shannon & Wilson Inc., in Seattle.
This article is based on a paper ( https://doi.org/10.1061/9780784479902.066) the authors presented at the 14th Triennial International Conference, held in New Orleans, June 12-15, 2016, which was sponsored by PIANC and the Ports and Harbors Committee of the Coasts, Oceans, Ports, and Rivers Institute of ASCE.
Engineer of record (structural/geotechnical work): Parsons Corp., Seattle office; COWI North America Inc., Seattle office; and Shannon & Wilson Inc., Seattle office
Owner: Seattle Department of Transportation
Design prime consultant: Parsons
Structural designers: Ben C. Gerwick Inc./COWI; Exeltech Consulting Inc., Seattle office
Geotechnical engineer: Shannon & Wilson
General contractor/construction manager: M.A. Mortenson Co./Manson Construction Co. JV,
Seattle Seawall habitat and public space: Magnusson Klemencic Associates, Seattle office
Precast concrete supplier: Oldcastle Precast
Ground improvement contractor: Hayward Baker