Pier Reconfiguration—A Seismic Focus
Local ports must evolve to meet industry needs, or ships and jobs will simply go elsewhere. The Northwest Seaport Alliance (the Ports of Tacoma and Seattle) needed to accommodate some of the largest shipping vessels in the world. A 18,000 twenty-foot equivalent unit (TEU) Ultra-Large Container Ship is longer than two Space Needles, and the Port of Tacoma’s General Central Peninsula couldn’t accommodate such a vessel. Part of the reason was that Pier 4 stuck out diagonally into the Blair Waterway, narrowing the waterway and creating a chokepoint.
Pier 4 had to be completely reconfigured and aligned with Pier 3 to create a single 3,000-foot-long berth. This required demolition of the existing pier structure and cutting back the shoreline to accommodate the reconfiguration.
Hart Crowser supported multi-discipline engineering firm KPFF Consulting Engineers on this project, which involved demolishing the existing wharf, dredging 460,000 cubic yards of sediment (a volume equivalent to a football field stacked with 215 feet of sediment), constructing a new 1,750-foot-long concrete wharf structure, erecting a new 7,000-square-foot, two-story marine operations building, and improving utilities throughout the site.
Brand-New Seismic Code for Wharves and Piers
The Port of Tacoma Pier 4 Reconfiguration project was the first major port development in the Pacific Northwest to follow the new code standard. The new code standard for seismic design of piers and wharves (ASCE 61-14 ) encourages using “performance-based seismic design” to calculate how the soil and structures will act and interact during an earthquake.
It’s usually the differential deformation (the difference in movement from one location of the structure or soil to another location) that makes a structure unusable after an earthquake. Hart Crowser used dynamic finite element modeling and close collaboration with KPFF engineers to analyze the forces and displacement in the structure caused by earthquake ground motions. Hart Crowser’s work was used to provide input to KPFF’s structural analysis and to optimize the stone column ground improvement layout. This analysis makes the resulting design safer, more resilient, and less expensive than alternative design methods.
Ground Improvement for Seismic Stability
The soil at port and waterfront sites is often weak, and the Port of Tacoma is no exception. In many places there are river deposits or large areas of fill that have been deposited in the past. To strengthen the soil, the team installed stone columns, which are basically pillars of rock inserted into the ground. A hollow probe (or tube) is vibrated into the soft ground to the specified depth then filled with gravel which is injected into the surrounding soil. This vibration and volumetric displacement makes the surrounding soil denser and stronger. When the probe is removed, the gravel stays behind, forming underground columns. This can mitigate earthquake-induced liquefaction—in other words, keep the soil from turning to quicksand in an earthquake, and keep the underwater slope from failing in an earthquake.
The ability to design and install stone columns is not common and requires knowledge and experience. Plus, some stone columns needed to be installed on land and some in water, and there was a high water table to contend with. Construction had to be timed for dealing with tides so that workers could get close enough to create the hole before the tide came in. During the stone column installation, old undocumented pier structures were found. Hart Crowser made recommendations for over excavation of the structures in some places and a revised stone column layout to avoid underground obstructions in other places.
A Successful Outcome
The American Council of Engineering Companies of Washington awarded Hart Crowser a 2019 Silver Best in State Award for complexity for this project. The reconfigured dock now supports the largest container cranes on the West Coast and can accommodate some of the largest container vessels in the world. These new ships use 35 percent less fuel per box than smaller vessels and produce around 50 percent less carbon emissions. Hart Crowser was honored to be a part of the new Pier 4 – a world class marine cargo facility that will serve the Port of Tacoma, the Northwest Seaport Alliance, and the community for many years to come.