NAVFAC Drydock Emergency Repairs: Finite Element Analyses Confirmed Design Solution in Real Time

Abstract: The KAJV Team completed an accelerated progressive design-build retrofit of three drydocks on the Kitsap Peninsula in the Pacific Northwest. The retrofit, necessitated by analyses completed for the U.S. Navy indicating that the drydock walls were vulnerable to seismic loading, including potential collapse, was designed and constructed in 4 months. The U.S. Navy identified a protected zone where retrofit was required. The design concept consisted of a pattern of ground anchors drilled through the walls and into underlying very dense, glacially overridden soils. With limited subsurface information and a highly accelerated schedule, the KAJV Team used a detailed testing program to confirm the capacity of the ground anchors. KAJV completed two- and three-dimensional dynamic numerical modelling of the drydocks to support the study, design, and/or construction of the drydocks. The analyses included a site-specific probabilistic seismic hazard analyses (PSHA) and the development of spectrum-compatible time histories. Detailed plans were developed and analyses completed to address earth pressures on the walls (including seismic loading), liquefaction, anchor testing, corrosion of ground anchor elements, and potential collapse of walls beyond the protected zone. The project required quick response for updated design elements was necessitated throughout the project where ground anchors encountered and penetrated crane rail piles, utilities, and sheet piling that could not be avoided to complete the mission critical work.
The KAJV Team completed preliminary Finite Element Analyses (FEA) of the retrofitted drydock walls and other drydock walls to inform the U.S. Navy of the risk for other drydock walls to collapse under the design earthquake event. The dynamic FEA analyses combined knowledge of soil properties with structural behavior under a Level 2 seismic event (2 percent exposure in 50 years or 2,475 years) to determine the seismic performance of the walls with a performance objective of no collapse with repairable damage. The analyses consisted of nonlinear time-history soil-structure analyses to predict the maximum seismic design loads. The maximum seismic design loads were used as inputs to the pseudostatic pushover analysis to predict the overall seismic behavior and determine critical locations that the drydock may experience damage or failure. Models were developed to study typical retrofitted and unretrofitted wall sections, end zone behavior (for partial retrofits), headwall regions, pumpstation interface and walls near the caisson region. Based upon the pushover analysis results, structural design check of the drydocks were completed.
The general findings of this seismic performance evaluation were that prior to retrofitting the drydocks, the drydocks showed a very high likelihood of collapse during Design Earthquake Level 2 ground motions. After the retrofits, we found that the drydocks would have a substantial upgrade and survive the Level 2 ground motions. The analyses also evaluated the relative levels of seismic risks among the six drydocks. The work helps to develop a rational basis of prioritizing the seismic retrofit of the drydocks in the future.

How does this project / topic contribute to the advancement of the industry and profession?: Engineering analyses of seismic performance and retrofit of drydocks in complex geological conditions and multiple seismological sources in the northwest U.S. have not been previously performed and reported in professional publication. The paper will address state-of-the-art seismic analyses including site-specific probabilistic seismic hazard analyses (PSHA) of major earthquake from the crustal, intraslab and interface seismic sources, nonlinear soil-structure interaction time history analyses and nonlinear pushover analysis of the drydocks with advanced analytical approaches. The analytical work significantly advanced the knowledge of seismic behavior of drydocks with or without retrofitting in a major earthquake.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This project employed a synergistic combination of multiple sophisticated analysis models and software to achieve in-depth understanding of complex seismic performance of drydocks with improved reliability and confidence. Based upon ground motion time histories from PHSA of various seismic sources, nonlinear time history analysis was performed to predict the maximum seismic loads on the drydock. Then, nonlinear pushover analysis of refined 3-dimensional finite element model of drydocks was performed to identify the seismic behavior and failure mechanism of the drydocks. The innovative analysis approach substantially enhanced reliability and confidence of the drydocks seismic risk assessment and seismic retrofit work.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The advanced seismic analysis was a part of the fast-track design-build project for emergency seismic retrofitting of the drydock at the PSNA and TRFB sites in northwest U.S. Within a period of about 3 months, PSHA and advanced finite element analyses of 6 six drydocks were completed for various conditions including the existing un-retrofitted drydocks, partially retrofitted drydocks, and fully retrofitted drydocks. The design-build team closely cooperated to produce high quality work in a very tight schedule.

Who is the target audience for this paper?: The target audience for this paper are individuals involved in the design, evaluation, permitting, and construction of drydocks and retaining walls in highly seismic areas. For engineers and designers, the paper will describe an innovative approach and advanced engineering analysis to achieve reliable and effective seismic retrofit design. For the general audience, the paper will demonstrate how sophisticated engineering analyses can be integrated into a fast-track design-build project.