Soil Structure Interaction and Its Impact on Seismic Analysis of Dry Docks

Abstract: Soil-structure interaction (SSI) is a critical phenomenon in civil engineering, bridging the gap between geotechnical and structural disciplines. It refers to the complex interplay between a structure’s response and the behavior of the supporting soil. This interaction is particularly significant for waterfront structures, where the surrounding soil can greatly influence the structural integrity and performance.
The first section of this paper provides an in-depth exploration of SSI analysis methodologies. It covers a range of methods varying from the simplest approaches, which often involve basic assumptions about soil behavior and structural responses, to more sophisticated techniques that capture the nuanced interactions between soil and structure. The discussion encompasses a variety of behaviors, and how these factors are incorporated into different analysis models.
In the second section, the focus shifts to a case study examining the seismic evaluation of Dry Dock A (generalized at request of Navy for operational security reasons), a significant naval structure on the west coast. The structure was designed as a pressure-relieved dry dock with cast-in-place reinforced concrete floors and walls, resting on glacial deposits at its north end and mechanically consolidated fill material at its south end. The dry dock is surrounded by a large extent of liquifiable material.
The case study contrasts two distinct levels of SSI analysis applied to seismic assessment of Dry Dock A. The first is a standard analysis employing an equivalent force approach, which simplifies the soil-structure system into discrete soil springs and elastic structural elements. This method is evaluated against the spectrum of techniques discussed earlier, providing a benchmark for comparison.
The second analysis method described is a more advanced timehistory analysis. This approach models the soil as a continuum of elements with nonlinear material behavior, paired with an elastic structure whose effective stiffness is calibrated from an independent nonlinear pushover analysis. The advanced method’s ability to simulate the soil and structure’s dynamic behavior during seismic events offers a more realistic depiction of potential deformations and stresses.
Both analytical methods are then directly compared. The standard model, while useful for conceptual estimates, falls short in capturing the global deformations that can occur during seismic events. The comparison of internal forces within the dry dock, subjected to a seismic event with a 2475-year return period, further illustrates the differences between the two methods. The advanced analysis, which considers the full time-history of motion, provides a comprehensive envelope of moment demands. This contrasts with the standard analysis, which only accounts for an assumed seismic force distribution.
In conclusion, the paper acknowledges the value of both the standard and advanced SSI analysis methods. While the standard approach offers a quick and accessible means of assessment, the advanced method delivers a more detailed and accurate representation of the soil-structure system’s behavior under seismic loading. The findings underscore the importance of selecting the appropriate analysis technique based on the complexity of the structure and the level of detail required for the evaluation.

How does this project / topic contribute to the advancement of the industry and profession?: Soil-structure interaction is a frequent analysis consideration in the maritime industry. This paper presents a summary of the ways it can be accounted for as well as an example case using two different analysis techniques. Insights gained by comparing these two methods and results are informative to other design professionals considering how to capture SSI.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This site is in a seismically active region with challenging soil conditions. Structures being analyzed include the structure itself with a focus on the moles, mole sheet pile walls, pump well, and entrance structure. Advanced laboratory testing and in-house site characterizing tools are being used to support the calibration of the advanced soil constitutive models. Different seismic and water (flow/tide) conditions are being considered in the analyses. Successful collaboration between the geotechnical and structural engineers was facilitated by efficient post-processing and transferring data obtained from the SSI model for the subsequent structural analyses.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: There are few existing examples of advanced seismic analysis of dry docks of this magnitude. Existing code provisions provided insufficient guidance and it was necessary to apply engineering judgement on how to meet performance criteria. Further, software tends to focus on either structure or soil, which required close collaboration between the structural and geotechnical engineering team to coordinate results.

Who is the target audience for this paper?: The primary audience for this paper would be practicing marine structural and geotechnical engineers.