Integrated Analysis of Liquefaction Induced Pile Downdrag Loads Using SAP2000

Abstract: In recent years the existing port infrastructure of the Municipality of Skagway (MOS) began to reach the end of its useful life. The Skagway Assembly and Ports & Harbors Advisory Board worked to develop a strategy for a resilient multi-modal port that would serve the community for decades to come. By 2021 the MOS completed the Port of Skagway Master Plan which describes the Port developments and infrastructure investments to support cruise and industrial partners. The Ore Peninsuala Redevelopment project is one part of the ongoing efforts to revitalize the waterfront with the installation of a floating cruise dock, an industrial wharf, and a Roll-On Roll-Off (RORO) dock.

The to be constructed industrial wharf, referred to as the Marine Service Platform (MSP) is designed as a plumb pile supported wharf to be used for industrial applications such as ore transfer to ships. The MSP is supported by 24-inch diameter steel plumb piles which were selected based on cost, availability, and ease of procurement. These piles vertically span more than 150-feet before reaching competent soil; there is a 70-foot free height from the bottom of wharf to the mudline, then 80-feet through liquefiable soils. This pile selection created a design challenge when considering the length of pile in conjunction with seismic lateral and axial loads.

Southeastern Alaska has several major faults that contribute to the seismic hazard in Skagway. These faults are a result of the interactions between the Pacific Plate boundary and the North American plate and have the potential of producing earthquakes with mean magnitudes up to 8.2. The soil conditions at the project site generally consist of submerged loose to medium dense granular deposits, providing the perfect conditions for liquefaction when coupled with this seismic setting. As the excess porewater pressure within liquefied soils dissipates towards the end of shaking, the settling soil mass exerts a net downward load (downdrag) on the embedded piles.

Conventional methodologies to account for downdrag are detailed in design codes such as AASHTO, IBC, and ASCE 61. These procedures estimate the total downdrag load for the full depth of liquefiable soils, apply the load to the pile, and ignore strain compatibility between the pile and soil. While these simplified approaches result in straightforward and fast design, they conservatively ignore neutral plane theory and the potential for some soil support of the pile within the liquefied body. For deep deposits of liquefiable material, this likely results in overly conservative designs and excessive pile lengths, increasing project costs and construction duration.
The design of the MSP required the development of new analysis methods to directly integrate a neutral-plane analysis of the soil settlement in the SAP2000 structural model. This new analysis method makes use of the existing readily available software to estimate downdrag loads more accurately and realize project cost savings. The new analysis method will be compared to the methods currently detailed by design codes as well as the latest research at UC Davis.

How does this project / topic contribute to the advancement of the industry and profession?: This topic serves to provide a simple integrated method to account for the presence of downdrag loads due to soil settlement caused by earthquakes without the need to make conservative load assumptions or conduct more rigorous iterative calculations external to the structural model. This proposed methodology serves to streamline the design process required to account for soil-structure-interaction by directly integrating the downdrag analysis into existing structural modeling practices of using p-y and t-z springs resulting in a reduction in design schedules and budgets while still developing a more refined design.

Does the project / topic implement new and innovative techniques, materials, technologies, and delivery methods?: This topic implements a new and innovative analysis technique to directly integrate neutral plane analysis into existing structural modeling practices to capture downdrag . This analysis technique expands on the currently evolving practice of downdrag evaluation and can be easily performed by using the readily available SAP2000 structural analysis software.

What was the most challenging aspect of your project / topic and how did you handle it to ensure success?: The design schedule for this project was compressed due to a variety of issues that plagued the aging infrastructure of the Port as well as the desire to complete construction for certain other structures of the project in time to support the upcoming cruise season. This analysis technique was developed to eliminate the need to conservatively assume the downdrag pile loads potentially increasing the pile sizing, as well as the need for a time-consuming iterative process between structural and geotechnical disciplines that would normally be required to perform a more refined neutral plane analysis.

Who is the target audience for this paper?: The target audience for this paper is structural and geotechnical engineers who may consider implementing the proposed analysis technique to save time and budget on their client’s projects.