Seismic Vulnerability Analysis of Buried Steel Pipes in Liquefiable Soil Sites

Literature Overview

This paper by Liang Zetian, Zheng Shansuo, Liu Xiaohang, Wu Xingxia, and Yang Feng from Xi'an University of Architecture and Technology investigates the seismic vulnerability of buried steel pipes in liquefiable soil sites. Published in the Journal of Tianjin University (Science and Technology) in 2023, the research was funded by the National Key R&D Program of China (2019YFC1509302) and the National Natural Science Foundation of China (52278530). The study addresses a critical infrastructure resilience issue: buried pipelines in seismically active regions with liquefiable soils face a dual threat from seismic shaking and soil liquefaction-induced buoyancy, which can lead to severe structural damage and service disruption.

Numerical Modeling Methodology

The research employs a sophisticated numerical modeling approach that combines several advanced constitutive models and finite element techniques. The soil behavior is described using the Matasović nonlinear constitutive model coupled with the Byrne-corrected Martin pore pressure increment model to capture both the nonlinear stress-strain response and the liquefaction behavior of sandy soils. A user-defined material subroutine (UMAT) was developed in the ABAQUS finite element platform to implement these constitutive models.

The validity of the UMAT implementation was verified by comparing numerical simulation results with shaking table test data, demonstrating the reliability of the computational framework. The pipe-soil interaction is modeled using two complementary approaches: a solid nonlinear contact model and a shell-equivalent soil spring model. These two models are integrated to create a comprehensive pipe-soil contact-soil spring numerical model that captures both the local contact behavior and the global soil-pipe interaction.

The following table summarizes the key modeling parameters and approaches used in the study:

Vulnerability Assessment Results

The study conducted incremental dynamic time-history analyses for buried steel pipes under various conditions, including three different liquefied zone lengths and three different burial depths. The seismic response demand models were established for each configuration, and the vulnerability curves were developed by correlating the pipe damage indicators with seismic intensity indicators.

The key findings from the vulnerability analysis are as follows:

These results have important implications for pipeline design and siting in seismically active regions. The finding that greater burial depth increases vulnerability in liquefiable soils is counter-intuitive compared to conventional wisdom, and it can be explained by the fact that deeper burial places the pipe in a zone where liquefaction-induced buoyancy forces are more pronounced, leading to greater lateral and vertical displacements of the pipe.

Engineering Practice Implications

For pipeline engineering practice, this research provides several actionable insights. First, the selection of burial depth for pipelines in liquefiable soils should not be based solely on conventional factors such as protection from surface loads and freeze depth. The seismic vulnerability analysis indicates that excessive burial depth in liquefiable soils can be detrimental, and an optimal burial depth should be determined through site-specific seismic vulnerability assessment.

Second, the length of the liquefiable zone is a critical parameter that directly influences pipeline vulnerability. In seismic hazard assessment for pipeline corridors, the spatial extent of liquefiable soils should be mapped and used as an input parameter for vulnerability analysis. Mitigation measures such as soil improvement (vibro-compaction, stone columns, deep mixing) to reduce the extent of liquefiable zones can significantly improve pipeline seismic performance.

From a steel pipe specification and fabrication standpoint, the vulnerability analysis results inform the selection of pipe wall thickness, material grade, and joint design. In high-vulnerability zones, thicker-walled pipes with higher material grades and more robust joint designs (such as full-penetration butt welds with post-weld heat treatment) should be specified to enhance seismic resilience. The pipe material should also be selected for good ductility and toughness, particularly at the expected minimum service temperature, to accommodate the large deformations that may occur during seismic events.

Study Insights and Outlook

This research represents a significant advancement in the seismic vulnerability assessment methodology for buried pipelines in liquefiable soils. The integration of advanced constitutive models, validated UMAT implementations, and comprehensive vulnerability analysis provides a robust framework that can be adapted to different pipeline configurations and site conditions. Future research should extend this framework to include the effects of pipe joint types (banded joints, welded joints, mechanical couplings) on seismic performance, as joint behavior is often the critical factor in pipeline failure during earthquakes. Additionally, the incorporation of aging and corrosion effects on pipe material properties and joint integrity would make the vulnerability assessment more realistic for existing infrastructure. The methodology developed in this study should be integrated into pipeline design codes and seismic hazard assessment guidelines to improve the seismic resilience of critical underground infrastructure.