Technical Insights by Mark Havekost, PE
West Coast water providers have undertaken numerous projects in recent years to create a water system more resilient to earthquake hazards, including projects that focus on creating seismically robust backbones for water transmission systems. A major seismic hazard for existing and new pipelines is permanent ground deformation (PGD), and the typical design approach for dealing with PGD is a combination of strategies to avoid, resist, or accommodate movement.
Extreme seismic loading can result in complex pipeline interactions that generate high local strains and deformations. These may reach or exceed the yield strength limit of a pipe material, which leads to permanent (inelastic) deformation of the pipe. Therefore, the traditional pipeline design approach that limits the stress level in the steel by utilizing allowable specified stresses (usually fractions of yield strength) may be inadequate. This is because design features necessary to maintain stress within elastic limits and also meet serviceability objectives are often challenging to implement without significant ground modification or additional structural reinforcement. These measures can result in adverse impacts and are often costly. Instead, under certain circumstances, a strain-based design approach* can take advantage of the strain characteristics of the pipe material, resulting in a resilient design that is more economical.
Tensile and compressive strain design criteria are typically developed with owner input based on pipeline performance objectives. A typical performance objective for compressive strain is early onset of pipe-wall wrinkling as a limit for continued safe operation.
McMillen Jacobs Associates utilizes finite element analysis (FEA) to provide the means to understand steel pipeline behavior in the ground when ground movement pushes the pipe beyond its elastic limit. The analysis models the soil behavior with a system of nonlinear soil springs that represent the principal directions of buried pipeline restraint: vertical-uplift, horizontal-lateral, vertical-bearing, and longitudinal-axial. The soil-pipeline interaction is then simulated by applying ground deformations to the soil springs to determine the load demands on the pipeline. We also carefully select stress-strain curves that represent the expected pipe material behavior throughout the range of strains of interest. Important lessons learned from implementing a strain-based design include:
• Obtain stress-strain curves of the actual pipe material to understand the potential for stress-strain plateau behavior and its impact on pipeline performance.
• Consider steel materials typical in the oil and gas industry for analysis because they can often provide a more desirable stress-strain behavior and increase confidence in pipe modeling results.
• Couple the appropriate soil reaction force with the axial frictional force in FEA models, to avoid underestimating axial load demands on the pipe. Recent full-scale testing at Cornell University† indicates that frictional force along the pipe is directly related to the reaction force between the pipe and adjacent soil.
• Capture initial load state in the pipeline (e.g., from internal operating pressures) to which seismic load demands are additive.
While multiple pipeline design strategies are available to achieve seismic resiliency, strain-based design has proven successful in addressing extreme seismic loading. Our involvement in pipeline analysis and design of water conveyance pipelines throughout the West Coast of the United States has included development of seismic design guidelines, performance design criteria, customized analysis methodologies, and joint design optimization to address a wide range of complex loading. Through targeted and appropriate analyses, owners can have the information to confidently select cost-effective and resilient solutions.
*In addition to welded steel pipe, these methods can be used for other pipe materials such as HDPE.
†O’Rourke, T.D., J.K. Jung, and Christina Argyrou. 2016. Underground pipeline response to earthquake-induced ground deformation. In Soil Dynamics and Earthquake Engineering 91(11).