Probabilistic Evaluation of the Pipeline Response to Seismic Displacement at Balboa Boulevard during the 1994 Northridge Earthquake

Soil displacement along Balboa Boulevard during the 1994 Northridge earthquake ruptured natural gas transmission and distribution pipelines as well as two pressurized water trunk lines. Four other buried pipelines in the ground displacement zone were not damaged. This study probabilistically assesses the performance of the buried pipelines in the framework of performance-based earthquake engineering. The main aspects of pipeline performance follow from the geotechnical characteristics of the site. Uncertainty in each of the key soil-pipeline system parameters is estimated, including length of the seismic ground displacement zone, amount of seismic ground displacement, soil-pipeline interface shear stress, pipe steel yield strength and Young’s modulus, and shapes of the pipe steel stress-strain curves. Monte Carlo simulations are performed with an analytical model to assess the pipe strain response. New fragility functions are proposed to evaluate pipeline performance in response to tensile or compressive longitudinal strain. The resulting probabilities of failure are compared with the results of a conventional analysis in which the modeled pipeline strains are evaluated with respect to the critical strains that cause either tensile or compressive failure. The failure probabilities compare well with the pipeline performance observed during the Northridge earthquake, except for one natural gas transmission line. A sensitivity analysis is performed for this line to investigate the reasons for the discrepancy. Advantages and limitations of probabilistic analyses are discussed.