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.

The significant reduction in the stiffness of liquefied soil is accompanied by a decrease in the shear wave velocity, which ultimately results in the softening of the liquefied site. Time-frequency response analysis can identify the sudden drop in the frequency of the liquefied site, which has been widely employed to determine the onset of liquefaction. However, using the modal frequency (corresponding to the maximum power at each time step) to identify the timing of liquefaction (tL) captures the reduction in frequency during earthquakes, but it does not encompass the entire range of frequencies that have changed. Furthermore, previous literature defines tL as the boundary separating the modal frequency into pre- and postliquefaction time segments, but this estimate does not consider the generation of pore water pressure. Two representative case histories are presented to highlight the limitations of identifying tL by solely relying on the modal frequency approach that uses a two-step function. As a result, this study introduces an innovative method to identify tL utilizing the spectral energy ratio (SER), which captures the entire frequency shift. A step-by-step procedure using SER is detailed, and the new estimates of tL are compared with those derived from previous literature using 30 case histories. To validate the approach, a sensitivity analysis was performed using centrifuge test data from the Liquefaction Experiment and Analysis Projects. Results indicated that incorporating a ramp that accounts for pore water pressure buildup in the trilinear function improved tL estimation. An optimized SER value of 0.92 was determined for the proposed method. The notable contribution of this study is an enhanced approach of identifying the timing of liquefaction triggering by only utilizing acceleration records without requiring pore water pressure responses.

The magnitude (Mw) 8.3 Tokachi-oki earthquake occurred in September 2003, causing extensive damage in Hokkaido, Japan, and triggering extensive soil liquefaction in the region. The Port of Kushiro was one of the locations where surficial evidence of liquefaction was observed but was also a well-instrumented location with four pore-water pressure transducers installed in the backfill of the quay wall. However, all of the sensors malfunctioned during the earthquake. As a result, the pore-water pressure response recorded by those sensors were inaccurate and unusable with regard to evaluating liquefaction triggering and extent. This study introduced the energy-based soil liquefaction evaluation to estimate the excess pore water pressure responses at the Port of Kushiro based on the cumulative strain energy of the soil during the 2003 Tokachi-oki earthquake. In order to apply the energy-based method to this case history, this study explored the empirical equation describing a relationship between normalized cumulative energy and excess pore water pressure ratio while incorporating the bidirectional shaking effect on strain energy development. Although the energy-based method allowed for the estimation of the time needed to trigger liquefaction at a target site, it was derived using the empirical coefficients that were developed for a different soil from those at the site of interest. This indicated that an adjustment to the estimated timing of liquefaction was needed, which was accomplished by additional evaluation through a Stockwell transform and Arias intensity-based liquefaction assessment. Both procedures indicated a similar timing of liquefaction at the site. Based on the updated time of liquefaction triggering, the empirical coefficient was recalibrated to estimate the excess pore water pressure ratio, and the result provided reasonable excess pore water pressure responses at the backfill of the Port of Kushiro during the 2003 Tokachi-oki earthquake.

Increasing demand for transportation has forced new infrastructure to be built on weak subgrade soils such as estuarine or marine clays. The application of heavy and high-frequency cyclic loads due to vehicular movement during the operational (post-construction) stage of tracks can cause (i) cyclic undrained failure, (ii) mud pumping or subgrade fluidisation, and (iii) differential and excessive settlement. This keynote paper presents the use of prefabricated vertical drains (PVDs) to enhance the performance of tracks. A series of laboratory experiments were carried out to investigate the cyclic response of remoulded soil specimens collected from a railway site near Wollongong, NSW, Australia. The results of the laboratory tests showed that beyond the critical cyclic stress ratio (CSRc), there is an internal redistribution of moisture within the specimen which causes the top portion of the specimen to soften and fluidise. The role that geosynthetics play in controlling and preventing mud pumping was analysed by assessing the development of excess pore water pressure (EPWP), the change in particle size distribution, and the water content of subgrade soil. The experimental data showed that PVDs can prevent the EPWP from building up to critical levels. PVDs provide shorter-radial drainage for EPWP to dissipate during cyclic loading, resulting in less accumulation of EPWP. Moreover, PVDs cause soil to behave in a partially drained rather than an undrained condition, while geotextiles can provide adequate surficial drainage and effective confinement at the ballast/subgrade interface. Partially drained cyclic models were developed by adopting the modified Cam clay theory to predict the behaviour of soil under cyclic loadings. The Sandgate Rail Grade Separation project case study presents a design of short PVDs to minimise the settlement and associated lateral displacement due to heavy-haul train loadings.