The design of steel catenary risers (SCRs) is mainly affected by fatigue performance in the touchdown zone (TDZ), where the riser cyclically interacts with the seabed. This cyclic motion leads to seabed soil softening and remoulding. However, over an extended period of riser operations, the seabed soil undergoes a drainage because of small motion amplitudes of the floating vessel during calm weather or a limited contact with the seabed due to vessel relocation. This may cause recovery of the soil strength associated with excess pore pressure dissipation resulting in an extra fatigue damage accumulation in the TDZ. In the current study, a global SCR analysis has been conducted using a series of coded springs along the TDZ to model advanced SCR-seabed interactions. The instantaneous undrained shear strength of the soil is determined by using a recently developed effective stress framework. The effects of soil remolding and consolidation were integrated during both the dynamic motion of the SCR and intervening pause periods within the critical-state soil mechanics. The model updates the SCR-soil interaction spring at every time increment of dynamic analysis, calculating the cross- stress range while taking into account the overall configuration of the riser on the seabed. The study showed that the consolidation may result in an increased fatigue damage of about 23 %, which is currently neglected by the existing non-linear SCR-soil interaction models.

Seismic fragility analysis can quantitatively evaluate the seismic performance of structures from a probabilistic viewpoint and accurately characterize the relationship between the degree of structural damage and ground motion intensity. This study investigates the seismic fragility of shield tunnels in three different liquefiable and non-liquefiable soils. A plane-strain finite element model of the saturated soil and shield tunnel is established via the OpenSees computational platform employing the multi-yield surface elastoplastic PressureDependMultiYield and PressureIndependMultiYield models to simulate the constitutive behaviour of liquefiable and non-liquefiable soils. The developed model is utilized to conduct nonlinear dynamic effective stress time history analyses to generate the seismic fragility curves and surfaces based on the incremental dynamic analysis method. Meanwhile, appropriate scalar- and vector-valued intensity measures are identified based on their correlation, efficiency, practicality and proficiency. Compared with the fragility curves based on scalar-valued intensity measures, the fragility surfaces based on the vector-valued intensity measures can better describe the effect of ground motion characteristics on the structural seismic demand, and thus can more accurately assess the structural seismic performance. The seismic damage probabilities derived from the fragility curves and surfaces reveal that the seismic damage risk of the shield tunnel in sandwiched liquefiable soil deposit is higher than that of the tunnel structure located in entirely liquefiable or non-liquefiable soil profiles. This finding underscores the importance of carefully evaluating the seismic safety of shield tunnels situated in sandwiched liquefiable soil deposits.

In earthquake-resistant design, the characteristics of ground motion and soil conditions play a crucial role. Soil liquefaction, a critical issue in earthquake engineering, leads to significant ground deformations, including lateral spreading, settlements, and shear strain accumulation. While extensive research has focused on single-fault rupture, the impact of multiple-fault ruptures on liquefiable soils remains underexplored. This study examines the dynamic behavior of liquefiable soils subjected to single and multiple-fault ruptures through two-dimensional nonlinear fully coupled effective stress analyses within the Open Source Earthquake Engineering System (OpenSees) framework. The seismic response of saturated sandy soils with varying relative densities is simulated by the Pressure Dependent Multi Yield Material 02 (PDMY02) model. Three seismic records (Antakya record from the 2023 Kahramanmara & scedil; earthquake, Sakarya record from the 1999 Kocaeli earthquake, and Izmit aftershock record from the 1999 Kocaeli earthquake) were analyzed to assess settlements, lateral spreading, and excess pore water pressure. The results demonstrate that multiple-fault ruptures induce more complex and severe soil responses than single ruptures. These findings enhance the understanding of soil behavior under seismic loading, emphasizing the necessity of considering multiple-fault ruptures in liquefaction analysis for improved earthquake resilience.

The Makran Subduction Zone (MSZ) represents a convergent plate boundary where the Arabian Plate is subducting beneath the Eurasian Plate. This study assessed liquefaction susceptibility and ground response in Gwadar region, located on the eastern side of MSZ. A comprehensive dataset of seismic records, compatible with Pakistan design code BCP: 2021 rock spectrum, was used as input motions at bedrock. A series of one-dimensional (ID) non-linear effective stress analyses (NL-ESA) was conducted using DEEPSOIL v7 numerical tool. The findings revealed that pore water pressure ratio (r(u)) exceeded the threshold value for liquefaction onset (r(u) > 0.8) at various depths within the site profiles. A significant de-amplification of peak ground acceleration values was observed at liquefiable depths in soft soils. The liquefied stratum exhibited a non-linear response, with high shear strain values manifesting plastic deformations. A comparison of computed design spectra with code spectra revealed significant discrepancies. It is demonstrated that BCP: 2021 underestimated site amplification for site class D profiles in the 0.1 to 0.8 s period range, while overestimating it for site class E profiles across the entire period range up to 1.6 s. The findings will benefit infrastructure development in the region, particularly within the China-Pakistan Economic Corridor.

In this study, the one-dimensional nonlinear Matasovic model was extended to the two- and three- dimensional stress space by replacing the shear strain with the generalized shear strain. The extended Matasovic model was subsequently combined with Dobry's excess pore water pressure generation model, and the reliability of the proposed loosely coupled effective stress analysis model was verified through undrained cyclic triaxial tests and a one-dimensional site response analysis. Finally, this method was applied to an engineering site in China to evaluate the nonlinear seismic response of liquefiable sites. The results show that the proposed effective stress analysis method can capture the dynamic behavior of the soil and the generation of pore water pressure during strong shaking. Moreover, there are differences between the proposed effective stress analysis method and the total stress analysis method for liquefiable sites, which indicates the need for careful selection in practical applications.

Proper consideration of variations in soil properties and their effects is necessary to enhance the seismic safety of structures. In this study, the effect of spatial variations in the cyclic resistance ratio on seismic ground behavior was investigated. Initially, dynamic centrifuge model tests were conducted on sandy ground featuring a 20% mixture of weak zones with low relative density and on homogeneous sandy ground with no mixture of weak zones. Subsequently, an effective stress analysis was performed by modeling the distribution of weak zones in the centrifuge model tests. Finally, after confirming the validity of the parameter settings, several analytical models with different weak-zone distributions were generated and numerically analyzed using random field theory. The results indicate that a local mixing of approximately 20% weak zones has only a limited effect on overall ground behavior. However, differences were observed in the rate of increase and dissipation of the excess pore water pressure ratio and in the residual horizontal displacement.

Historical data has shown that soil-structure systems exhibit increased severity when subjected to earthquake sequences, attributed to the accumulated instability of soil deposits and the cumulative damage of structures. This study analyzes seismic responses of multi-story buildings and mechanical behavior of liquefiable soil deposits under repeated shake-consolidation process. This is achieved through a series of numerical simulations using a finite element-finite difference (FE-FD) code, namely DBLEAVE-X. Sequential earthquakes are obtained from the NGA-West2 PEER ground motion database and recalibrated relied on various aspect ratios, including peak ground acceleration ratios (rPGA) and consolidation time (Tgap). The numerical results reveal that shearinduced and residual settlements of buildings during sequential earthquakes might be notably larger than that during single earthquakes. The repeated shake-consolidation process has a significant impact on development and dissipation of excess pore water pressure (E.P.W.P), notably influencing the deformation response of both buildings and ground deposits. The findings also provide valuable insights into effects of both complete and partial consolidation processes on seismic mechanisms of entire liquefiable soil-structure systems. Numerical observations suggest that multi-story buildings under sequential earthquakes might be more vulnerable, underscoring the necessity of integrating sequential earthquakes into earthquake-resistant building design.

In earthquake-resistant design, amplitude and frequency content of ground motions (GMs) have been considered using spectral matching techniques; however, duration effects remain insufficiently explored in designing buildings in liquefiable soils. This study investigates the influence of ground-motion duration on seismic response of shallow-founded buildings under strong earthquakes. Buildings in liquefiable soils are analyzed using nonlinear dynamic analysis with coupled u-p formulations. The numerical code and calibrated constitutive parameters of Toyoura sand are validated through dynamic centrifuge testing. Two ground-motion suites, including 30 pairs of long and short-duration events, are scaled to the target PGA of 0.3 g, and then selected to be spectrally equivalent to isolate duration measures from the others. Comparative results show that longer duration events result in greater settlements and tilt compared to shorter events. Therefore, this study emphasizes the importance of considering duration measures in assessing seismic responses of buildings. Furthermore, correlation between settlements and peak transient tilt, and intensity measures (IMs) of GMs are comprehensively analyzed. It is found that employing compound IMs can lead to notable improvements in predictive accuracy for settlement and peak transient tilt compared to single common IMs. The compound IMs, namely CAV2/3 x Ds5-951/3 and SMV x Ds5-95 3, are newly proposed for use in order to achieve best correlation with the shear-induced settlements and peak transient tilt, respectively.