In this study, the effect of near-field and far-field ground motions on the seismic response of the soil pile system is investigated. The forward directivity effect, which includes a large velocity pulse at the beginning of the velocity time history of the ground motion is the most damaging phenomenon observed in near-field ground motions. To investigate the effect of near-field and far-field ground motions on the seismic response of a soil-pile system, a three-dimensional model consisting of the two-layer soil, liquefiable sand layer over dense sand, and the pile is utilized. Modeling is conducted in FLAC 3D software. The P2P Sand constitutive model is selected for sandy soil. Three fault-normal near-field and three far-field ground motion records were applied to the model. The numerical results show that near field velocity pulses have a considerable effect on the system behavior and sudden huge displacement demands were observed. Also, during the near-field ground motions, the exceeded pore water pressure coefficient (Ru) increases so that liquefaction occurs in the upper loose sand layer. Due to the pulse-like ground motions, a pulse-like relative displacement is created in response to the pile. Meanwhile the relative displacement response of the pile is entirely different due to the energy distribution during the far-field ground motions.

Underground structures may be buried in liquefiable sites, which can cause complex seismic response mechanisms depending on the extent and location of the liquefiable soil layer. This study investigates the seismic response of multi-story underground structures in sites with varying distributions of liquified soil employing an advanced three-dimensional nonlinear finite element model. The results indicate that the extent and location of liquefied soil layers affect the seismic response characteristics of underground structures and the distribution of their damage. When the lower story of the subway station is buried in liquefied interlayer site, the structure experiences the most serious damage. When the structure is located within a liquefiable interlayer site, the earthquake ground motion will induce greater inter-story deformation in the structure, resulting in larger structural residual displacement. When all or part of the underground structure is buried in the liquefiable soil layer, the structural failure mode should be assessed to ensure that the underground rail transit can quickly restore functionality after an earthquake. Meanwhile, permeability effects of liquefiable soil have a significant impact on the dynamic response of subway station in the liquefiable site.

Fiber reinforcement has been demonstrated to mitigate soil liquefaction, making it a promising approach for enhancing the seismic resilience of tunnels in liquefiable strata. This study investigates the seismic response of a tunnel embedded in a liquefiable foundation locally improved with carbon fibers (CFs). Consolidated undrained (CU), consolidated drained (CD), and undrained cyclic triaxial (UCT) tests were conducted to determine the optimal CFs parameters, identifying a fiber length of 10 mm and a volume content of 1 % as the most effective. A series of shake table tests were performed to evaluate the effects of CFs reinforcement on excess pore water pressure (EPWP), acceleration, displacement, and deformation characteristics of both the tunnel and surrounding soil. The results indicate that CFs reinforcement significantly alters soil-tunnel interaction dynamics. It effectively mitigates liquefaction by enhancing soil stability and slowing EPWP accumulation. Ground heave is reduced by 10 %, while tunnel uplift deformation decreases by 61 %, demonstrating the stabilizing effect of CFs on soil deformation. The fibers network interconnects soil particles, improving overall structural integrity. However, the increased shear strength and stiffness due to CFs reinforcement amplify acceleration responses and intensify soil-structure interaction, leading to more pronounced tunnel deformation compared to the unimproved case. Nevertheless, the maximum tunnel deformation remains within 3 mm (0.5 % of the tunnel diameter), posing no significant structural risk from the perspective of the experimental model. These findings provide valuable insights into the application of fibers reinforcement for improving tunnel stability in liquefiable foundations.

This study examines the fragility response of an earthen embankment supported on a liquefiable deposit subjected to pulse and nonpulse ground motions. Fragility curves are developed based on two key parameters, namely, median seismic intensity and overall variability in the analysis. Such curves represent the vulnerability of an earthen embankment under two distinct types of ground motions. Numerical simulations are performed using two-dimensional finite-element analysis under plane strain conditions. The saturated sandy deposits in the foundation are modeled with the UBC3D-PLM constitutive model and calibrated with appropriate parameters. Two damage indexes are introduced: normalized embankment settlement and lateral embankment deformation. Nonlinear incremental dynamic analysis is performed for various ground motions, and fragility parameters are developed for different damage levels. The results show that pulse-type earthquakes cause more serious damage to earthen structures than nonpulse-type earthquakes, increasing the vulnerability. Further, the liquefiable layer thickness in the foundation soil plays a significant role in the vulnerability assessment of the embankment. The foundation liquefiable layer with less thickness may lead to an early onset of damage and lower the seismic demand on the embankment structure at lower damage levels. With an increase in the layer thickness, seismic demand reduces, with the drainage path playing a critical role.

This study quantifies the seismic fragility assessment of shallow-founded buildings in liquefiable and treated soils, enhanced by drainage and densification, considering both short-and long-term behaviors. A conceptual framework is proposed for developing seismic fragility curves based on engineering demand parameters (EDPs) of buildings subjected to various earthquake magnitudes. The framework for establishing seismic fragility curves involves three essential steps. First, nonlinear dynamic analyses of soil-building systems are performed to assess both the short-term response, which occurs immediately following an earthquake, and the longterm response, when excess pore water pressure completely dissipates, and generate a dataset of building settlements. The seismic responses are compared in terms of excess pore water pressure buildup, immediate and residual ground deformation, and building settlement to explore the dynamic mechanisms of soil-building systems and evaluate the performance of enhanced drainage and densification over short-and long-term periods. Second, 38 commonly used and newly proposed intensity measures (IMs) of ground motions (GMs) are comprehensively evaluated using five statistical measures, such as correlation, efficiency, practicality, proficiency, and sufficiency, to identify optimal IMs of GMs. Third, fragility curves are developed to quantify probability of exceeding various capacity limit states, based on structural damage observed in Taiwan, for both liquefaction-induced immediate and residual settlements of buildings under different levels of IMs. Overall, this study proposes a rapid and straightforward probabilistic assessment approach for buildings in liquefiable soils, along with remedial countermeasures to enhance seismic resilience.

In seismic regions, many underground structures are inevitably partially embedded in liquefiable sites, which may cause complex seismic response mechanisms due to the varying distribution of liquefiable soil layers. This study investigates dynamic interaction between underground structures and liquefiable soils employing three-dimensional nonlinear finite element models. The seismic response of both standard and connection sections of the subway station-tunnel of underground structures in liquefiable sites is evaluated to reveal the seismic response patterns of the soil-structure system under different liquefiable soil distribution forms. The results revealed that compared to homogeneous liquefiable sites, liquefiable interlayer sites can cause greater seismic damage to underground structures, potentially leading to failure along the entire length of the subway station. Therefore, the post-earthquake failure modes of the structure and site should be comprehensively considered based on the site layers distribution characteristics.

A series of large-scale shaking table tests were carried out to investigate the seismic performance of different cement-soil reinforced pile groups in liquefiable sands. Specifically, sinewave scanning was performed on three cement-soil reinforced 3 x 3 pile groups and one conventional (unimproved) 3 x 3 pile group. In this study, the bending moment of group piles, the horizontal displacement of the superstructure, pore water pressure into soils, and the settlement and acceleration response of piles and the ground under different earthquake intensities were recorded. The natural frequency of the ground and the dynamic stress-strain relationship of the soils around piles were obtained. The results show that the acceleration response of the improved pile groups before soil liquefaction is significantly smaller than that of the unimproved pile group. However, the acceleration attenuation of the unimproved pile groups after soil liquefaction is substantially greater than that of the improved pile group. In addition, the lateral displacement of the superstructure, the settlement of pile heads, the bending moment of pile shafts, and the dynamic shear strain of the soils around piles in improved cases are all smaller than those in the unimproved case. In particular, the improved cases significantly suppressed the pile bending moment at the interface between the liquefied layer and the non-liquefied layer. The spatial layout of cement-soils significantly impacts the natural frequency and stress changes of the pile-soil Winkel elastic foundation beam systems.

The connection between subway stations and tunnels in subway systems is a critical consideration in the design of underground transportation systems. Expansion joints may be introduced between the station and tunnel to reduce the stress and deformation transmitted to the structure and mitigate the potential structural damage. However, adverse conditions such as large deformations in liquefiable sites and extreme earthquakes can severely impact the integrity of this connection. This study employs three-dimensional finite element numerical models of dynamic soil-structure interaction in liquefiable sites to investigate the seismic response of the subway station-tunnel connection structure under different distributions of liquefied soil layers and considering various structural connection methods. The results demonstrated that subway station-tunnel structure placed in liquefied interlayer sites experiences greater seismic damage compared to structures with their upper parts embedded in homogeneous liquefiable sites. In addition, using expansion joints between the station and tunnel can indeed reduce the seismic stresses and deformations transmitted to the structure, which can mitigate the extent and severity of its damage. However, the expansion joint can lead to misalignment between the subway station and the tunnel. The findings provide theoretical references for seismic design and disaster mitigation measures for subway structures in liquefiable sites.

The use of permeable piles as an effective drainage method in liquefiable sites has become widely accepted. In this study, the seismic response of both the liquefiable soil and the pile was simulated using FLAC3D software to validate the anti-liquefaction performance of the permeable pile. A group of permeable piles designed according to the China foundation code were numerically modeled with various opening ratios (i.e. area of openings/total surficial area). The numerical results showed that the permeable pile is able to enhance liquefaction resistance by dissipating excess pore water through the drainage holes. The bending moments and axial force of the permeable pile decrease but the ultimate bearing capacity increases in the process of drainage. It is found that the excess pore water pressure ratio (EPWPR) of soil around permeable pile under seismic loading reduces rapidly with increasing opening ratio, but the excess pore water pressure tends to keep nearly a stable level once the opening ratio is beyond a critical value of 0.5%. As a result, the critical value of the opening ratio may be considered as the optimum parameter to design the permeable pile against liquefaction.

Focusing on a T-shape cantilever retaining wall in a liquefiable site, a series of shaking table model tests were conducted to investigate the seismic stability characteristics of the wall when using EPS composite soil isolation piles (WEP), EPS composite soil isolation walls (WEW), and backfilled natural fine sand from Nanjing (WSS). The seismic response characteristics of the model ground soil and the retaining wall for the three models were comparatively analyzed regarding the acceleration, displacement, dynamic earth pressure and excess pore water pressure ratio. Moreover, the seismic performance of anti-liquefaction measures in the liquefiable ground with EPS composite isolation structures were discussed from the view of the phase characteristics and energy consumption. The results indicate that under the same peak ground acceleration, the excess pore water pressure in the WEP and WEW models is significantly lower than that in the WSS model. Different from WSS, WEP and WEW exhibit a segmented distribution with the buried depth in acceleration amplification factors. The embedding of isolation structures in liquefiable sites can reduce the wall sliding and rotational displacements by approximately 25%-50%. In addition, the out-of-phase characteristics of dynamic earth pressure increment are evidently different among WEP, WEW and WSS. There is an approximate 180 degrees phase difference between the dynamic earth pressure behind the wall and the inertial force in the WEP and WEW models. EPS composite soil isolation structures show good energy dissipation characteristics, and especially the isolation wall is better than isolation pile. The displacement index of WSS retaining wall is significantly larger than that of WEW and WEP, indicating that EPS composite isolation piles and wall play an important role in the mitigating damage to the retaining wall. This study can provide references for the application of isolation structures in the liquefiable ground soil regarding the seismic stability.