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.

As a potential source of damage, earthquake-induced liquefaction is a major concern for embankment safety and serviceability. Densification has been a popular method for improving the performance of liquefiable soils. Understanding embankment settlement mechanisms plays a fundamental role in determining densification remediation. In this work, nonlinear dynamic analysis of embankments on liquefiable soils is conducted by the finite-difference program FLAC3D (version 6.0) with the simple anisotropic sand constitutive model. Numerical models are validated via dynamic centrifuge test results reported in the literature. The effects of densification countermeasures on the mean and differential settlements are explored in this study. Furthermore, the effects of the densification spacing and width are investigated to optimize the geometry of the densified regions. The development of pore pressure and the movement of the surrounding loose soil are discussed. The results show that both the mean settlement and differential settlement should be simultaneously utilized to comprehensively assess the overall effectiveness of densification treatment. The mean settlement is influenced by the densification spacing and width, but the differential settlement is highly associated with the inner edge of the densified region. This study provides insight for improving the design of the location and lateral extent of densification regions to prevent excessive embankment settlement.

After sand liquefaction, buried underground structures may float, leading to structural damage. Therefore, implementing effective reinforcement measures to control sand liquefaction and soil deformation is crucial. Stone columns are widely used to reinforce liquefiable sites, enhancing their resistance to liquefaction. In this study, we investigated the mitigation effect of stone columns on the uplift of a shield tunnel induced by soil liquefaction using a high-fidelity numerical method. The liquefiable sand was modeled using a plastic model for large postliquefaction shear deformation of sand (CycLiq). A dynamic centrifuge model test on stone column-improved liquefiable ground was simulated using this model. The results demonstrate that the constitutive model and analysis method effectively reproduce the liquefaction behavior of stone column-reinforced ground under seismic loading, accurately reflecting the time histories of excess pore pressure ratio and acceleration. Subsequently, numerical simulations were employed to analyze the liquefaction resistance of saturated sand strata and the response of a shield tunnel before and after reinforcement with stone columns. Additionally, the effects of densification and drainage of the stone columns were separately studied. The results show that, after installing stone columns, the excess pore pressure ratio at each measurement point significantly decreased, eliminating liquefaction and mitigating the uplift of the tunnel. The drainage effect of the stone columns emerged as the primary mechanism for dissipating excess pore pressure and reducing tunnel uplift. Furthermore, the densification effect of stone columns effectively reduces soil settlement, particularly pronounced around the stone columns, i.e., at a distance of three times the diameter of the stone column.

Repetitive traffic loads lead to particle rearrangement and breakage in granular trackbeds and roadbeds, resulting in irreversible deformations that hinder normal operations. Current models for cyclic deformation primarily address rearrangement-induced densification but overlook breakage-induced degradation. Particle breakage disrupts inter-particle interlocking and creates finer debris, promoting volumetric contraction and weakening material stiffness. This study introduces a novel cyclic constitutive model for predicting plastic strain in coarse granular materials, emphasizing the role of particle breakage. The model extended from the existing cyclic densification model within the framework named Shakedown Surface Model, integrating breakage effects. In this model, shakedown thresholds, associated with material densification, are influenced by both plastic strain and breakage-induced loosening. Parameters for shearing contraction/dilation decrease with breakage, capturing breakage-induced contraction. Additionally, the model accounts for principal stress rotation from moving loads. Implemented via an implicit Euler-backward algorithm and Newton-Raphson iteration, the model was validated against cyclic triaxial and full-scale tests, demonstrating its accuracy in predicting the permanent deformation of granular materials under traffic loads.

Improvement of the mechanical properties of soil, such as density, compressibility and shear strength, is typically due to the variation in its inherent microstructure. This paper presents a microscopic study of the densification mechanism in granular soils subjected to impact loading. Both model test and discrete element simulation were carried out to quantitatively analyze the fabric evolution from a particle-scale perspective. Irregularly shaped particles were used in the simulation, on basis of which realistic packing structural information such as average contact number, contact area and branch vector length could be learned. The results reveal the microscopic densification mechanism that impact loading not only promotes the increase of contact number, but also enhances the contact area around per particle. Increasing of contact area has enriched the distribution of sutured contacts to form more steady mechanical status of soil.