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To consider the influence of epistemic uncertainties in both seismic hazard and numerical simulation on seismic risk of subway station structures, the uncertainties mentioned above are uniformly characterized as the epistemic uncertainty of seismic demand of subway station structures from the perspective of uncertainty propagation in this paper. On this basis, the analytical formulations of seismic risk considering simultaneous aleatory and epistemic uncertainties are derived. The validity of the derived analytical formulations is verified by the Monte Carlo simulation and the influences of epistemic uncertainty on the seismic risk of subway station structure are further discussed. [ABSTRACT FROM AUTHOR]

This paper presents the design and commissioning of a novel pseudo‐static test apparatus for underground structures that accounts for soil‐structure interaction by simulating the soil with suitably designed springs. The developed apparatus was employed to conduct 1:10 large scale tests on a two‐story three‐span prefabricated subway station structure. Two comparative cyclic load tests were conducted: one involved the developed springs‐structure system; and one involved the structure alone (no springs). The test results demonstrated important differences in the damage location, damage degree, bearing capacity, and deformation capacity of the prefabricated subway station structure under the two loading conditions (i.e., with and without springs). The presence of springs (i.e., soil‐structure interaction) enhanced the lateral collapse resistance of the underground structure and affected the inter‐story displacement ratio (IDR) between the upper and lower layers of the two‐story prefabricated subway station structure. However, it did not affect the deformation coordination of the walls and columns of each layer. A finite element model of the prototype station was also established to conduct dynamic time history analysis simulating the soil‐structure interaction. The results from the dynamic analysis validated the effectiveness of the pseudo‐static test method employing the spring‐structure system. The excellent agreement between the calculated dynamic responses and the responses obtained from the pseudo static tests confirmed the ability of the developed apparatus to conduct seismic tests on complex large‐scale underground structures such as prefabricated subway stations. Thus, this test methodology might be utilized to attain valuable insights into the seismic performance of prefabricated subway stations at a relatively low cost and effort. [ABSTRACT FROM AUTHOR]

In this paper, we study the seismic behavior of pile group bridges in inclined liquefiable soil and reveal the failure mechanism by conducting large-scale shaking table tests. Two bridge models were supported by two foundations: a group pile in an inclined liquefiable site and a rigid foundation. Typical results of the model test under a strong event (Tabas 0.3 g) are illustrated, and the effects of soil–group pile–bridge interaction are explored. The inertial and kinematic effects of the pile–pier curvature are evaluated, and the seismic failure mechanisms of the pile–bridge system are revealed. The results demonstrated that the near-pile shallow soil exhibited significant shear dilation response during the occurrence of strong earthquakes, which induced acceleration spikes for both soil and structure. The interaction state was soil pushing the pile and pile pushing the soil during the first and the subsequent strong earthquake, respectively, due to the bridge P−Δ effect. The liquefaction-induced lateral spreading increased the kinematic effect and reduced the inertial effect on the pile head curvature. In addition, the inertial effect on the pile curvature decreased gradually from the shallow layer to the middle of liquefiable soil, while the kinematic effect increased gradually. The results also demonstrated that the rigid foundation assumption overestimated the acceleration demand of the bridge during strong earthquakes; however, it seriously underestimated the lateral displacement. Finally, the lateral spreading shifted the vulnerable position of the pile group–pier system from the pier bottom to the pile head and bottom, and the leading piles sustained more damage than the trailing piles. [ABSTRACT FROM AUTHOR]

The expression of effective stress proposed by Terzaghi has always been questioned. Many correction formulas are modification of pore pressure term. The pore pressure factor is related to porosity, contact area and other factors. When the particles are in point contact, the expression of the effective stress is that proposed by Terzaghi, while for the surface contact particles, the actual effective stress increases the stress produced by pore pressure passing through the contact surface based on the Terzaghi effective stress. There are many factors that affect the development of contact area and pore pressure, therefore, a series of the drained triaxial tests were carried out on four groups of sand samples with different initial hydrostatic pressures to study the influence of different initial hydrostatic pressures on the effective stress due to the term of contact area (σα). The test results show that the shear strength is increases with the initial hydrostatic pressure under the same effective confining pressure, which indirectly indicates that the initial hydrostatic pressure increases the contact area stress.

A one-g shake table experiment was performed to reveal the failure mechanism of the pile group in sloping liquefiable soils with crusts. The results show that the shallow saturated sand exhibits the alternate behavior of dilatancy and shrinkage, which induces high-frequency peak acceleration. In addition, larger PGA tends to transfer vulnerable positions of the pile in the liquefiable layer from the shallow layer to the interface. Moreover, the pile group effect mainly manifested in piles at the bottom of saturated sand and pile-head during strong earthquakes. [ABSTRACT FROM AUTHOR]

Offshore wind turbine (OWT) structures are subject to wave, wind, and seismic loading. Due to the cyclic nature of these loads, OWT foundations can be vulnerable to cumulative deformation and liquefaction triggered by waves and earthquakes. The effects of cumulative deformation and liquefaction on the monopile-supported OWT are not fully appreciated. This paper develops a three-dimensional numerical model for analyzing the seismic performance of large monopile-supported OWT under the long-term effect of cyclic loading. The numerical model was established employing FLAC3D and utilizing SANISAND constitutive model to simulate the soil behavior. The numerical model was validated by comparing its predictions with the results of dynamic triaxial tests and centrifuge tests. A simplified densification and subsidence site model was integrated into the numerical model to facilitate considering the long-term effect of cyclic loading. The numerical model was then used to conduct a comprehensive study to evaluate the influence of long-term cyclic loading on the natural frequency and seismic response of OWT structure. The results demonstrated that the densified subsidence zone around monopile increased the liquefaction resistance. However, the horizontal displacement of pile and the response acceleration of tower-top increased due to soil subsidence around monopile. [ABSTRACT FROM AUTHOR]

The seismic performance of monopiled offshore wind turbine (OWT) structures was evaluated numerically. The aim was to analyze offshore wind farm sites on complicated layered seabed with high seismicity. Following pile soil analysis (PISA) model, three-dimensional (3D) numerical evaluations were undertaken for two separate locations under two independent seismic events. The effects of pile diameter, depth, site impact owing to transverse soil layering, uni-directional and multi-directional seismic loading and seismic acceleration magnitude are presented. Dynamic impedance depth variation and site response analysis via lateral displacement, lateral soil response and Fourier response amplitude are explored in frequency and time domain. In a seismic zone with stratified soil, pile diameter has a greater influence. Variations in soil profile affect wind turbine performance and seismic sensitivity. This research will give a strong platform for later studies to recommend a safe wind farm site based on simulation results. [ABSTRACT FROM AUTHOR]

The impact of crushing on the macroscopic mechanical characteristics of granular materials from a microscopic perspective has recently been revealed using the discrete element method (DEM). The influence of the self-organized, mass conservation and volume conservation of daughter particles in the fragment replacement method (FRM) on particle breakage is complex and crucial. In this paper, 14-ball Apollo directly filling method (DFM) and expansion method are combined to keep balance among particles before and after crushing, considered to establish DEM to simulate granular material. By considering the influence of volume loss on the micro-fracture condition of the samples, the macro-responses of mechanical and deformation characteristics, the gradation change and the critical state are studied. The effect of inter-particle interaction on macroscopic properties is also analyzed from the microscopic perspective. The rationality of the volume expansion method is verified. The comparisons of samples with volume conservation and volume loss show that the stress development process and the volumetric strain are affected. The existence of the balance point of volume effect is related to the stress level which lead to the difference in stress and strain. Through analysis of relative breakage index (Br) and the statistical number of microscopic particles, it can be seen that the gradation deviation comes from the two-way influence of the volume loss on the crushing of particles. The volume loss has no effect on stress ratio in the critical state, but has a significant effect on stress change during shearing process, and results in the critical state void ratio to decrease with increasing confining pressure. The rationality of daughter particles' volume conservation are analyzed from the motion, displacement development and force chain evolution. [ABSTRACT FROM AUTHOR]

Gas production by depressurization can significantly increase the effective stress in hydrate-bearing sediments. Therefore, strength and deformation characteristics of sediments under high effective confining pressure should be fully understood before large-scale extraction. In this study, a series of triaxial tests on artificial methane hydrate-bearing specimens were conducted under effective confining pressures of 0.2–20 MPa, and the effects of effective confining pressure and hydrate saturation on the strength parameters and the stress–dilatancy characteristics were discussed. The results demonstrate that the strength and stiffness of hydrate-bearing sediments increase with increasing effective confining pressure. Shearing under high effective confining pressure leads to significant breakage of host particles, which is independent of the hydrate saturation. The increase in the effective confining pressure decreases the internal friction angle while increasing the cohesion of hydrate-bearing sediments. By linking the effective confining pressure and hydrate saturation with strength parameters of Mohr–Coulomb criterion and Drucker–Prager criterion, the strength of sediments in a high range of effective stress can be accurately predicted. With increasing effective confining pressure, the shear–dilation transfers to shear–contraction, the critical stress ratio gradually decreases to attain a constant value, and the effects of hydrate saturation and effective stress on the dilatancy characteristics gradually become less notable. [ABSTRACT FROM AUTHOR]