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.

The application of prefabricated assembly technology in underground structures has increasingly garnered attention due to its potential for urban low-carbon development. However, given the vulnerability of such structures subjected to unexpected seismic events, a resilient prefabricated underground structure is deemed preferable for mitigating seismic responses and facilitating rapid recovery. This study proposes a resilient slip-friction connection-enhanced self-centering column (RSFC-SCC) for prefabricated underground structures to promote the multi-level self-centering benefits against multi-intensity earthquakes. The RSFC-SCC is composed of an SCC with two sub-columns and a series of multi-arranged replaceable RSFCs, intended to substitute the fragile central column. The mechanical model and practical manufacturing approach are elucidated, emphasizing its potential multi-level self-centering benefits and working mechanism. Given the established simulation model of RSFC-SCC-equipped prefabricated underground structures, the seismic response characteristics and mitigation capacity are investigated for a typical underground structure, involving robustness against various earthquakes. A multi-level self-centering capacity-oriented design with suggested parameter selection criteria is proposed for the RSFC-SCC to ensure that prefabricated underground structures achieve the desired vibration mitigation performance. The results show that the SCC with multi-arranged replaceable RSFCs exhibits a significant vibration isolating effect and enhanced self-centering capacity for the entire prefabricated underground structure. Benefiting from the multi-level self-centering process, the RSFC-SCC illustrates a robust capacity that adapts to varying intensities of earthquakes. The multi-level self-centering capacity-oriented design effectively facilitates the target seismic response control for the prefabricated underground structures. The energy dissipation burden and residual deformation of primary structures are mitigated within the target performance framework. Given the replacement ease of RSFCs and SCC, a rapid recovery of the prefabricated underground structure after an earthquake is ensured.

Shallow cut-and-cover underground structures, such as subway stations, are traditionally designed as rigid boxes (moment-resisting connections between the main structural members), seeking internal hyperstaticity and high lateral (transverse) stiffness to achieve important seismic capacity. However, since seismic ground motions impose racking drifts, this proved rather prejudicial, with great structural damage and little resilience. Therefore, two previous papers proposed an opposite strategy seeking low lateral (transverse) stiffness by connecting the structural elements flexibly (hinging and sliding). Under severe seismic inputs, these structures would accommodate racking without significant damage; this behaviour is highly resilient. The seismic resilience of this solution was numerically demonstrated in the well-known Daikai station (Kobe, Japan) and a station located in Chengdu (China). This paper is a continuation of these studies; it aims to extend, deepen, and ground this conclusion by performing a numerical parametric study on these two stations in a wide and representative set of situations characterised by the soil type, overburden depth, engineering bedrock position, and high- and lowlateral-stiffness of the stations. The performance indices are the racking displacement and the structural damage (quantified through concrete damage variables). The findings of this study validate the previous remarks and provide new insights.

Previous studies have demonstrated that reducing earthquake-induced damage to central columns in underground structures can effectively prevent the collapse of overall structures. Truncated columns (TC) are less likely to experience severe damage during lateral deformation because the partial release of the constraints at both ends of the columns helps maintain their integrity. This approach can effectively enhance the seismic performance of the overall underground structures. In this study, pushover and shaking table tests were conducted to investigate the seismic performance of a subway station using TC columns compared to that using the cast-in-place columns (CC). These tests aimed to examine failure modes, structural stiffness, lateral deformation and load-bearing capacities, acceleration and deformation responses of the underground structures. The results from the pushover tests indicated that the initial stiffness of both structures-those with TC and with CC-was equivalent. On the other hand, the shaking table tests showed no significant differences in the dynamic responses of the two types of underground structures under small earthquakes. However, the vertical ground motions exacerbated damage to the structures. Although the lateral load-bearing capacity of the structure with TC is somewhat lower, the movements between the column ends and beams during loading enhance the structure's ability to adapt to the deformation of surrounding soil due to the release of column end constraints. As a result, the seismic resistance of the overall underground structures is improved. It is important to note that the ceiling slab and sidewalls in the structures with TC are more likely to crack during earthquakes, thus requiring additional efforts to prevent leakage. The findings of this study provide experimental evidence that supports the seismic control of underground structures.

The seismic resilience of underground structures is one of the critical issues for the development of resilient cities. However, existing assessing methods for assessing the seismic resilience of underground structures do not comprehensively address their seismic capacity and post-earthquake recoverability. This paper developed a seismic resilience index and framework for assessing the seismic resilience of underground frame structures by considering both the damage and functionality of underground structures caused by earthquakes, as well as the processes involved in repairs. The seismic resilience index was developed by quantifying the resist resilience and recovery resilience, which can be used to describe the robustness, redundancy, and resourcefulness of the seismic resilience. Then the assessing procedure for this method is presented step by step. Additionally, a case study was conducted to assess the seismic resilience of a frame subway station, focusing on the economic losses associated with earthquakes. The study also discusses the improvements in seismic resilience achieved through the use of reinforced concrete truncated (RCT) columns. Results indicate that RCT columns can significantly enhance the seismic resilience of underground structures. The reasonability and quantifiability of the developed method were compared with existing methods, demonstrating its effectiveness. Furthermore, the developed assessing method can be extended to assess the seismic resilience of underground structures after quantifying their operational functionality.

Although the influence of duration of ground motion on the seismic response of aboveground structures is clearly recognized, its influence on underground structures remains unclear. To this end, this study performs incremental dynamic analyses under both short and long duration ground motions, to quantify the significance of the duration effect on the seismic fragility of subway stations. A two-dimensional soil-structure system is established on the basis of the Daikai subway station, consisting of an elastoplastic soil model and a concrete damage plasticity model. A set of thirty spectrally matched ground motions with varying significant durations (D5-95) are employed. In particular, using the center column total compressive damage index (DTCD) and peak inter-story drift ratio (IDR) as structural demand measures (DM), the percentage difference in fragility curves between long and short duration is evaluated by accounting for six suits of damage state thresholds. Correlations between the two DMs and D5-95 show that ground motion duration affects significantly the seismic fragility of subway stations. Overall, the duration effect is not detected in the minor damage state and becomes more pronounced in the collapse state, suggesting that the duration effect increases as the damage state threshold increases. The median collapse capacity for long duration ground motions is up to approximately 60% or 37% lower than that for short duration ground motions, when a peak IDR or DTCD are adopted, respectively. The results of this study highlight the great importance of properly considering duration when selecting earthquake records for seismic fragility assessment of subway stations.

Investigations of seismic response of underground structures often assume homogeneous or layered homogeneous sites. However, significant spatial variability in soil parameters may lead to vastly different underground structure performance from that obtained for homogeneous sites. Based on random field theory, this study models the spatial variability of the soil elastic modulus, cohesion, and friction angle using the Karhunen-Loe`ve (K-L) expansion method. Target acceleration response spectra are generated according to standards, and the trigonometric series method is employed to create artificial seismic waves of four different intensities. Nonlinear dynamic analyses of underground structures under deterministic and random field conditions are conducted using ABAQUS software. The study comprehensively analyzes the structural damage state, internal forces, interstory displacement, and drift ratio to evaluate the station structure's performance under different seismic intensities. Results show that the spatial variability of soil parameters significantly impacts the dynamic response of underground structures, especially for stronger earthquakes. The variability of soil stiffness and strength parameters leads to greater fluctuations and uncertainties in displacement and internal force responses, exacerbating structural damage. It is recommended that when the peak ground acceleration (PGA) reaches or exceeds 0.5 g, the spatial variability of soil parameters should be incorporated into the analysis to ensure a reliable assessment of the structural seismic performance.

The aim of this study was to solve the problems of low retention rate and poor grouting effect caused by the strong slurry fluidity of traditional cement-based grouting materials in deep backfill strata. Metakaolin-cement-based materials were used as raw materials to examine the dispersion of nano-boron carbide (B4C) using a laser particle size distribution instrument. Nano-B4C was used as a modifier to perform rheological and macroscopic mechanical tests. The effect of nano-B4C dispersion on the performance of the cement-based composite grouting materials was analyzed. The modified materials were further characterized via microscopic tests, and the grouting modification mechanism was revealed. The results showed that the agglomeration of nano-B4C with poor dispersion resulted in an increase in the fluidity of the grouting material, a decrease in viscosity, and a decrease in early strength. The well-dispersed nano-B4C effectively improved the viscosity and early strength of the grouting material and decreased the fluidity, and the change range increased with increasing nano-B4C content. The performance of the modified grouting material was superior to that of the traditional grouting material. The results present new solutions to problems, such as poor grouting effects in deep backfill soil strata.

In the processes of seismic design of underground structures, selecting a reasonable input ground motion is very important, which can cause severe damage to underground structures. To quantitatively evaluate the seismic damage potential of ground motions on multi-storey underground structures and solve the problem that single intensity measures are inadequate in accurately indicating the seismic damage potential of ground motions, this paper taking the input ground motions in the seismic design of underground structures as the research object, and constructing some composite intensity measures that can effectively characterize the damage potential of input ground motion. Firstly, considering that the underground structures in different characteristic period-type sites will exhibit different seismic responses under the same excitation, the soil-structure system is divided into four-period bands. Then, four representative periods are selected from four-period bands respectively, and the corresponding four soil-structure system numerical models are established. Subsequently, 40 ground motions are selected for elastoplastic numerical analysis, and the results of the numerical analysis were used as sample data to construct composite intensity measures corresponding to soil-structure systems in each period band using the partial least squares regression method. Finally, 100 additional ground motions were used to verify the correlation between the composite intensity measures and the seismic damage of underground structures. The results show that the correlation coefficient between the composite intensity measures and the seismic damage of multi-storey underground structures is better than those of commonly used single intensity measures.

Internal soil erosion in urban environments is a significant factor contributing to the chronic uneven settlement of subway stations. This paper investigates the seismic failure mechanisms of subway stations affected by prior soil internal erosion. Erosion is modeled via a practical approach based on the Cap plasticity model. A 2D finite element model of a two-layer, three-span subway station is developed to simulate its seismic response under various factors, including the seismic incidence angle, soil erosion, and earthquake motions. The vertical load transfer and damage assessment of the vertical elements are thoroughly analyzed across all the scenarios. The results show that after the adverse internal force redistribution caused by soil erosion in the corners of the underlying soil, the subway station experiences a progressive seismic failure process. As the seismic incidence angle increases, the deformation mode of the station shifts from a bilateral shear mode to a unilateral pushover mode, requiring more seismic energy for structural collapse.