The seismic vulnerability of interaction system of saturated soft soil and subway station structures was explored in this paper. The coupled nonlinear numerical models of interaction system were established using the u–p formulation of Biot′s theory to describe the saturated two-phase media. A refined finite element model of interaction system was developed to study its nonlinear seismic responses and seismic hazard mechanism. In this study, the multi-yield elastoplastic constitutive model was adopted for the soil, while a fiber section elastoplastic constitutive model was used for the structure. The seismic response of the structure was calculated by inputting the artificial seismic wave obtained from the power spectrum-triangular series method. The maximum inter-story drift angle was taken as a structural performance parameter for the subway station structure. The structural demand cloud was obtained under random ground motion sequences. Based on the probabilistic seismic demand model analysis method, the seismic vulnerability curve of the subway station structure was plotted, and the seismic vulnerability curve was analyzed as per the vulnerability of performance parameters. With the increase of soil strength, the vulnerability index of subway station structure under different peak acceleration ground motion decreased correspondingly. Based on the above vulnerability theory and analysis methods, it can be found from the above vulnerability theory and analysis methods that the subway station structure with established buried depth in saturated soft soil site exhibits a certain degree of safety and reliability, and can meet the seismic fortification goal of "no damage in small earthquakes, repairable in medium earthquakes and no collapse in large earthquakes". The results of vulnerability analysis are in line with the actual seismic survey, and the vulnerability analysis method proposed in this paper can be applied to the vulnerability analysis of underground structures on saturated soft soil foundation.

For the complexity and difficulty of seismic research on subway station structure system embedded in soft soil foundation, the seismic research method is quite different from the ground structure. The methods of seismic research on subway station structures in soft soil were summarized, and relevant literature on this field in recent years were sorted out. The advanced progress of theoretical analysis and quasi-static simplification analysis, model test (shaking table test, centrifuge test), numerical simulation (the total stress method, the effective stress method), and dynamic reliability of underground structures were mainly introduced for seismic analysis of subway station embedded in the soft foundation. The advantages and disadvantages of each method and the development direction of this field were proposed briefly in order to better understand seismic analysis of underground structure engineering in soft soil.

To reveal the meso-mechanism of the axial compression failure of recycled aggregates stabilized by multi-source solid waste geopolymers and to overcome the limitation that traditional laboratory tests cannot observe the meso-state of the material, a three-dimensional numerical model of this material was established using PFC3D discrete element software to simulate an unconfined compressive strength test. The meso-contact parameters for the specific mix proportion were calibrated using the back-analysis method. The stress–strain curves obtained from the simulation and laboratory tests were compared and analyzed. The meso-rules of axial compression deformation and failure of the material were analyzed from the perspectives of crack evolution, particle movement, and force chain transmission. The results show that the descending branch and failure mode of the stress–strain curves from the discrete element simulation are highly consistent with those from laboratory tests, whereas slight differences exist in the ascending branch owing to model limitations and material randomness. The through-cracks of the specimen failure are mainly distributed in the middle and lower parts. The cracks initiate from the middle of both sides and propagate longitudinally. The increase in cementation bond breakage reached 349.5% during the rapid crack development stage. After failure, the particles in the middle and lower parts of the specimen flowed around, and the particles in the upper part moved downward slowly. The force chains were uniformly distributed in the elastic stage. However, a large number of force chains broke, and the high-force chains were concentrated in the middle and lower parts in the crack development stage. This study verifies the feasibility of the discrete element method in simulating the meso-mechanical behavior of this material and provides a meso-theoretical basis for its mix ratio optimization and engineering applications.

For the aim to study the effects of burial depth on seismic response in the subway station’s structure embedded in saturated soft soil, on the basis of the OpenSEES program, numerical models with four cases of burial depth for the nonlinear interaction system including subway station’s structure and saturated soft soil are developed with the u‐p formulations of Biot’s theory as it seeks to describe the saturated two‐phase media. The effects of burial depth on the seismic response and the failure pattern of the subway station structure are investigated. The numerical results show that (1) the burial depth has a significant effect on the nonuniform uplift of station’s structure due to the movement of soil from far field towards baseplate; (2) with the increase of burial depth, the inertial effect of overlaying soil of the subway station structure becomes more obvious, and the peak zone of the pore pressure ratio contracts from the outer area to the baseplate bottom with decreasing amplitude; (3) the dynamic index R decreases with the increasing burial depth of the station’s structure; and (4) in terms of the middle column, the burial depth of the station’s structure has the most crucial effect on the additional dynamic shear force and bending moment at the top end.

Sixteen groups of comprehensive tests were undertaken to examine the modifications in the mechanical characteristics of weak expansive soil resulting from the inclusion of a composite material comprising cement and jute fibers. The aim of the tests was to analyze the unconfined compressive strength and shear strength. The results reveal that (1) as the modified material content increased, the stress–strain curve of the modified soil progressively became steeper, with a more pronounced peak value. Under the same strain condition, the stress variation increased accordingly. After the stress reached its maximum, a sudden phenomenon of brittle axial compression failure occurred. (2) With the increase in cement content, the unconfined compressive strength and shear strength of the modified expansive soil gradually increased, reaching the highest shear strength when the cement content was 8%. With the increase in jute fiber content, the unconfined compressive strength and shear strength of modified expansive soil increased first and then decreased, reaching the highest value when the content was 0.5%. (3) The cohesion of modified expansive soil increased with the increase in cement content. When the cement content was 8%, the cohesion of the soil sample was as high as 298.29 kPa. The cohesion of modified expansive soil increased first and then decreased with the increase in fiber content. When the jute fiber content was 0.5%, the cohesion reached the maximum value. (4) The internal friction angle of the modified expansive soil increased with the increase in cement content. When the cement content was 8%, the internal friction angle of the soil sample increased by 2.64°. The cohesion of modified expansive soil increased first and then decreased with the increase in fiber content. When the jute fiber content was 0.5%, the internal friction angle reached the maximum value.

At present, the built or newly built underground structures are mostly concentrated in developed areas along the coast, lakes, and rivers, while deep soft soil is widely distributed in these areas. The cold heat treatment method is one of the advanced foundation treatments and has broad application prospect. However, geothermal exchange and temperature field change are complex Stefan problems with mobile boundaries, internal heat sources, and phase changes. It is important to determine the freezing area and freezing temperature reasonably under the premise of ensuring the safety of the frozen construction. Therefore, this paper summarizes the research methods of freezing method construction of cross passage and introduces the research on the constitutive model of freezing undisturbed soil, the theory of freezing wall calculation, and the evolution mechanism of the temperature field. Then, a brief evaluation is made in view of the lack of research, and the development direction of this field is put forward with the research characteristics. It is to deepen the understanding of the deformation mechanism of cross passage embedded in soft soil caused by the freezing and melting.