This paper aims to investigate the tunnelling stability of underwater slurry pressure balance (SPB) shields and the formation and evolution mechanisms of ground collapse following face instability. A laboratory SPB shield machine was employed to simulate the entire tunnelling process. Multi-faceted monitoring revealed the responses of soil pressure, pore water pressure, and surface subsidence during both stable and unstable phases. The morphological evolution characteristics of surface collapse pits were analyzed using three-dimensional scanning technology. The experimental results indicate that: (1) The key to stable tunnelling is balancing the pressure in the slurry chamber with the tunnelling speed, which ensures the formation of a filter cake in front of the cutterhead. (2) The torque of the cutterhead, soil pressure, and surface subsidence respond significantly and synchronously when the tunnel face becomes unstable, while the soil and water pressures are relatively less noticeable. (3) Excavation disturbance results in a gentler angle of repose and a wider range of collapse in the longitudinal direction of the collapsed pit. (4) A formula for predicting the duration of collapse is proposed, which effectively integrates the evolution patterns of the collapse pit and has been well-validated through comparison with the experimental results. This study provides a reference for the safe construction of tunnel engineering in saturated sand.

Employing soil improvement techniques to mitigate and prevent the detrimental effects of liquefaction on foundations often leads to a significant increase in construction costs in engineering projects. Developing simple, cost-effective, and eco-friendly liquefaction mitigation methods has always been one of the main concerns of geotechnical engineers. Researchers introduced the induced partial saturation (IPS) method to increase the liquefaction resistance of the saturated foundations, which is based on decreasing the saturation degree of the saturated sand. In this study, hollow cylinder torsional shear tests were conducted on loose saturated and desaturated calcareous sand to assess the liquefaction behavior of desaturated sand. Soil compressibility is the primary parameter affecting the liquefaction behavior of desaturated sand. As saturation degree, back pressure, and effective confining pressure significantly influence soil compressibility, their effects on the liquefaction resistance of desaturated sand were investigated. The pore pressure development during cyclic loading reveal that, unlike saturated samples, desaturated samples do not exhibit an excess pore pressure ratio reaching one, even when the double amplitude shear strain surpasses 7.5 %. Finally, the test results demonstrated a notable correlation between liquefaction resistance ratio, maximum volumetric strain, and the maximum generated excess pore pressure ratio, and a pore pressure model was proposed.

As a cost-effective and environmentally friendly technique for enhancing the liquefaction resistance of sandy soils, the air-injection method has attained widespread application in multiple soil improvement or desaturation strategies. This study reports undrained cyclic loading experiments on reconstituted, slightly desaturated sand specimens under either isotropic or anisotropic consolidation to examine the effects of the presence of injected air and initial stress anisotropy on the energy-based assessment of pore pressure and liquefaction resistance. The results exhibited three different cyclic response patterns for the saturated/desaturated specimens with distinct deformation mechanisms, revealing that the sand has a higher degree of stress anisotropy and lower degree of saturation typically being more dilative and less susceptible to cyclic liquefaction. The energy-based liquefaction potential evaluation indicates that the accumulative energy is mathematically correlated with the pore pressure, thus establishing a unified energy-pore pressure relationship for both saturated and desaturated sand. Furthermore, the energy capacity for triggering cyclic failure demonstrates a consistently rising trend with an increase in the consolidation stress ratio and a reduction in the degree of saturation, which seems closely linked to the cyclic liquefaction resistance. This result signifies the potential applicability of an energy-based approach to quantify the liquefaction susceptibility of desaturated in situ soils using strength data from conventional stress-based analyses.

After the construction of the frozen wall of the vertical shaft is completed, it will undergo a long thawing process. Accumulation of damage under load may lead to the rupture of frozen walls and cause engineering accidents. The changes in mechanical properties during the thawing process of frozen rocks are key issues in controlling the stability of frozen walls. In view of the instability problem of the frozen wall of the vertical shaft, this article chooses the saturated sandstone of the Cretaceous system as the research object. Conduct triaxial compression tests under different temperature and confining pressure conditions. Obtain relevant parameters for analysis. And nuclear magnetic resonance technology was used to detect the changes in pore water content in saturated sandstone at different temperatures. The results indicate that: (1) At room temperature, pore water mainly exists in the form of free water, while at low temperatures, pore water mainly exists in the form of adsorbed water. (2) Compared with frozen soil, frozen rocks also exhibit significant supercooling phenomena. (3) According to the variation of unfrozen water content in saturated sandstone at different temperatures, it can be divided into three stages: freezing cessation (- 20 degrees C similar to - 6 degrees C), stable freezing (- 6 degrees C similar to - 2 degrees C), and rapid freezing (-2 degrees C similar to 20 degrees C). (4) As the temperature increases, the closure level of saturated sandstone gradually increases, while the initiation and expansion levels gradually decrease. (5) There is an exponential relationship between the unfrozen water content and the peak strength of saturated sandstone, with a good correlation. And show the same trend of change under different confining pressures. The research results can provide theoretical support and experimental basis for evaluating the instability and failure induced by thawing of frozen walls.

The cyclic response in saturated sand is gaining increasing interest owing to the soil-structure interaction in seismic regions. The evolution of the pore water pressure in liquefiable soil can significantly reduce soil strength and impact the structural dynamic response. This paper proposes a semi-analytical solution for a cylindrical cavity subjected to cyclic loading in saturated sands, incorporating an anisotropic, non-associated SANISAND model. The problem is formulated as a set of first-order partial differential equations (PDEs) by combining geometric equations, equilibrium equations, stress-strain relationships and boundary conditions. Due to the non-self-similar nature of this problem, these PDEs are solved by the hybrid Eulerian-Lagrangian approach to determine the cyclic response of the cavity. Then finite-element simulations with a user-defined subroutine are performed to validate the proposed solution. Finally, parametric studies are presented with the focus on soil parameters and cyclic loading history. It is found that the cyclic responses of the cavity in saturated sands are sensitive to the initial void ratio, and the at-rest coefficient of earth pressure primarily affects the monotonic response but marginally affects the cyclic response. Cylindrical cavities are more likely to liquefy when the sands are compacted in a loose state and under lower displacement amplitudes. The proposed solution has potential use for future research on the cyclic response of the soil-structure interaction in geotechnical engineering.

Although universal in practical engineering, the soil arching effect induced by tunnel face unloading (TFU) in the unsaturated sandy ground (USG) hardly receives academic concerns for its complicacy. In this study, a physical model and a discrete element method (DEM) incorporating the interparticle capillary water force (ICWF) were established and verified. With the combination of experimental and numerical TFU, the intrinsic mechanism of soil arching effect in the USG was innovatively investigated from macroscale to mesoscale. The results indicate that the tunnel face limit support pressure in sandy ground decreases firstly, and then increases with the increase of saturation degree and its minimum value can be less than 22% of that in the dry sandy ground (DSG). Meanwhile, distinct from the global collapse in DSG, a self-stabilized soil arch emerges above the tunnel crown in USG and prevents the loosening zone from further development. With more effective stress transfer under the stronger soil arching effect, the cover-ratios of transition zone and weak deflection zone for the major principal stress in USG can decrease to 24% and increase to 47% respectively as compared to those in the DSG. Additionally, the coordinate number, weak contact proportion, porosity, and contact anisotropy can effectively reflect the meso-mechanical characteristics of soil arching effect in the USG. This work provides precious evidence for evaluating the tunnel face stability in the USG.

To quantify the influence of basic physical properties and cyclic loading conditions on the liquefaction properties of sandy soils, this study uses a combination of physical experiments and numerical simulations to investigate the liquefaction behavior of saturated sandy soils under undrained conditions and their relationship to physical property parameters and external loads. A numerical model with discrete elements was created based on cyclic triaxial tests. A numerical study and predictive analysis of liquefaction of common bulk samples were carried out in conjunction with a PSO-BP neural network prediction model. Using a multivariate analysis of variance and a random forest model, the complexity of the influence of physical parameters and external loads on soil liquefaction was investigated. Quantitative results indicate that particle size distribution, external loads and effective internal friction angle have a significant influence on the liquefaction of saturated sandy soils. In summary, the results of this study provide new insights into understanding the liquefaction behavior of sandy soils.

Historical earthquake-induced damage and studies have shown that the impact of vertical earthquake motions on sand liquefaction cannot be ignored in liquefiable sites with underground structures. Therefore, this study performed a finite element-finite difference (FE-FD) coupling numerical method to compare the influence of different seismic component excitations (vertical, horizontal, and bidirectional) on sand liquefaction with and without subway stations and to further explore the uplift mechanism of the subway station under vertical earthquake motion. The results revealed that the liquefaction response of the foundation soil is much different in the model with and without the subway station. In a liquefiable site with a subway station, the vertical seismic component may also trigger soil liquefaction due to soil structure interaction, while in the unstructured site, it cannot trigger liquefaction. The vertical seismic component will aggravate the degree of liquefaction and horizontal acceleration response of soil near subway stations, and the extent of this influence decreases with the increase of horizontal distance between the soil and the side wall of the subway station. In addition, the influence of vertical earthquake motions on the uplift of subway stations is related to seismic wave characteristics and the value of Arias intensity. The mechanism of the effect of vertical earthquake motion on the uplift of subway stations is to reduce friction between structure and soil and increase the flow deformation of the soil.

The occurrence of earthquake-induced soil liquefaction poses a significant threat, leading to extensive damage to building foundations and other structures, resulting in substantial economic repercussions. The seismic performance of geotechnical systems is markedly influenced by the saturation level of the soil. This study examines the impact of dynamic response on Palar sand. Cyclic triaxial tests were conducted on partially saturated finegrained loose sand with a relative density of 35 % and a degree of saturation ranging from 65 % to 75 %. These tests were carried out at a strain rate of 0.1 % and confining pressures of 50 and 75 kPa. The study findings reveal that an increase in back pressure corresponds to a rise in the excess pore water pressure ratio of the sand. Additionally, the sand undergoes liquefaction as the number of cycles increases, and the degree of saturation decreases for different confining pressures at frequencies of 0.75 and 1 Hz. It was observed that soil liquefies more rapidly at lower strain rates with an increase in effective confining pressure. Conversely, at higher frequencies, soil liquefaction occurs in a smaller number of cycles. Comparing the effects of confining pressure and frequency, a damping ratio of 13 % and a shear modulus of 40 MPa were achieved at a frequency of 0.75 Hz and a confining pressure of 50 kPa. The shear modulus of partially saturated sand decreases with an increase in the initial degree of saturation due to specific characteristics of the Palar sand and the loading conditions.

Proper consideration of variations in soil properties and their effects is necessary to enhance the seismic safety of structures. In this study, the effect of spatial variations in the cyclic resistance ratio on seismic ground behavior was investigated. Initially, dynamic centrifuge model tests were conducted on sandy ground featuring a 20% mixture of weak zones with low relative density and on homogeneous sandy ground with no mixture of weak zones. Subsequently, an effective stress analysis was performed by modeling the distribution of weak zones in the centrifuge model tests. Finally, after confirming the validity of the parameter settings, several analytical models with different weak-zone distributions were generated and numerically analyzed using random field theory. The results indicate that a local mixing of approximately 20% weak zones has only a limited effect on overall ground behavior. However, differences were observed in the rate of increase and dissipation of the excess pore water pressure ratio and in the residual horizontal displacement.