International Journal of Geomechanics

Compared to traditional tunnel construction, artificial freezing involves two distinct stages: freezing and thawing. However, this process can pose a risk to the surrounding environment if it is not possible to accurately analyze the frost heave and thawing settlement during the freezing process. The thermomechanical coupled mathematical model of formation frost heave was established by considering boundary conditions, such as formation temperature and convective heat transfer, and introducing the parameters of instantaneous volumetric strain and denaturation characteristic coefficient. In addition, the numerical analysis method for the whole construction process of the tunnel by the horizontal freezing method was established by programming the user subroutine that takes into account the characteristic coefficients of frozen soil relying on the secondary development technology of ABAQUS (version 2022). Then, the method was applied to the horizontal freezing engineering of a double-line tunnel, and the distribution laws of the freezing temperature field and frost heave displacement field were obtained and compared with the field measurement results. The numerical analysis method for determining the deformation law of the ground surface has been shown to be reliable through comparison with field measurements. This method can serve as a reference for designing effective ground surface heaving control schemes during the freezing construction period of tunnels in complex environments.

Seismic retrofitting of existing bridges has been in practice for years to meet the stringent seismic requirements set forward by revised design codes. For retrofitting, however, bridge piers are often prioritized while less attention is given to the bridge foundations, which are equally prone to damage under seismic loadings. The current work presents a series of experimental studies in assessing the performance of 2 × 2 pile groups reinforced with micropiles in terms of head-level stiffness and damping under low-to-high levels of static and dynamic loadings, encompassing the influence of loading-induced soil nonlinearity. Practical micropiles inclinations of 0°, 5°, and 10° with respect to the vertical are studied. Experimental results reveal that the head-level stiffnesses of pile groups reinforced with micropiles, contrary to the general expectations, become smaller than the pile group without micropiles at higher levels of applied loading. To elucidate the governing mechanism for such experimentally obtained results, three-dimensional nonlinear finite-element analyses were carried out. Results from the numerical analyses support the experimental results, suggesting that the presence of micropiles may not always increase the head-level stiffness of soil–foundation systems, particularly at higher levels of applied loading where the soil nonlinearity generated at the vicinity of piles and micropiles governs the overall head-level stiffnesses.

In this study, a subsidence simulator, also known as a trapdoor, is used to evaluate stages of soil arching (a load transfer mechanism), and related failure features are imaged using time-lapse surface-wave analysis in near real time. The trapdoor was reinforced with a nonwoven geotextile to simulate void roof sagging due to increased loading during void development. This soil behavior was uniquely monitored with three methodologies, namely, multiple horizontal layers of earth pressure cells, photography, and time-lapse seismic surface-wave surveys. Pressure measurements were recorded after every 1 mm of trapdoor downward displacement, along with time-lapse photographs, to visually monitor the development of failure features. Time-lapse seismic surface-wave surveys were conducted after every 7 mm of displacement with pseudo-2D shear-wave velocity (Vs) sections generated for each survey using surface-wave inversion techniques. Vs variations were consistent with the progressive development of shear planes that formed an arch above the trapdoor. A simplified ground reaction curve was then derived from pressure measurements with the estimated stages of soil arching supported by each experimental method. Based on this physical model, Vs variations from surface-wave surveys can be used to successfully and noninvasively image and monitor stress-field variations related to progressive void roof collapse and associated deformation features.

The effectiveness of hydraulic fracturing is determined by the location of fracture initiation and the propagation. Previous research paid more attention to the control effect of geostress and construction-related parameters on the fracturing and propagation of fractures induced by hydraulic fracturing but ignored the important influence of fracturing fluid flow distribution when soft coal and hard coal interbed. In view of this, a model for the amount of fracturing fluids entering soft–hard coal interbedded strata was established. Meanwhile, the relationship among the strength differences between soft and hard coals, the perforation aperture, the coal seam thickness, the thickness ratio of hard coal, and the ratio of fracturing fluids entering hard coal was analyzed. The effect of the ratio of fracturing fluids flowing into hard coal on fracturing and gas production was discussed. On this basis, the fracturing construction suggestions under different thickness ratios of soft and hard coals were proposed. The results reveal that fracturing fluids always preferentially enter the soft coal with a lower strength, which causes an increased frictional resistance of fracturing fluid migration in soft coal, resulting in the redistribution of fracturing fluids entering coal seams and finally achieving the distribution balance. The strength differences between soft and hard coals, the perforation aperture, and seam thickness are negatively correlated with the proportion of fracturing fluids flowing into hard coal. The thickness of hard coal is positively correlated with that of hard coal; when the reservoir gas production potential is similar and the proportion of fracturing fluids entering hard coal does not exceed 30%, stable daily gas production is unlikely to exceed 800 m³/day; when the proportion of fracturing fluids entering hard coal exceeds 70%, daily gas production exceeds 1,000 m³/day. The results provide theoretical support for optimizing hydraulic fracturing schemes when soft and hard coals are interbedded.

Large-scale triaxial tests are popular for evaluating the mechanical properties of railroad ballast. The literature shows that different compaction methods have been used to prepare the samples. This research investigates the potential variation caused by different compaction methods, even when the samples are compacted to the same density. A series of triaxial tests are conducted, and the results confirm that the way in which ballast is compacted significantly affects its stiffness, but not its peak strength, as long as the density is consistent. Discrete-element method simulations are performed to explore this phenomenon and discover that stiffness variations are caused by different ways in which particles interlock and arrange themselves during compaction. Based on the findings from this study, it is recommended to consider the influence of compaction methods when conducting large-scale triaxial testing on railroad ballast or when evaluating the test results.

The excavation of rock masses in deep engineering can induce stress concentration or unloading, which can significantly affect the long-term stability of engineering. To describe the creep mechanical behavior of deep hard rocks after stress adjustment, true triaxial creep tests were conducted through multistage loading and unloading. The effects of σ₁ loading, σ₂ unloading, and σ₃ unloading on creep deformation of granite were investigated. Meanwhile, the changes in creep, acoustic emission (AE) counting, and energy release rates during creep stages were evaluated. The creep failure mode and mechanism of granite were revealed. The results of the true triaxial test showed that both σ₁ loading and σ₂ unloading accelerate the creep of granite, while σ₃ unloading promotes the creep in the σ₃ direction, but the creep in σ₁ and σ₂ directions is not promoted to any significant extent. In the first three creep stages of σ₂ unloading, the strain rate of granite changes significantly. Upon approaching the stage of instability fracture, the changes in the AE count rate and energy release rate are more pronounced compared to the strain rate. By analyzing creep curves, a comprehensive three-dimensional nonlinear viscoelastic–plastic damage creep model specifically for granite was established. The consistency between the experimental data and the predicted result obtained from the model shows that the three-dimensional nonlinear viscoelastic–plastic damage creep model can demonstrate the creep behavior of granite under true triaxial σ₁ loading and σ₂ and σ₃ unloading conditions, thereby serving as a valuable reference for assessing the long-term stability of deep rock masses.