The cyclic secant shear modulus is essential for assessing the dynamic response of saturated compacted loess prior to large deformation and flowslide. However, its evolution characteristics remain incompletely elucidated, and a criterion based on this modulus to evaluate the dynamic stress states of compacted loess is still lacking. To address the issues, a series of undrained cyclic shearing tests was conducted, and the results were analyzed in the δN-γ, δN-u, CSR-Nγ, CSR-Nu, and p′-q planes. Test results indicate that cyclic secant shear modulus is essential for assessing the state of compacted loess under cyclic shearing. The compacted loess at a low CSR is in a stable state, as its effective stress remains far away from the instability line (IL) in p′-q plane. This state can be characterized by the stiffness index δN fluctuating around 1.0 under cyclic shearing. In contrast, the compacted loess at a high CSR is in an unstable state, as the effective stress is approaching IL and CSL. The unstable state can be characterized by a decrease in δN from 1.0 to 0.3 under cyclic shearing. At a medium CSR, the compacted loess is in a metastable state, as its effective stress state is approaching the IL during cyclic shearing. This state can be characterized by a stiffness index fluctuating around 1.0 for a limited cyclic number, after which it decays continuously. The compacted loess in the metastable state may exhibit a stiffening-decay transition behavior; that is, the stiffness index first increases to a peak value ( δ_N > 1.0) before falling to a residual value ( δ_N ≈ 0.3), which could be separated by a stiffening-decay line (SDL) in the p′-q plane. Furthermore, a criterion formulated on the basis of the modulus within the framework of bounding surface plasticity theory was presented to evaluate the dynamic stress states of compacted loess. Below the SDL, compacted loess may stiffen due to the interparticle strengthening under cyclic shearing. Above the SDL or IL, its stiffness decays as pore water pressure rises and effective confining pressure falls. The results provide a basis for evaluating the dynamic stress states of compacted loess under cyclic undrained shear conditions.

Understanding the strength and deformation behavior of recompacted loess is essential for the stability assessment of man-made geotechnical structures in loess regions. While numerous studies have examined the macroscopic mechanical properties of unsaturated recompacted loess, the underlying particle-scale micromechanical deformation mechanisms remain inadequately explored. In this study, a series of drained triaxial tests were conducted on unsaturated recompacted loess under varying matric suctions and mean net stresses to investigate their effects on strength and deformation behavior. The experimental results indicate that higher matric suction increases deviatoric stress due to suction-induced apparent cohesion, whose influence weakens with increasing mean net stress. Moreover, all specimens exhibit continuous volumetric contraction during shearing, consistent with the strain-hardening behavior observed in the stress–strain curves. To interpret these observations from a micromechanical perspective, the original Hill contact model within the discrete element method (DEM) framework was modified by incorporating suction-dependent micromechanical parameters. The modified model was calibrated and validated against laboratory data, showing good agreement in reproducing both stress–strain and volumetric deformation behaviors. Further micromechanical analysis reveals that suction enhances capillary bonding at low net stress, but its influence diminishes as the mean net stress increases. Specifically, at low net stress, higher suction results in a greater proportion of tensile (i.e., capillary) contacts and slightly reduced particle displacements, indicating stronger interparticle bonding. As the mean net stress increases, particle displacements become slightly larger and contact stability is primarily governed by vertical contact forces rather than suction effects. These micromechanical insights are consistent with the experimental observations, thereby establishing a clear link between particle-scale interactions and macroscopic mechanical responses.

The span of pile foundations beneath metro depots typically ranges from 10 to 20 m, exhibiting a notably large span. This structural characteristic results in the pile foundations bearing a more concentrated upper load, while the interstitial soil between the piles bears minimal force. Concurrently, global climate change and enhanced urban greening initiatives have led to a significant increase in rainfall in northwest China, a region traditionally characterized by arid and semi-arid conditions. This climatic shift has precipitated a continuous rise in groundwater levels. Furthermore, the extensive distribution of collapsible loess in this region exacerbates the situation, as the rising groundwater levels induce loess collapse, thereby adversely affecting the mechanical behavior of the pile foundations. In light of these factors, this study utilized the pile foundations of a metro depot in Xi’an as a prototype to conduct static load model tests under conditions of rising groundwater levels. The experimental results reveal that the load–settlement curve of the pile foundations in the absence of groundwater exhibited a steep decline with distinct three-stage characteristics, and the ultimate bearing capacity was determined to be 5 kN. When the groundwater level is situated below the loess stratum, the settlement of both the pile foundations and the foundation soil, as well as the axial force, skin friction, and pile tip force, remains relatively stable. However, when the groundwater level rises to the loess stratum, there is a significant increase in the settlement of the pile foundations and foundation soil. Negative skin friction emerges along the pile shaft, and the bearing type of the pile foundation transitions gradually from a friction pile to an end-bearing pile. The influence range of the pile foundation on the settlement of the foundation soil is approximately three times the pile diameter.

Ultra-high performance concrete (UHPC), as a recent advancement in concrete technology, boasts exceptional durability and mechanical properties. Its widespread application in housing, bridges, and tunnels significantly enhances the overall structure and durability of buildings. This paper summarizes the material composition and working performance (Compressive, tensile, impact and durability properties) of ultra-high performance concrete. In view of its important role in underground engineering field, it has excellent effect in shield tunnel segment reinforcement. Therefore, the reinforcement methods of shield tunnel segment and the reinforcement process of Ultra-high performance concrete are discussed, and the failure modes of Ultra-high performance concrete reinforcement are evaluated. The overall structural characteristics, segment joints, positive and negative bending moments of the segment are analyzed respectively. At present, it is concluded that long-term performance and related features need to be further studied. This study will help alleviate the shortcomings of shield segment reinforcement technology and methods, and further encourage the wide application of Ultra-high performance concrete in tunnel engineering.

The mechanical behavior of structured soils is influenced by both inter-particle bonding and fabric arrangements. Existing constitutive models primarily account for soil structure through fabric arrangements. In this study, we first present experimental investigations on intact loess samples, including isotropic compression (IC), conventional consolidation undrained (CU), and consolidation drained (CD) triaxial tests, which reveal the complex structural properties of the soil. Next, we employ the work done by strain energy to comprehensively account for soil structure, incorporating both inter-particle bonding and fabric arrangements. Subsequently, a new strain work constitutive model for structured soils is presented within the critical state framework. Specifically, a linear decreasing function between strain power and mean effective stress is introduced to capture structural degradation, and a new hardening rule is derived from the relationship between strain work and mean effective stress. Compared to traditional structured soil models, the proposed model offers clear physical meaning, and its parameters are easily obtainable. The model’s simulation results are validated against experimental data, demonstrating its ability to capture key mechanical and deformation characteristics, such as strain softening under CU conditions and strain hardening under CD conditions. Finally, we compare our model with the structured cam clay (SCC) model, and the results show that our model provides a better fit to the experimental data, further confirming its accuracy and effectiveness.

In this study, we selected ancient building timber as the research object. A series of static load tests were conducted to analyze the different performances of timber under tensile and compressive loads. After that, vibration fatigue tests on ancient timber samples were carried out under different upper limit stress ratios. Finally, a dynamic constitutive model of ancient timber was established based on the Ramberg–Osgood model. The static load test results show that the tensile strength was approximately 80% of the compressive strength. Meanwhile, the samples that failed under compressive pressure had obvious residual strength, and their failure strains were also much larger than those under tensile stress. In the vibration fatigue tests, the stress–strain curves were analyzed and the results showed that the curves displayed a trend moving to sparse from dense during the loading process. Meanwhile, the curves moved right with the increase in the upper limit stress ratios. The relationship between axial strain and the number of cycles appeared to be characterized by a three-stage form, i.e., damage occurrence, damage expansion, and damage penetration, and this relationship was formulated by a nonlinear function model. Finally, a dynamic constitutive model with high accuracy in describing the vibration fatigue characteristics of ancient timber was established by converting constant parameters to the variable parameters of the Ramberg–Osgood model.

AbstractAdvancing and receding water contact angles, often denoted as the maximum and minimum apparent water contact angles, are crucial parameters reflecting a soil's water holding capacity. These parameters play an important role in establishing theoretical soil–water characteristic curves (SWCCs) for unsaturated soils. However, pre‐assuming constant advancing and receding contact angles during soil wetting and drying processes may be erroneous due to their close correlations with the water content and void ratio. To address this research gap, systematic laboratory measurements were conducted on a loess with different void ratios and water contents. Apparent water contact angles were acquired using an axisymmetric drop shape analyzer, enabling a comprehensive dataset. Analysis of variance was employed to assess the statistically significant differences between void ratios and water contents. The results reveal a significant increase in the observed water contact angle as the void ratio decreases and a decrease with increasing water content. Although both the void ratio and water content influence the water contact angle, the latter has a more pronounced effect. The relationship between the receding water contact angle and water content/void ratio is observed to be linear. The identification of this linear relationship offers insights into the fitting of the SWCC for loess across varying void ratios. This study serves to enhance theoretical methodologies, particularly in the adaptation of contact angles, thus facilitating the development of more precise SWCC models.

Metro transit construction has begun to develop rapidly in northwest China because of the acceleration of urbanization. Accordingly, metro depots are also regarded as an essential auxiliary facility for stopping, operation, and maintenance of trains. Meanwhile, many commercial buildings are constructed over metro depots to improve the utilization rate of land due to the increasingly scarce urban land resources, known as transit-oriented development (TOD). These buildings have a large covered area and transfer concentrated loads to the bases. Therefore, pile bases under metro depots have the bearing characteristics of undertaking large concentrated loads, while lesser loads are placed on the soil between the adjacent pile bases. Additionally, the main ground in northwest China is collapsible loess, so the collapsibility should also be considered. Based on the above background, this research performed static loading tests with and without immersion in a reduced scale of adjacent pile bases under a metro depot in Xi’an. The remolding process of natural loess could destroy its structure and the anisotropy of natural loess could also affect the test results. Therefore, four kinds of artificial collapsible loess with different mass ratios of barite powder, kaolin, river sand, cement, industrial salt, and calcium oxide were made by the free-drop method. This method could make the artificial loess simulate the structure of natural loess reasonably. Then, the artificial loess with the most similar properties to intact loess was selected by comparison. Finally, static loading tests with this artificial loess were implemented. The results showed that the ultimate bearing capacity was 4.5 kN. At the same time, the axial force decreased along depth, since the pile shaft friction was positive, and the load sharing ratio of pile tip force increased to 0.58 when the load exceeded 4.5 kN in the situation without immersion; the settlement of pile bases increased significantly after immersion, while the negative shaft friction occurred at the depth of −8 cm~−35 cm, and the load sharing ratio of pile tip force reached 0.92.

The contribution of denitrifying anaerobic methane oxidation (DAMO) as a methane sink across different habitats, especially those affected by anthropogenic activities, remains unclear. Mining and industrial and domestic use of metals/metal-containing compounds can all cause metal contamination in freshwater ecosystems. Precipitation of metal ions often limits their toxicity to local microorganisms, yet microbial activity may also cause the redissolution of various precipitates. In contrast to most other studies that apply soluble metal compounds, this study investigated the responses of enriched DAMO culture to model insoluble copper compounds, malachite and covellite, in simulated sedimentary environments. Copper ≤ 0.22 µm from covellite appeared to cause immediate inhibition in 10 h. Long-term tests (54 days) showed that apparent methane consumption was less impacted by various levels of malachite and covellite than soluble copper. However, the medium-/high-level malachite and covellite caused a 46.6–77.4% decline in denitrification and also induced significant death of the representative DAMO microorganisms. Some enriched species, such as Methylobacter tundripaludum, may have conducted DAMO or they may have oxidized methane aerobically using oxygen released by DAMO bacteria. Quantitative polymerase chain reaction analysis suggests that Candidatus Methanoperedens spp. were less affected by covellite as compared to malachite while Candidatus Methylomirabilis spp. responded similarly to the two compounds. Under the stress induced by copper, DAMO archaea, Planctomycetes spp. or Phenylobacterium spp. synthesized PHA/PHB-like compounds, rendering incomplete methane oxidation. Overall, the findings suggest that while DAMO activity may persist in ecosystems previously exposed to copper pollution, long-term methane abatement capability may be impaired due to a shift of the microbial community or the inhibition of representative DAMO microorganisms.

The non-unique critical state of soils with time-dependent behaviors is a significant issue in geotechnical engineering problems. However, previous bounding surface plasticity models cannot predict accurately the non-unique critical state of soils, because the distance between the compression line and critical state line charged by strain-rate effect is basically neglected. To fill this gap, a generalized spacing ratio of soils is defined in the elasto-viscoplastic framework, and a bounding surface visco-plasticity model is formulated and verified, which can consider the generalized spacing ratio. Specifically, the generalized spacing ratio of soils reflects the distance between the compression line and the critical state line of soils with time-dependent behaviors. Then, the generalized spacing ratio is introduced into an improved anisotropic bounding surface. A new expression of the visco-plastic multiplier is derived by solving the consistency equation of an anisotropic bounding surface. In the expression, a strain rate index is proposed to account for the strain-rate effect on visco-plastic strain increment, and a visco-plastic hardening modulus is derived to predict the visco-plastic response of soils in overconsolidation conditions. The model is then verified through constant strain rate tests and creep tests. Notably, it can capture the non-unique critical states of soils with time-dependent behaviors due to the generalized spacing ratio and the creep rupture of soils due to the visco-plastic multiplier that considers the stress ratio and visco-plastic strain rate.

Inherent anisotropy is often observed in natural clays. This paper presents a hypoplastic constitutive model to describe the inherent anisotropy of normally and overconsolidated clays. A stress transformation technique is used to include the nonlinear failure criterion. A fabric tensor for inherent anisotropy is introduced, and its evolution provides an important link between the loading direction and the fabric orientation. Moreover, an updated internal tensor is coupled with the stress tensor to reflect the consolidation history and fabric evolution. The model is able to reproduce the salient behaviour of normally and overconsolidated clays with different bedding angles under various loading conditions.

The non-unique critical state represents the distance between the critical state line (CSL) and the isotropic consolidation line (ICL) that significantly varies with stress paths and particle size distribution of soils. A structural bounding surface plasticity model with spacing ratio r (SBSP-R model) was implemented using an explicit algorithm. However, the explicit algorithm did not well capture the non-unique critical state of soils with a large spacing ratio r, which prevented the soil mechanics research on non-unique critical state via finite element analysis. To overcome the limitation, the implicit algorithm of the SBSP-R model is formulated, and it mainly includes elastic prediction and plastic correction. The plastic correction is realized using the Newton–Simpson scheme with a controlling equation set related to consistency condition, plastic flow, hardening parameter, structural bounding surface, plastic modulus, and mapping rule. Case studies indicate that the implicit algorithm of the SBSP-R model is right and stable in predicting non-unique critical states. Comparisons between predicted and tested results indicate that the implicit algorithm of the SBSP-R model not only captures the critical state, stress-strain, and stress paths of various soils but also shows higher computational accuracy and efficiency compared with the previous explicit algorithm. These results indicate that the formulated implicit algorithm of the SBSP-R model is an alternative approach to the previous explicit algorithm.

The control parameter of the complex underground space structure with the Track Panel Tunnel and wall columns was studied. An analysis method of the control parameter was established based on the complementary energy principle with mixed variables. The general analytical solution of the structure under the trapezoidal load was obtained. Then, the correctness of the solution was verified by two-dimensional finite element simulation. The three-dimensional global model is built to analyze models with different story heights, the distance of two adjacent wall columns, and the thickness of the earth covering; the consequences prove the assumptions’ rationality and the engineering applicability of the analytical solution. The sensitive region of the control parameter is found through the analytical solution, which is meaningful in determining the reasonable stiffness ratio of column and beam for structural design optimization with cost savings. It can be the reference for complex underground space engineering designs.

Natural loess soils generally possess loose structures and strong water sensitivity, which leads to various geological disasters upon wetting. This study aims to improve the anti-permeability and stability of loess under wetting conditions by using a novel hydrophobic power. We first treated loess specimens with hydrophobic powers at various concentrations, and a series of triaxial permeability tests were performed to study their permeability behaviour. Furthermore, soil-water contact angle tests and scanning electron microscope tests were carried out to investigate the mechanism of the hydrophobic powder reducing the permeability of loess from both micro and macro perspectives.

双剪统一边界面塑性本构模型(BUST模型)能反映低偏应力水平下岩土材料的塑性应变累积行为,这不同于经典双剪统一弹塑性本构模型.为了将BUST模型应用于滑坡或边坡的静、动力响应分析,构建并编译了 BUST模型的有限差分算法,且通过数值模拟分析验证了该算法的可靠性.首先,推导BUST模型在一般应力空间和主应力空间的有限差分计算格式,给出了边界面、加载面、映射变量、塑性乘子、塑性修正、角点奇异性处理等的具体数学形式;其次,编写BUST模型的C++程序,并在FLAC3D软件平台开展一系列数值计算,比较BUST模型与双剪统一弹塑性本构模型结果差异性,预测软岩三轴剪切应力-应变关系,模拟黄土-泥岩边坡坡体内部加速度响应.结果表明所构建的双剪统一边界面塑性本构模型有限差分算法计算结果可靠,可用于分析边坡或滑坡的静、动力响应.

The strength of jointed soft rock changes in a certain geo-stress field around the tunnel. To explore the behavior, specimens with a single joint plane are triaxially compressed under different consolidation times and confining pressures. On the one hand, jointed soft rock shows strength healing. The residual strength, peak strength, and secant modulus of jointed soft rock increase as isotropic consolidation time increases. On the other hand, jointed soft rock shows special strength decay because of the joint plane, that is strain-softening curve of jointed soft rock is linear. The strength decay occurs for jointed soft rock under normal consolidation condition. To predict the strength decay and healing behavior of jointed soft rock, a modified structural bounding surface plasticity (modified SBSP) model is formulated by introducing an improved expression of elastic modulus for jointed soft rock. The modified SBSP model contains four parameters corresponding to linear strain-softening, compressive hardening, strength, and secant modulus increasing induced by consolidation. The modified SBSP model well predicts the strength decay and healing of jointed soft rock by comparing the results from modified SBSP model, SBSP model, and experiment. Especially, the modified SBSP model overcomes the limitation that compressive hardening and elastic modulus of jointed soft rock cannot be captured by bounding surface plasticity model like SBSP model. The results are important for reasonable supporting time and optimum supporting strategy.

岩土体结构的破坏是由于施加能量超过变形能阈值所造成,基于能量的结构性参数更有利于反映结构性的本质特征.对不同含水状态,不同干密度的延安桃花山原状与压实重塑黄土进行了侧限压缩试验,发现孔隙比e与竖向压力P在ln(1+e)-lgP双对数坐标系内有良好的分段线性关系.在此基础上,基于应变能密度理论,推导提出了侧限压缩条件下的结构性参数映射能,并将该参数推广至复杂加载条件.进一步通过多个区域内,原状与重塑,不同含水率,不同干密度,不同埋深深度,不同粒度土体的侧限压缩试验结果,以及不同含水率土体的等向压缩试验结果,对提出结构性参数的合理性进行了验证.结果表明提出的结构性参数映射能物理意义明确,且能准确地定量表征不同状态,不同试验条件下土体的结构性.

Natural soils usually exhibit microstructure effects that significantly influence their engineering properties; however, existing hypoplastic constitutive models rarely consider structural evolution during loading. This paper presents a hypoplastic constitutive model for structured soils. A structural factor that combines the initial structure degree and the overconsolidation ratio is introduced to describe the structure effects induced by the soil deposition and stress histories, respectively. A structure degradation law is adopted to capture the destructuring process during loading. Thus, this model can describe the mechanical behaviours of both structured and reconstituted soils with normal consolidation and over-consolidation at various stress levels. The model is further verified by comparing numerical predictions with experimental data. The comparison suggests that the proposed model can reproduce the salient behaviours of structured soils under various testing conditions.

The soil-water characteristic curve (SWCC) describes the relationship between water content (or saturation) and matric suction (or soil-water potential) in unsaturated soils. It is worth noting that the amount of water stored in soils during the drying process caused by evaporation or gravity drainage is more than that during the wetting process caused by infiltration and capillary rise under the same matric suction level. This phenomenon is described by the hysteretic behavior of the SWCCs. Based on existing knowledge, the contact angle and the ink-bottle effect caused by the grain-size distribution (or pore-size distribution) are the two main factors that affect the hysteretic behaviors of the SWCCs. First, the behaviors of a liquid drop sitting on an inclined surface are investigated by the numerical method, and a relationship between the contact angle and the water content during the wetting and drying processes is proposed based on the principle of the minimum potential energy. Second, the expression for the volume of the liquid bridge is modified to make the Young-Laplace equation keep valid in calculating the matric suction in the boundary affected zone, transition zone and the part of the residual zone on the w-lns plane. Finally, an analytical model for analyzing the hysteretic behaviors of SWCCs is proposed after introducing the Roshin-Rammler grain-size distribution function. The model has a clear physical meaning, and the parameters can be obtained easily. Moreover, the model can describe the hysteretic behaviors of the SWCCs. Taking the SWCC of recompacted loess in a foundation pit in Qujiang, Xi'an as an example, the model is verified. The results show that the proposed model can well reflect the nonlinear relationship between the matric suction and the water content. The model can also be used for studying the shear strength and the constitutive model for unsaturated soils.

工科专业课程思政和专业教学容易出现"两张皮"特征,两者的有机融合方式和效果被广泛关注.线上-线下混合式教学是一种以深度学习为目的的新学习范式,与课程思政育人理念同向同行.分析了混合教学模式下的工科课程思政教育的可行性和效果,并以《土力学》课程为例,探讨了其课程思政育人总目标,探索了基于线上-线下混合教学模式的课程思政教学,初步构建了混合教学下《土力学》课程思政教学体系,有效提高了《土力学》课堂教学的思政育人效果.