Unsaturated soils exhibit significant time-dependent characteristics under long-term loading and environmental changes.Accurate prediction of their rate-dependent behavior and time effects is crucial for deformation control and long-term stability analysis of engineering structures.Based on the isotach approach for unsaturated soils,a coupled relationship among yield stress,matric suction,and viscoplastic strain rate is established for unsaturated time-dependent soils.To address the discontinuity of viscoplastic rate-dependent parameters during the transition between saturated and unsaturated states,an improved parameter evolution law is proposed.Using bounding surface plasticity theory as the framework and extending the non-stationary flow rule to unsaturated conditions,a constitutive model applicable to triaxial stress states is developed.In this model,the evolution of bounding surface size is governed by viscoplastic volumetric strain,matric suction,and viscoplastic volumetric strain rate.This model effectively captures the coupled effects of viscoplastic strain rate and matric suction on the time-dependent behavior of unsaturated soils.The rationality and effectiveness of the model are verified by comparisons with experimental data from isotropic compression tests,triaxial shear tests,and creep tests on various time-dependent soils under different matric suction levels and axial strain rates.Validation results demonstrate that the model accurately predicts the rate-dependent behavior and creep characteristics of unsaturated soils under specified suction levels.

We conducted a one-dimensional vertical infiltration test on a loess soil column considering the effects of vibration to examine how combinations of vibration frequency and amplitude affect the temporal variation of volumetric water content and the evolution of the wetting front.Scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP)tests were used to reveal microscopic evolution.Using the experimental data,we determine the unsaturated permeability coefficient of loess with three methods—the VG-M model,the wetting-front advancement method,and the instantaneous-profile method—and perform a comparative analysis.The results indicate that vibration accelerates the water infiltration rate.Under vibration,the depth of the wetting front increases with higher vibration frequencies,and the time required to transition from unsaturated to stable infiltration decreases as the vibration frequency increases.All three methods effectively capture the enhancement of the permeability coefficient due to vibration.The results from the instantaneous profile method and the VG-M model are relatively close,while the unsaturated permeability coefficient calculated using the wetting front advancement method is slightly lower.In the initial stage of the loess soil column's microstructure evolution,medium pores dominate the soil's internal structure.During the intermediate stage,under the coupled effects of vibration and seepage,the proportion of large pores increases,forming seepage channels.In the later stage,the soil's microstructure stabilizes,with a significant reduction in large pores and an increase in small and micro pores.

The shear deformation characteristics of the pile-soil interface is significantly influenced by the water content due to the structural strength and water-sensitive nature of loess,leading to strain-softening behavior during shear deformation.Effective saturation and Bishop's effective stress were employed as direct driving variables to reflect the effects of saturation on the structural strength of loess,based on the water-stress coupling characteristics of the pile-loess interface.Structural parameters such as cohesion,friction angle,and compression index,along with their evolution equations,are developed to reflect the degradation of structural strength with plastic strain and effective saturation.On the basis,by equating the plastic deformation of unsaturated structural loess with saturated non-structural loess under lateral confinement,a load-collapse function is developed for the pile-loess interface in the effective stress-effective degree of saturation space.An elastoplastic hydro-mechanical coupling model for the pile-loess interface is developed by integrating a soil-water characteristic curve.The model is validated using direct shear test data from unsaturated structural Lanzhou loess and field pile test data from Shanxi unsaturated loess.The results show that the proposed model effectively represents the hydro-mechanical coupling behavior of the pile-unsaturated loess interface,reflects the effects of saturation on shear strength,and captures the variation of strain-softening characteristics at the pile-soil interface with saturation.The model offers an effective approach for disaster prevention design,analysis,and assessment of the load-carrying behavior of piles in unsaturated loess.

Undrained torsional shear tests were carried out on remolded loess specimens using a hollow cylinder apparatus to investigate the deformation behavior and noncoaxiality (where the direction of the plastic strain increment is not coaxial with the direction of the principal stress) of remodeled loess when the magnitude and direction of the principal stress change simultaneously. It can be found that the deformation behavior of tested samples is significantly influenced by the intermediate principal stress coefficient and the rotation range of the principal stress. The influences of elastic strain, rotation range of principal stress, intermediate principal stress coefficient, and cycle period on the noncoaxiality were also studied. Analysis of the test results demonstrate obvious noncoaxiality of the remolded loess. The noncoaxiality shows segmentation characteristics. The noncoaxiality will be overestimated if the elastic strain is considered and is negative when the principal stress rotates in the reverse direction. Reversal of the principal stress leads to abrupt changes in noncoaxiality. However, the noncoaxiality is similar in the process of forward rotation and reverse rotation of the principal stress. With the same cycle period, the noncoaxiality angle decreases with an increase in intermediate principal stress coefficient, but the effect of intermediate principal stress coefficient decreases with an increase in the cycle period. Increases in the cycle period increase the noncoaxiality of the remolded loess, while the influence of the rotation range of the principal stress on the noncoaxiality is not significant.

在后疫情时代,网络教学达到前所未有的新高度,"互联网+教育"也成为当前教育信息化新阶段的一个特征.近年来,由于新型冠状病毒感染疫情影响,导致部分国际留学生面临返校难、复课难等难题.因此,针对上述难题及传统基础工程课程教学过程中的上课时间少、内容多、理论多于实践、教学方法和手段单一等问题,该文围绕国际学生专业基础课程基础工程的教学理念、教学内容、教学模式及评价指标构建基于OBE理念的国际学生课程体系,这种教学体系在一定程度上转换传统教学的教学理念,改变传统教学模式的不足,提高后疫情时代环境下国际学生的教学效果,并可为未来国际留学生教育模式提供新思路.

The special engineering characteristics of loess will inevitably bring hidden trouble to the construction of shallow buried subway tunnel. The purpose of this paper is to investigate the variation law of surrounding rock stress, lining stress, and surface settlement in loess tunnel construction and further explore the optimum construction method in the soft loess area. A model test considering the different excavation methods and prereinforcement measures was conducted to measure the surrounding rock pressure, lining stress, and surface settlement under different working conditions. The results show that the sequence of tunnel excavation had a great influence on the stability of the tunnel. As the experimental steps increased, the surrounding rock stress and lining stress at each monitoring section gradually increase. Especially, stress concentration could easily occur at the arch foot of the tunnel. The pre-reinforcement measure, especially the WSS grouting, was more crucial than the excavation method. Considering the number of elements, the combination of DSDM excavation method and WSS grouting reinforcement method was proposed as the final construction method of the tunnel in soft loess area in this paper. The research results can effectively guide the design and construction of a shallow tunnel in the soft loess area.

To investigate the influence of temperature and humidity on the mechanical behavior of earth materials, a triaxial apparatus, which can accurately control the temperature and relative humidity of earth materials, was independently developed. Based on the conventional triaxial pressure chamber, a deformation measuring system was set up with the assistance of eddy-current type noncontact deformation sensor, thereby fulfilling the capture of continuous strain variation of earth specimen under the change of environmental condition. A series of cyclic loading-unloading triaxial tests was carried out on the earth material located in Lyon, France, while considering the effects of humidity, confining pressure and temperature. The failure strength, stress-strain curves and the volumetric change were measured through the aforementioned tests, and the Young's modulus and residual strain under different stress levels were also studied by means of hysteresis loop occurred in the experimental results. Additionally, a vapor migration test of earth material under relative humidity action was also conducted. The results show that the new developed instrument can well control the environmental condition variation and simulate the real working state of earth specimen. The mechanical and environmental indexes are accurately measured, which preliminarily verifies the reliability and accuracy of this apparatus. The relevant research results can provide technical support for understanding the thermal-hydro-mechanical coupling deformation mechanism and constitutive modelling of unsaturated earth materials.

公路隧道在建设过程中易受到地理环境等因素的影响,山体结构的不稳定可能会产生潜在的安全隐患,而隧道沉降量是反应隧道结构变化的一项重要指标,因此提出一种基于贝叶斯优化XGBoost的隧道沉降监测量预测模型.由于隧道施工场景复杂干扰严重,给数据采集和后期沉降变化分析带来困难,本文首先对原始沉降监测数据进行时间尺度统一,然后融合时域和空域信息对数据中的异常值、缺失值进行数据修复,在此基础上,提出贝叶斯优化的XGBoost集成模型对隧道监测的周边收敛、地表沉降和拱顶沉降数据分别进行分析.通过与优化前模型以及时序预测模型预测结果进行对比,发现贝叶斯优化的XGBoost模型精度最高,对拱顶沉降、地表沉降、周边收敛的平均预测精度可以达到0.979 4.该模型能够对隧道沉降变化过程进行有效的监测与预测,对于隧道安全问题的监管具有重要的实际应用价值.

针对隧道施工过程中沉降量精准预测问题,提出了一种基于时空特征区域神经网络的施工隧道沉降量预测方法.依据当前隧道地表下沉量,通过有效融合多维空间特征量,对未来的演化趋势做出合理预测.以白家庄隧道栾川端的地表观测数据为例,对所提方法的预测性能进行算例分析.结果表明:所提预测方法对隧道地表沉降量数据均有较准确的预测效果,且预测结果也具有一定的鲁棒性.研究可应用于实际隧道施工的监测管理过程.

隧道建设是公路建设中一类重要的工程,沉降是隧道质量监测的一项重要指标.为预防沉降带来的安全隐患,确保后期隧道正常运行,在建成初期对隧道的沉降累积量进行实时监测并对后期沉降作出较为准确的预测是很有意义的.采用随机森林模型对隧道建成初期的隧道累计沉降量进行预测,并将该模型的预测结果与深度神经网络模型做对比分析.结果表明,随机森林模型对沉降累积量的预测精度更高,能够对隧道建成初期的累计沉降量进行有效的预测,为隧道的安全监测提供数据支持.

The safety of partially exposed concrete structures is threatened by the complicated corrosive sources in sulfate-rich saline areas. Experimental tests were conducted in the present study, taking internal sulfate, external sulfate-magnesium multiple combined attack into consideration, to simulate the corrosion process of partially exposed concrete. The diameter, weight and compressive strength were measured during the immersion period. Sulfate concentration of concrete samples was measured by chemical method. Microstructural and mineral properties were analyzed by SEM, XRD and TG tests after immersion. Results show that internal sulfate attack speeds up corrosion process to a large extent, bring great expansion and damage. Partially exposed condition further accelerates the degradation and failure process of concrete. Magnesium in the environments retards the degradation caused by both internal and external sulfate attack, enhancing the resistance ability of cast-in-situ concrete against cracking. Also, the diffusion of sulfates is slowed by magnesium induced corrosion products since the diffusion paths are blocked. Partially exposed working condition and internal sulfate attack greatly accelerate the degradation induced by sulfate attack. Magnesium in the environments could retard the degradation process and enhance the resistance ability of cast-in-situ concrete against sulfate attack. (c) 2021 Elsevier Ltd. All rights reserved.

The variation of relative humidity induces the migration of vapor, which has an important effect on the strength and deformation of earth. The sorption-desorption and migration of vapor inside the earth are of great significance for revealing the disease breeding mechanism of earth structure. Aiming at two different earth materials named STR and CRA chosen from a few existing construction sites located in Lyon region in the southeast of France, the sorption-desorption tests were conducted through two different methods, saturated salt solution method and dynamic vapor sorption (DVS) separately, and the experimental results were compared accordingly. In addition, an innovative apparatus with high-precision strain measuring system was invented and used for monitoring the vapor migration, the variation of relative humidity and gas pressure with time at the top and the bottom of earth specimens was explored, and the deformation characteristics of earth specimens during vapor migration were also analyzed. The experimental results indicate that the sorption curve obtained through DVS is in accordance with that from saturated salt solution method. The sorption curve can be divided into three parts: monolayer adsorption, multilayer adsorption and capillary condensation. Meanwhile, the retention characteristics of earth increases with the rise of clay activity. The retention characteristics have important effect on the relative humidity at the outlet and relative gas pressure at the bottom of specimens. The dilatancy phenomenon occurs during the vapor migration process.

In this study, a new approach based on DEM was developed to simulate the damage of water-rich rock after freeze–thaw cycles. In this way, water-rich rock samples at low temperatures were simplified as rock particles, ice particles, rock–rock contacts, rock–ice contacts, and ice–ice contacts. The volume of the ice particles changed as the temperature changed. The change characteristics were determined by the relationship between the temperature and the unfrozen water content. The developed approach was proven to be effective by comparing the simulation results with the laboratory test results. The physico-mechanical behaviors of water-rich rock samples after freeze–thaw cycles were studied. The results showed that the volume and porosity significantly increased after the freeze–thaw cycles, especially after 15 freeze–thaw cycles, and the increase in the radius was significantly larger than the increase in height. The uniaxial compressive strength, elastic modulus, and peak strain had an exponential reduction as the number of cycles increased. In uniaxial compression tests, the tensile failure rate of the sample after freeze–thaw cycles increased compared with that of the sample without freeze–thaw cycle treatment. With the increase in the number of freeze–thaw cycles, the distribution of cracks in the rock sample was more homogeneous. However, overall, the cracks that formed due to the freeze–thaw cycles were more distributed near the surface of the sample.

Pile composite foundation can make good use of the bearing capacity of the soil and pile, which is widely used in the Chinese northwest loess area. However, the theory of pile composite foundation is far from sufficient, hindering its long-term development. Aiming at this problem, a laboratory model test of pile composite foundation in the loess area was conducted to explore the common working mechanism and variations of each bearing stage. Besides, the settlement of the single pile composite foundation was calculated by using the modified tangent modulus method, and the result was compared with the experimental data. The main results of this paper are as follows: Both in the single pile and single pile composite foundation, loading-settlement curves showed a trend of "elastic to elastoplastic to plastic," accompanied by the appearance of plummeting point. Influenced by the pile group effect, the loading-settlement curve of the group pile composite foundation showed a slow-varying trend without an obvious breakdown point. Pile axis stress increased with the growth of upper load. At the beginning of loading, the pile axis stress indicated such a distribution that stress on both ends of the pile was larger than that in the middle of the pile. When reaching a certain load, the location of the biggest pile axis stress transferred to the pile top, and the pile axis stress decreased gradually as the pile became deep. The side friction resistance in the static load test of the single pile was always positive, whereas in the composite foundation of a single pile and a group of piles, negative side resistance appeared in the upper side of the neutral point. Pile-soil stress ratio in the depth of 12 cm changed with the upper load. The outcome calculated by the modified tangent modulus method had a relatively better consistency with experimental data if the upper load was not too large.

Excavation and earth surface processes (e.g., river incision) always induce the unloading of stress, which can cause the failure of rocks. To study the shear mechanical behavior of a rock sample under unloading normal stress conditions, a new stress path for direct shear tests was proposed to model the unloading of stress caused by excavation and other processes. The effects of the initial stresses (i.e., the normal stress and shear stress before unloading) on the shear behavior and energy conversion were investigated using laboratory tests and numerical simulations. The shear strength of a rock under constant stress or under unloading normal stress conforms to the Mohr Coulomb criterion. As the initial normal stress increases, the cohesion decreases linearly and the tangent of the internal friction angle increases linearly. Compared with the results of the tests under constant normal stress, the cohesions of the rock samples under unloading normal stress are smaller and their internal friction angles are larger. A strength envelope surface can be used to describe the relationship between the initial stresses and the failure normal stress. Shear dilatancy can decrease the total energy of the direct shear test under constant normal stress or unloading normal stress, particularly when the stress levels (the initial stresses in the test under unloading normal stress or the normal stress in the test under constant normal stress) are high. The ratio of the dissipated energy to the total energy at the moment failure occurs decreases exponentially with increasing initial stresses. The direct shear test under constant normal stress can be considered to be a special case of a direct shear test under unloading normal stress with an unloading amount of zero.

In order to have a better understanding of the real contact area of granular materials, the white light interference method is applied to explore the real surface morphology of clay soils under high stress. Analysis of the surface profile indicates that there exists a support point height z0 with the highest distribution frequency. A concept of a real contact region (from z0 to z0 + d90; d90 represents the particle size corresponding to 90% of the volume fraction) is proposed by combining a surface profile with the particle size distribution of clay soil. It was found that under the compressive stress of 106 MPa–529 MPa, the actual contact area ratio of clay soil varies between 0.375 and 0.431. This demonstrates an increasing trend with the rise of stress. On the contrary, the apparent porosity decreases with an increasing stress, varying between 0.554 and 0.525. In addition, as the compressive stress increases, the cumulative frequency of apparent profile height (from z0 − d90 to z0 + d90) has a concentrated tendency with a limited value of 0.9.

在长期自然因素和人为因素的作用下,黄土中含盐量发生变化,使得土的力学性质发生改变.文中通过自主研发一种水平方向的去易溶盐淋洗装置,开展黄土室内淋溶试验,结果表明该装置能有效去除土体试样中的易溶盐成分;同时提出一种能测量黄土试样淋溶变形的试验方案.

The hydromechanical behaviour of compacted earth samples was experimentally analysed with a triaxial apparatus at controlled hygrometry and temperature. The results show that the relative humidity at which the samples were stored have a strong impact on the mechanical characteristics of earthen material: both the maximum deviator stress and Young's modulus decrease with the increase of relative humidity, meanwhile, more plastic characteristics are observed. Non-negligible swelling/shrinkage phenomena induced by variations of relative humidity are also observed. These results are eventually analysed in the light of a fully coupled hygrothermal–hydromechanical model.

This paper presents a new constitutive model on the poroplastic behaviour of earthen materials accounting for stiffness degradation, using the approach of continuum damage mechanics. The poroplastic behaviour is modelled based on the bounding surface plasticity (BSP) theory and the concept of effective stress while isotropic damage is modelled using a scalar variable. Plastic flow and damage evolution occur simultaneously in a coupled process which take into account the impact of suction. The model was successfully validated against results of triaxial compression tests performed at different relative humidities and confining pressures. Despite the relatively small number of material parameters, this model can reproduce the essential features of earthen materials behaviour observed experimentally: suction-induced hardening and stiffening, post-peak softening, as well as the progressive transition from contractant to dilatant volumetric behaviour. Use of the BSP theory allows to reproduce a smooth stress–strain relation as experimentally observed, instead of an abrupt change upon plastic yielding predicted by classic elastoplastic models. Furthermore, the present model also furnishes a quantitative description on the degradation of elastic properties hitherto not accounted for, thanks to the additional scalar damage variable.

This paper investigates the impact of clay and moisture contents on the shear behavior of compacted earth taking into account loading-unloading cycles. Fine sand was added to a natural soil, thereby obtaining three different soils with clay contents of 35%, 26%, and 17%, respectively. A series of triaxial tests was conducted on samples previously equilibrated at three different values of relative humidity (RH). The evolution of failure strength fc, Young's modulus E, and residual strain εres was investigated according to the clay content and the RH, the last two parameters being measured during the loading-unloading cycles. Firstly, the relative humidity at which the samples were fabricated and conditioned was seen to have a strong impact on the mechanical characteristics of the earthen material. An increase in RH led to a decrease in both failure strength fc and Young’s modulus E, and an increase in plastic strain. The tendencies were found to depend on the clay content of the samples. Secondly, with an increasing stress level, a progressive decrease in Young’s modulus and an increase in residual strain εres (after a loading-unloading cycle) appeared. Thirdly, within the range of the investigated clay contents, both failure strength fc and residual strain εres increased with an increasing clay content at constant values of RH and confining pressure, the rate of this increase being a function of the RH. Young’s modulus E was relatively insensitive to changes in the clay content, its variation being less than 20% for all cases. Finally, based on a particular definition of Bishop's effective stress, involving a specific functional form χ(s), the failure states of all the samples were observed to lie approximately on a unique failure line crossing the origin in the (p′-q) plane regardless of the matric suction and confining pressure.