Mechanical properties of artificially structured soil and Binary-medium-based constitutive model under undrained conditions.

To investigate the mechanical properties and constitutive models of structured soil under undrained conditions, triaxial compression tests on initially anisotropic structured soil, isotropic structured soil, and remolded soil were conducted under consolidation undrained conditions at confining pressures of 25, 50, 100, and 200 kPa, respectively. The results demonstrate that the samples of structured soils with strong structural characteristics have an obvious yield strength when the consolidation stress is low. At this time, the pore water pressure in structured soils increases at the beginning of loading. As the axial strain increasing, it turns to reduce. When failure, the samples have obvious shear band. With the consolidation stress increases, the mechanical properties and deformation mechanism of structured soils are near to the remolded soil. Combining the Binary-medium theory with the analysis and discussion of the mechanical properties and deformation mechanisms of structured soil, the rationality of the corresponding Binary-medium model was verified, which shows that the constitutive model can reflect the characteristics of dilatancy and strain softening, volumetric contraction and strain hardening under the conditions of low and high confining pressure respectively. At the same time, the constitutive model can also reflect the differences in the stress-strain characteristics of the two structural soils caused by the structural differences. In general, the results agree with the experiment relative well.

Mechanical properties of rooted soil under freeze-thaw cycles and extended binary medium constitutive model.

In seasonally frozen soil, soil sometimes is affected by freeze-thaw cycles and root systems. In order to study its mechanical characteristics, a series of consolidation drained triaxial tests under different confining pressures (25, 50, 100, 200 kPa), different freeze-thaw cycles (N = 0, 1, 5, 15) and different root-containing conditions (r = 0, 1, 3) were carried out. The test results show that the specimens exhibit strain softening behavior and volumetric dilatancy phenomena and shear failure under lower confining pressure, and strain hardening and volumetric contraction, bulging failure under higher confining pressure. With the increase of freeze-thaw cycles, the bearing capacity of the sample decreases and the volume strain increases. With the increase of volume ration of roots in the sample, the bearing capacity increases and the volume strain decreases. Based on the binary medium model, the soil is abstracted into bonded elements and frictional elements. At the same time, the bonded elements are transformed into frictional element when the bonded elements are broken during the loading process. Also, the root is abstracted into another non-destructive bonded elements material, which bears the load together. The linear elastic constitutive model is used for root and bonded elements, and the double-hardening model is used for friction elements. Considering the influence of freeze-thaw cycles, the extended binary model is derived here. Finally, the experimental results show that the predicted results of this model are in good agreement with the experimental results, and the new model can relatively well simulate the strain softening and volumetric dilatancy phenomena.

Experimental study on moisture and heat migration and deformation properties of unsaturated soil column under a temperature gradient during rainfall infiltration.

Mountainous areas in southwest China are rainy in summer. The rainfall infiltration process involves complex soil thermal-hydraulic-mechanical (THM) coupling problems. The researches on soil THM coupling are mostly focused on numerical simulations, whereas the corresponding model tests are relatively few, and the existing model test studies often ignore the effect of temperature gradients in the soil. However, temperature gradients in the soil can cause water migration and affect the THM behavior of soil, so it cannot be ignored. This paper describes an experimental device that can test the changes of temperature, moisture and displacement in unsaturated soil columns with temperature gradients under rainfall infiltration conditions. By using the apparatus, the model tests of homogeneous soil column (H), homogeneous soil column with infiltration (HI), and preferential flow soil column with infiltration (P) under different temperature gradients are respectively conducted, and the results of moisture and heat migration and deformation properties in soils under different conditions are presented and discussed. A rainfall of low intensity and long duration is applied in the experiments, and the temperature of infiltration rainwater is consistent with that of the soil upper boundary. The results show that: (1) The infiltration of rainfall will increase the temperature of the soil column. The appearance of preferential flow results in faster heat transfer within the soil column, but causes the steady-state temperature to be lower than that of the homogeneous soil (HI); (2) Under infiltration conditions, the preferential flow soil column has an earlier outflow time but a later time for water field to reach steady state, while its water distribution is different from that of the homogeneous soils, with accumulation occurring near the end of preferential flow channel; (3) Under the action of temperature gradient, water migration occurs in homogeneous soil column (H), accompanied by soil settlement, while the infiltrated columns (HI and P) exhibit an increase in both water content and top displacement. In addition, the larger the temperature gradient, the more obvious the thermally induced hydraulic-mechanical response. The research results in this paper can provide experimental evidence for the theoretical study and numerical simulation of the soil THM coupling problems.

Influence of inorganic and organic salts on the hydration mechanism of montmorillonite based on molecular simulation.

The molecular dynamics method is used to further reveal, from the molecular point of view, the mechanisms of salt inhibiting the hydration of Na-MMT. The interaction between water molecules, salt molecules, and montmorillonite are calculated by establishing the adsorption models. According to the simulation results, the adsorption conformation, interlayer concentration distribution, self-diffusion coefficient, ion hydration parameters, and other data are compared and analyzed. The simulation results show that the volume and basal spacing increase in a stepwise manner with the increase of water content, and water molecules have different hydration mechanisms. The addition of salt will enhance the hydration properties of compensating cations of montmorillonite and affect the mobility of particles. The addition of inorganic salts mainly reduces the adsorption tightness between water molecules and crystal surfaces, thereby reducing the thickness of water molecules layer, while the organic salts can better inhibit migration by controlling interlayer water molecules. The results of molecular dynamics simulations reveal the microscopic distribution of particles and the influence mechanism when the swelling properties of montmorillonite are modified by chemical reagents.

Unified transient creep constitutive model based on the crack evolution of micritic bioclastic limestone.

The surrounding rock at the exit of the No. 1 drainage tunnel of the Artashi Water Conservancy Project is micritic bioclastic limestone with 55% bioclastic material. This rock underwent unpredictable large and time-dependent deformation during excavation. To date, the mechanical behaviour of this kind of rock has rarely been studied. In this study, traditional triaxial compression tests and multilevel creep tests were conducted on micritic bioclastic limestone, and the results clarified the instantaneous and time-dependent mechanical properties of the rock. Considering that the essence of rock failure is crack growth, the crack strain evolution properties were revealed in rock triaxial compression tests and multilevel creep tests. Based on triaxial compression tests, the evolution of axial cracks with increasing deviatoric stress ratio Rd (ratio of deviatoric stress to peak deviatoric stress) was observed, and an axial crack closure element and new crack growth element were proposed. To simulate the creep behaviour of a rock specimen, the relationship of the rock creep crack strain rate with Rd was studied. A creep crack element was created, and the creep crack strain evolution equation was obtained, which closely fit the experimental data. Combining the 4 element types (elastic element, crack closure element, crack growth element, and creep crack element), a unified transient creep constitutive model (Mo's model) was proposed, which represented both the transient and time-dependent mechanical properties of the micritic bioclastic limestone.

Research on Mechanical Behavior of the Steel-Concrete-Steel Composite Structures Subjected to High Temperature of Fire.

A new type of steel-concrete-steel composite structure was adopted and widely used in the immersed tunnel of the Shenzhen-Zhongshan access. The research on the mechanical behavior of the new composite structure under a high temperature of fire is of great engineering significance to the fire protection design of the structure. Both the model test and a numerical simulation were adopted to study the mechanical behavior and damage characteristics of the new composite structure under fire. The RABT standard temperature rise curve was used to simulate the temperature rising law under fire (it reflects the characteristics of temperature rise in case of fire in an enclosed environment: rapidly raised to 1200 °C within 5 min, maintained at 1200 °C for 120 min, then it is cooled to normal temperature after 110 min). The temperature distribution law inside the structure, the deformation development law of the roof and the crack distribution were analyzed based on the thermal-mechanical coupling analysis method. The results showed that the internal part of the composite structure close to the fire surface was directly affected by the high temperature, and the temperature presented a step distribution law, while the part far from the fire surface was affected by the lag effect of the temperature transfer, and the temperature presented a continuous growth law. The roof deformation presented a three-stage model of "rapid growth-deformation stability-deformation recovery" with time. The overall cracks of the composite structure showed a symmetrical distribution under fire. The composite structure presented a shear failure mode as a whole. The results could provide a reference for the study of fire resistance for the new composite structure and support the structural fire protection design of the immersed tunnel of the Shenzhen-Zhongshan access.

Mechanical behavior and constitutive model for loess samples under simulated acid rain conditions.

Acid rain is mainly composed of sulfuric acid and nitric acid aqueous solutions, which can deteriorate the mechanical properties of soil and thus threaten the safety of soil engineerings. In this paper, the influence of sulfuric acid rain on mechanical properties of loess soil samples was studied. The diluted sulfuric acid solution has respectively pH 5.0, 4.0 and 3.0 to simulate the acid rain condition, and the triaxial compressional tests and scanning electron microscope were carried out to investigate the deteriorated properties and evolution of the microstructure of the saturated loess samples. The results demonstrated that acid rain made the porosity of loess samples larger, and changed the pore distribution and contacts of soil grains, so that the mechanical properties of loess samples varied in some degree. With the decrease of pH value, both the peak value of the deviatoric stress and volumetric contraction of loess samples decreased, which reduced the parameters of shear strength of loess samples. Furthermore, a framework of the chemical-mechanical model for loess under the action of acid rain was established, in which loess was considered as a porous medium material, and the influences of acid rain with different pH values were taken into account in the double-hardening constitutive model, and the model was also verified by the triaxial test results finally.

Influences of freezing-thawing actions on mechanical properties of soils and stress and deformation of soil slope in cold regions.

Freezing-thawing actions can affect the mechanical features of soil greatly, which is vital for the stability of soil slope in cold regions. Firstly, triaxial compression tests on sand samples under undrained conditions were performed to investigate the influences of freezing-thawing cycles, which shows that the freezing-thawing actions can weaken their strength and stiffness, and with the increasing freezing-thawing cycles, both the deviatoric stress and pore water pressure decrease gradually. Then, the double hardening constitutive model was revised to model the influences of freezing-thawing cycles in consideration of the influences of freezing-thawing actions, and the model was also validated by the test results. Finally, the proposed constitutive model was incorporated into a finite element code to numerically simulate the distribution of displacement and pore water pressure of sand slope subjected to freezing-thawing cycles, which shows that the freezing-thawing actions accelerate the dissipation of the pore water pressure and enlarge the displacement of the slope. The study here can provide a help in designing and construction of civil engineering in cold regions.

A method of calculating the bearing capacity of sand pile composite foundations in a mucky soil layer considering consolidation.

In engineering practice, the measured bearing capacity of a sand pile composite foundation in a mucky soil layer is much larger than the design value. Based on the sand pile construction and the load application process, a method of calculating the bearing capacity of the foundation based on the effective stress was proposed. Cavity diameter expansion in sand pile construction was simplified into a planar problem, and the cavity expansion theory was used to establish the expression of the rate of displacement and the horizontal stress increase. Based on the e-p curve and the calculation of the degree of consolidation, the relationships between the horizontal and vertical effective stress and the void ratio were obtained. According to the close relationship between the bearing capacity of the foundation in a mucky soil layer and the water content, an expression describing the relationships between the bearing capacity of the foundation, effective stress, void ratio, and water content was established. For the temporary engineering foundation treatment project, which needs a high bearing capacity but allows large foundation deformation, the design of sand pile composite foundations uses these relationships to take the consolidation effect of mucky soil into consideration, thereby reducing the replacement rate and lowering the construction cost.

Spatial modeling of gully head erosion on the Loess Plateau using a certainty factor and random forest model.

Gully head erosion significantly contributes to land degradation, and affects gully dynamics on the Loess Plateau of China. Modeling with a gully head erosion susceptibility map (GHEM) is an essential step toward appropriate mitigation measures. This study evaluates the effectiveness of two varieties of advanced data mining techniques-a bivariate statistical model (certainty factor (CF)) and a machine learning model (random forest (RF)) for the accurate mapping of gully head erosion susceptibility taking the Dongzhi Loess Tableland of China as an example at a regional scale. A database comprising 11 geographic and environmental parameters was extracted with 415 spatially distributed gully heads, of which 70% (291) were selected for model training and 30% (124) were used for validation. An accuracy evaluation using the area under the curve (AUC) value revealed that the CF model was 84.1% accurate, while the AUC value of the RF model map was 88.8% accurate. According to the RF method used to assess the relative significance of predictor variables, the most significant factors influencing the spatial distribution of the GHEM were the slope angle, slope aspect, topographic wetness index, and slope length. The GHEM can ultimately aid in decision making with respect to soil planning and management and sustainable development of the study area, which can be applied to other similar loess regions.

Magnetization switching by magnon-mediated spin torque through an antiferromagnetic insulator.

Widespread applications of magnetic devices require an efficient means to manipulate the local magnetization. One mechanism is the electrical spin-transfer torque associated with electron-mediated spin currents; however, this suffers from substantial energy dissipation caused by Joule heating. We experimentally demonstrated an alternative approach based on magnon currents and achieved magnon-torque-induced magnetization switching in Bi2Se3/antiferromagnetic insulator NiO/ferromagnet devices at room temperature. The magnon currents carry spin angular momentum efficiently without involving moving electrons through a 25-nanometer-thick NiO layer. The magnon torque is sufficient to control the magnetization, which is comparable with previously observed electrical spin torque ratios. This research, which is relevant to the energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.

Evaluation of Rice Resistance to Southern Rice Black-Streaked Dwarf Virus and Rice Ragged Stunt Virus through Combined Field Tests, Quantitative Real-Time PCR, and Proteome Analysis.

Diseases caused by southern rice black-streaked dwarf virus (SRBSDV) and rice ragged stunt virus (RRSV) considerably decrease grain yield. Therefore, determining rice cultivars with high resistance to SRBSDV and RRSV is necessary. In this study, rice cultivars with high resistance to SRBSDV and RRSV were evaluated through field trials in Shidian and Mangshi county, Yunnan province, China. SYBR Green I-based quantitative real-time polymerase chain reaction (qRT-PCR) analysis was used to quantitatively detect virus gene expression levels in different rice varieties. The following parameters were applied to evaluate rice resistance: acre yield (A.Y.), incidence of infected plants (I.I.P.), virus load (V.L.), disease index (D.I.), and insect quantity (I.Q.) per 100 clusters. Zhongzheyou1 (Z1) and Liangyou2186 (L2186) were considered the most suitable varieties with integrated higher A.Y., lower I.I.P., V.L., D.I. and I.Q. FEATURES: In order to investigate the mechanism of rice resistance, comparative label-free shotgun liquid chromatography tandem-mass spectrometry (LC-MS/MS) proteomic approaches were applied to comprehensively describe the proteomics of rice varieties' SRBSDV tolerance. Systemic acquired resistance (SAR)-related proteins in Z1 and L2186 may result in the superior resistance of these varieties compared with Fengyouxiangzhan (FYXZ).