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The aim of this study is to investigate the engineering properties and solidification mechanism of loess through the use of alkali-activated coal gangue powder with sodium silicate. Experimental methods and comprehensive analysis were employed to examine the effects of different proportions of alkali-activated coal gangue powder with sodium silicate on the engineering properties of loess, including mass shrinkage, compressibility, and shear strength. Additionally, scanning electron microscopy was utilized to gain in-depth insights into the interaction and solidification mechanism between loess and alkali-activated coal gangue powder. The results show that the sodium silicate alkali-activated gangue powder curing loess has significantly improved the compressive strength and shear strength of the loess. With a ratio of 7 : 2 : 1, the 28 days compressive strength of solidified loess is 1.7 MPa, and the shear strength is 67.92 kPa, which is 1.91 and 2.13 times the 28 days compressive strength and shear strength of unmixed gangue powder and sodium silicate specimens respectively. The hydration–hydrolysis reaction, ion-exchange reaction, and volcanic ash reaction of the gangue powder under an alkaline environment generated hydrides that filled the pores between soil particles, enhanced the interparticle cohesion, and made the internal structure of the specimens denser, improving the engineering performance of loess solidification. The proposed sodium silicate alkali-activated gangue powder curing loess mechanism can provide a theoretical reference for the engineering application of gangue powder and the curing modification of loess. [ABSTRACT FROM AUTHOR]

Loess has unique water sensitivity due to its distinct formation environment. The structure of loess is undercompactness, weak cementation, and porousness. The water sensitivity of loess directly leads to many environmental problems and geological hazards, including subgrade subsidences, slope collapse or failures, and building cracking. To reveal the relationship between water sensitivity and loess structure, confined-compression collapsibility tests and triaxial-collapsibility tests were performed on loess in different areas. The collapsibility coefficient, porosity ratio, and collapsibility rate were analyzed. Results show that the collapsibility process of loess can be divided into three stages: wetting, softening, and settling. The collapsibility sensitivity of loess is determined primarily by its structural and hydraulic state. [ABSTRACT FROM AUTHOR]

Microbially induced carbonate precipitation (MICP) has been utilized as a new method to improve loess soil strength. In this study, we investigated the influence of the main parameters on the shear strength of MICP-treated loess specimens. Initially, culture media with different formulas and pH values were examined to identify the most efficient medium for loess soil. To explore the shear behavior of MICP-treated loess under general stress levels, unconfined compressive strength (UCS) tests and triaxial tests relevant to the compression strength and vertical loads were performed on MICP-treated loess with different calcium sources, cementation concentrations, and curing periods. Subsequently, calcium chloride was selected as the optimal calcium source based on the ultimate strength of the MICP-treated loess. The effective cementation concentration in the loess soil was between 1.0 and 1.25 M. The ultimate strength of the MICP-treated loess was 3.6 times of the untreated loess. The stress-strain curves indicate that a higher cementing effect can be expected with an increase in the curing period. The formation process of calcium carbonate and the micromorphology of the MICP-treated loess samples were examined using scanning electron microscopy. In this study, we present an environmentally friendly technique for improving loess soil strength. [ABSTRACT FROM AUTHOR]

In this study, the loess hill site of an elevated bridge section in Tongwei-Qin'an of the Baolan high-speed railroad was selected as the research object, and the vibration acceleration of the loess hill site under the elevated bridge was tested in the field under the train operating load. The results show that under the same intensity of train load, the time range of vibration acceleration observed by field test and numerical simulation decays linearly with increasing distance from the source, while the amplification effect appears in the loess hill site at a greater distance, and the vibration duration also appears to increase. The vibration acceleration waveforms at each observation point observed by field tests and numerical simulations are similar, and the peak vertical acceleration at each observation point obtained from numerical simulations is overall greater than the peak acceleration at each point obtained from field tests, with a Simulated − max / a Observated − max values ranging from 1.04 to 1.63. The Fourier spectrum frequencies recorded by numerical simulation and field test are mainly concentrated in the range of 1∼40 Hz, but the difference between the main frequencies recorded by the two is large. The main frequency of the energy spectrum recorded by the numerical simulation is around 15 Hz, which is the same as the main frequency of the energy spectrum vibration of the input vibration wave, and the main frequency of the energy spectrum vibration recorded by the field test is around 25 Hz. [ABSTRACT FROM AUTHOR]

Compacted loess is widely used as a construction material in engineering practices. The compaction dry density and compaction water content have significant effects on the hydromechanical behavior of compacted loess, thereby influencing the serviceability and safety of engineering structures. The former was widely studied in previous studies, while the latter is rarely known. In this study, the water retention, compression, and collapse behavior of compacted loess at different compaction water contents are investigated through pressure plate and oedometer tests. Microstructure analysis was carried out for insight analysis of the test results. The air entry value and yield stress of compacted loess decreased by 64% and 50%, respectively, with the compaction water content increasing from 12.4% to 19.0%. This is due to the fact that the number of clods increases with increasing the compaction water content, leading to many large-sized pores (i.e., diameter > 1,000 μm) and weaker soil skeletons. The influence of compaction water content on the water retention and compression behavior of loess is more pronounced at lower compaction dry densities and lower testing water contents, respectively. In addition, the specimen shows smaller collapse indexes at higher compaction water contents, mainly because of the larger deformation induced by the initial compression. [ABSTRACT FROM AUTHOR]

Rainfall is an important factor affecting the stability of Xiashu loess slope, so it is particularly important to understand the infiltration law and induction process of rainfall in Xiashu loess slope. In this paper, a Xiashu loess slope model with a slope ratio of 1 : 1.2 is constructed in the model box. The rainfall infiltration law, vertical stress characteristics, pore-water pressure characteristics, and deformation failure mode of the slope under two different rainfall types and four different rainfall degrees are analyzed. The instability and failure mechanism of Xiashu loess slope with gravel layer under different rainfall conditions is studied. The test results show that the influence of rainfall intensity factors on the gravel soil slope is mainly reflected in the degree of erosion of the slope. With the increase in rainfall frequency and intensity, the rainfall infiltration rate gradually increased, reached the highest value after standing, and then decreased with the increase of rainfall intensity. Rainfall has a great influence on the pore-water pressure in the gravel soil slope, and the pore-water pressure at the monitoring point of the slope toe changes the most. The vertical stress curve will change abruptly in the later stage of intermittent rainfall and during continuous rainfall. [ABSTRACT FROM AUTHOR]

The strength of loess in seasonal frozen soil areas decreases evidently due to the freeze-thaw cycle, which even leads to some engineering and geological problems. Fibre reinforcement is very effective in improving the mechanical properties of soil. Triaxial tests under different water contents, freezing temperatures, and number of freeze-thaw cycles were carried out for fibre-reinforced loess to study the ability of fibre reinforcement to improve the resistance of loess to freeze-thaw damage; scanning electron microscopy (SEM) was also conducted. The effects of freeze and thaw deterioration and reinforcement were discussed by comparing the strength parameters under different test conditions, and the influence mechanism of freezing temperature, number of freeze-thaw cycles, and reinforcement on loess strength was explained. Results show that a lower freezing temperature indicates a more evident decrease in strength after thawing. Under the action of the first five freeze-thaw cycles, the degree of decrease is more prominent. The fibre reinforcement can reduce the fragmentation of aggregates in loess, thereby effectively restraining the deterioration effect of loess during freeze-thaw cycles. Finally, the strength parameter prediction model under different freeze-thaw cycles of reinforced loess is established, thereby providing theoretical support for engineering application and numerical simulation of fibre-reinforced loess. [ABSTRACT FROM AUTHOR]

Understanding the effects of in-situ drying–wetting pattern on the stress-dependent water retention curve of intact loess is vital for addressing geotechnical problems in loess regions. The principal objective of this study is to investigate the influence of in-situ drying–wetting on the stress-dependent water retention behavior of intact loess. To meet this objective, six drying–wetting tests were carried out using a suction- and stress-controlled pressure plate extractor. Intact loess was sampled from three different depths: 1.0, 3.0, and 5.0 m. For specimens from each depth, two vertical net stresses (i.e., 0 and 50 kPa) were applied prior to the drying–wetting cycle. Experimental results revealed that the in-situ drying–wetting pattern greatly affected various aspects of the water retention behavior, particularly the hysteresis. The hysteresis of the specimen from 5.0 m is about 82% and 77% larger than that of the specimens from 1.0 and 3.0 m, respectively. This is because the specimen from 5.0 m has some large-size pores (i.e., >400 μm), which were not found in specimens from 1.0 and 3.0 m. These large-size pores enhance pore nonuniformity and hence the hysteresis. Furthermore, specimens from different depths consistently showed a reduction of hysteresis when the stress was increased from 0 to 50 kPa. The reduction is the most significant for a specimen from 5.0 m due to the collapse of large-size pores under compression. [ABSTRACT FROM AUTHOR]

With the development of ultrahigh voltage transmission line construction in northwest China, an increasing number of overhead transmission lines are crossing the loess regions. Ensuring the stability of electricity transmission towers and mitigating the risk of collapse are crucial prerequisites for the construction of transmission lines. Taking the 750 kV demonstration electricity transmission line in Guanting–Lanzhou East as an example, representative loess samples were selected for compression and collapsing sensitivity tests. Four foundation treatment schemes were designed, and the wetting deformation characteristics were investigated through numerical simulation of the rainfall infiltration process. Corresponding optimisation schemes were proposed. The collapsing process was divided into four stages through laboratory collapsing tests. The relationship function between saturation and the Duncan–Chang model parameters of loess was established, which was introduced into the finite element model to consider the rainfall infiltration process. The analysis of the displacement field, and loess saturation field with time after rainfall infiltration for the four schemes showed that adding a lime-stabilised soil cushion layer and waterproof layer to the inclined column foundation had the best effect. Given that the design scheme needs to consider safety and economy, the thickness of the cushion layer was optimised, thereby reducing the use of lime-stabilised soil by 20%. Research findings provide meaningful guidance for the design of transmission tower foundations in loess areas. [ABSTRACT FROM AUTHOR]

To reveal the influence of water content on the strength characteristics and brittle-plastic failure process of Xiashu loess, the direct shear test and unconfined compressive strength test of Xiashu loess with different water content were carried out, and the influence of water content on its strength characteristics and brittle-plastic failure transformation characteristics was studied. Eight kinds of Xiashu loess with different moisture contents were designed, and a direct shear test and uniaxial compression test were carried out, respectively. The results show that with the increase in water content, the shear strength and unconfined compressive strength of Xiashu loess decrease continuously. The influence of water content on cohesion in the shear strength index is greater than that of the internal friction angle. The relationship curve between cohesion and internal friction angle and water content shows obvious segmentation. When approaching the optimal water content, the downward trend is slowed down. When the water content is constant, the shear strength of the sample will also increase with the increase of normal stress. When the water content is 12% to 15%, the failure mode of Xiashu loess is a brittle failure, and the unconfined compressive strength decreases by 43.23%. When the water content is 15% to 16%, the failure mode of Xiashu loess is a transitional failure, and the unconfined compressive strength decreases by 60.38%. When the water content is greater than 16%, Xiashu loess shows plastic failure, and the unconfined compressive strength decreases slightly. [ABSTRACT FROM AUTHOR]

To study the mechanisms of strong and weak combined water on the mechanical properties of in situ loess deformation and strength, triaxial compression tests with different combined water contents were carried out to in situ loess in a region of Shaanxi Province, China. The influence on the mechanical properties and structural variation of in situ loess was discussed from the perspective of combined water. The endogenous factors of deformation and strength properties of in situ loess in triaxial tests were investigated from a fine-scale perspective by combining numerical simulations of discrete elements. The results show that the failure specimen under the condition of strong combined water has an obvious shear band and shows brittle failure. The failure mode of soil samples under weak combined water is characterized by no obvious cracks and bulging, which is plastic failure. The water content at the boundary of strong and weak combined water is the boundary to distinguish weak and strong strain hardening. With the increase of combined water, the ultimate strength, the structural strength, and structural parameters decrease, and the initial structural parameters decrease nonlinearly, resulting in the decrease of soil structure and deformation resistance. Weak combined water is the main factor affecting soil structure. The particle flow numerical simulation explains the whole process of triaxial test of in situ loess from macro to meso and explains from the meso perspective that the structural strength change of intact loess under different combined water contents is mainly caused by the change of contact force and pore between particles. [ABSTRACT FROM AUTHOR]

In order to investigate the safety and stability of loess high slope under dynamic ramming, the MatDem software was used to simulate the process of heavy rammer compacting the spot which was 11 m away from the toe of loess high slope. The rammer was applied with different energies of 10000 kN·m, 8000 kN·m, and 6000 kN·m. In this way, the safety and stability of slope under the action of different dynamic tamping energies can be determined. The results show that the loess high slope presented circular landslide damage by dynamic compaction. Under the same ramming times, with the decrease of ramming energy, the damage degree of loess high slope gradually reduced. According to the displacement value of different monitoring points, the large horizontal and vertical displacement points in landslide were obtained. When the ramming energy was 10000 kN·m and 8000 kN·m, the maximum horizontal displacements were 15.45 m and 10.72 m, and the maximum vertical displacements were 17.43 m and 11.91 m. When the ramming energy was 6000 kN·m, the soil at the bottom of slope would produce slight vibration. Considering the actual project, when the ramming energy was 10000 kN·m and 8000 kN·m, the minimum safe distance was recommended to be 25 m and 20 m. When the ramming energy was 6000 KN·m, the slope remained stable as a whole, and the minimum safe distance suggested should not be less than 11 m. A safety distance of collapse of loess high slope under dynamic compaction was determined, which provided a strong safety guidance for loess high slope construction under dynamic compaction. [ABSTRACT FROM AUTHOR]

Loess foundations often affect the stability of superstructures due to their insufficient bearing capacities; thus, reinforcement measures are crucial for mitigating the uneven settlement of foundations. This study aims to investigate the settlement characteristics of composite foundations for loess foundations reinforced by splitting grouting piles. First, split grouting tests were carried out on loess foundations. After the test piles reached the design strength, static load tests and geotechnical tests were carried out to obtain the Q-s curves of the test piles. The pile-forming mechanism was analyzed via excavations at an on-site test pile. Furthermore, the traditional settlement calculation method of composite foundations was used to calculate the settlement of split grouting pile composite foundations, and the results were compared with field test results to explore a suitable settlement calculation method of split grouting pile composite foundations. Finally, the numerical model of split grouting pile composite foundations was established according to the pile forming mechanism, and the settlement deformation characteristics of single pile and pile group composite foundations were analyzed. The relative errors of the composite modulus method and the stress correction method for calculating the settlement of the composite foundations were 6.4% and 32.8%, respectively, and the composite modulus method was closer to the test values. The calculated curves of the numerical model were in good agreement with the measured curves and relevant literature research results, demonstrating the rationality of the modeling method; the findings of this study provide theoretical guidance for designing loess foundation splitting grouting reinforcement and for predicting settlement during later construction periods. [ABSTRACT FROM AUTHOR]

Large deformation, collapse, and destruction of supporting structure occurred many times due to rainfall in Xiaohegou tunnel, which is located at Taixing Railway in Shanxi Province. In this paper, the Xiaohegou tunnel is taken as an example, and the large deformation numerical simulation method is used to reproduce the collapse process of the slope at the entrance and the internal section of the tunnel. The reasons for the failure of the original initial support of the tunnel are analyzed, and the failure mechanism of the support structure of the expansive loess tunnel was clarified. The results show that the rainfall leads to the destruction of the tunnel support and the decrease of the bearing capacity of the support structure, which results in the collapse of the tunnel. [ABSTRACT FROM AUTHOR]

This paper investigates the dynamic properties of compacted loess under wetting and drying (W-D) cycles. A series of tests were conducted on compacted loess samples, namely, the soil dynamic triaxial test and the scanning electron microscopy (SEM) test. The test results showed that the dynamic stress-strain relationship of the compacted loess under the action of W-D cycles accords with the Hardin–Drnevich model. The initial dynamic shear modulus (G0) and the maximum dynamic shear stress (τy) of the compacted loess first decreased and then increased with the number of W-D cycles (n) increasing. The damping ratio (λ) increased linearly with the dynamic strain (εd) increasing in the semilogarithmic coordinate. The defined change rate of the damping ratio (η) first increased and then decreased with the n increasing. The macrostructure and microstructure characteristics of samples in the process of W-D cycles indicate that the increasing number of pores in the humidifying process and the cracks on the surface and inside of samples during dehumidification lead to the structural damage and dynamic properties reduction of compacted loess. The main reasons for structure strengthening and dynamic properties increasing are that soil particle structure develops to mosaic structure, pore structure develops to uniform small pore, and matrix suction makes soil sample tend to be dense. [ABSTRACT FROM AUTHOR]

The engineering properties of collapsible loess have significant uncertainty. Accurate prediction of collapsible deformation is crucial for the safety of engineering construction in loess areas. Taking the typical collapsible loess stratum as the research object in Xi'an, based on the random field theory, combined with the Monte Carlo strategy and modulus reduction method, the stochastic finite element analysis of loess self-weight collapsibility is carried out to study the influence of the spatial variability of compression modulus on the self-weight collapsibility of loess. The results show that the loess tends to be stratified and average along the depth direction with the increase of transverse correlation distance. The random field result of self-weight collapsibility considering the spatial variability of compression modulus is significantly greater than the deterministic result of layered average and the calculated value of loess code. Considering the low compression modulus dominance effect of compression modulus with positive skewed distribution of random field, the equivalent characteristic value of the compression modulus calculated by the layered average modeling for the collapsibility evaluation of typical loess strata in Xi'an area is proposed. [ABSTRACT FROM AUTHOR]

Focusing on the performance improvement of glass fiber-modified loess, a series of laboratory tests were carried out, and the macroscopic mechanical properties of glass fiber-modified loess were studied by conventional triaxial shear tests. With the help of scanning electron microscopy (SEM), the pores and cracks image analysis system (PCAS) and nuclear magnetic resonance (NMR) tests, the microscopic properties of the improved loess were studied qualitatively and quantitatively from three aspects: mode of structural contact, pore morphological characteristics and pore structure arrangement characteristics. Finally, the interface zone bond-slip model was used to analyze the internal mechanism of glass fiber and loess interface zone slip. The macroscopic test results show with the same glass fiber length, the shear strength, cohesion and internal friction angle of the modified loess all increased first and then decreased with increasing incorporation of glass fibers. And the optimal ratio of glass fibers is 0.6%. Under the same fiber ratio, the shear strength and cohesion of the improved loess continued to increase with increasing fiber length. The optimal fiber length used in the project was 9 mm. Qualitative analysis of the SEM images show that with the increase in the glass fiber incorporation ratio, the fiber increasingly sutures the cracks in the soil and tightens the soil at both ends of the cracks. Quantitative analysis shows that the proportion of pore area is the largest at the 0.8% mixing ratio, followed by the 0% mixing ratio, and the smallest at the 0.6% mixing ratio. It can be seen from the NMR test results that the T2 spectral distribution curve under the fiber ratios of 0.2%–0.6% is shifted to the left compared with the ratios of 0% and 0.8%. The peak-top signal intensity of the main spectrum peak when the fiber ratio is 0.6% is the weakest. [ABSTRACT FROM AUTHOR]

To reduce urban pressure, urban rail transit has become an effective way to reduce traffic congestion, mitigate traffic accidents, reduce environmental pollution, and improve commuting efficiency. Subway as the main means of urban public transport travel, in recent years by people's favor, although the construction industry of rail transit is developing rapidly and the industry scale is expanding, but because the construction of rail transit construction projects is very difficult, especially in the wet loess within the concealed excavation tunnel, but also frequent accidents, and the quality of the project is not easy to guarantee, so the underground railroad wet loess within the concealed excavation tunnel construction technology is poor. Therefore, it is especially important to study the construction technology and project quality management of underground railway concealed tunnel in wet loess. In this paper, based on the in-depth study of the basic principle of quantum particle swarm optimization calculation and the realization of key engineering technologies, the particle swarm optimization algorithm is programed using MATLAB software, and the coding scheme, operation specification, and operation parameters are designed. Then, combined with the particle swarm optimization algorithm and assisted MATLAB software, the main analysis of the construction and quality control of wet loess in concealed tunnels of subway projects was carried out, mainly by systematizing the relationship between the three major elements of subway project schedule, cost, and management and the construction and quality control of wet loess in concealed tunnels of a subway, and concluded that the construction and quality control of wet loess in concealed tunnels of the subway needed schedule. It is concluded that the construction and quality control of subway wet loess tunnel requires stable schedule, adequate cost budget, and management personnel. [ABSTRACT FROM AUTHOR]

The soil stabilizer of the Consolid system (content: 0%–2.46%) was used for the modification of collapsible loess. The consolidation test, compression test, collapsibility test, and strength test of modified loess were conducted. In addition, X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury injection tests were carried out to study the loess before and after modification. The results indicated that with an increase in the content of the stabilizers, the optimal water content of the modified loess increased, whereas the dry density decreased. Furthermore, as the content of the stabilizers increased, the compressibility and collapsibility of the modified loess decreased. When the stabilizer content was 0.86%, the modified loess exhibited almost no collapsibility. The unconfined compressive strength of the modified loess demonstrated an exponential relationship with the content of the stabilizers. The shear strength increased with the increase in the content of the stabilizers. When the stabilizer content reached 1.66%, the friction angle started to decrease. The microstructure analysis indicated that the cumulative pore volume of the modified loess decreased with the increase in the content of the stabilizers, which could facilitate the formation of a more stable soil structure and improve the impermeability and strength. [ABSTRACT FROM AUTHOR]

Taking China's first application of domestic shield tunneling in a water-rich sand layer for crossing a high-speed railway as research background, the influence of changes in the shield tunneling parameters on ground settlement is analyzed based on field tests, and a construction control method suitable for shield tunneling under a risk source in a water-rich sand layer is proposed. The test results show that the use of 10% sodium bentonite as the soil modifier for soil pressure balance shield tunneling into the water-rich sand layer has advantages, while adding 50% loess into the bentonite slurry in the gravel sand layer can greatly improve the impermeability of the soil; the settlement of soil can be effectively reduced by using the special segment with grouting holes for deep grouting and applying the adaptive transformation of the shield cutter. Based on the statistical analysis results, a reasonable range for the cutter head torque, cutter head speed, chamber pressure, bentonite injection volume, and advanced speed in the water-rich sand layer can provide a construction control basis for similar projects and provide data support for the compilation of relevant specifications. [ABSTRACT FROM AUTHOR]

Seismic compression is the accumulation of contractive volumetric strains in unsaturated soil from earthquake shaking. For Chinese loess, the behavior of seismic compression is extremely significant and has been recognized in historical earthquakes. As a main technical mean, dynamic triaxial apparatus has been employed to study the seismic compressibility of loess for a long time. However, dynamic simple shear apparatus is superior to dynamic triaxial apparatus in modeling the real stress condition of soil under earthquake. Meanwhile, dynamic triaxial apparatus could not measure the volumetric strain of unsaturated loess precisely. A series of cyclic compression tests are hereby performed on unsaturated and intact loess by the new cyclic direct simple shear device developed in Xi'an University of Technology. The properties of cyclic compression of loess specimens are analyzed under the combined conditions of four water contents, four vertical stresses, and six shear strains. The test results reveal that the volumetric strain is constantly compacted through the process of cyclic load-unloading and is much larger during loading half-cycle than unloading half-cycle. It has been found that the cyclic compressibility of loess is dependent upon density, water content, confining pressure, shear strain (or stress), and cycles. An empirical formula to estimate the increment of cyclic volumetric strain per cycle is expressed in terms of the amplitude of shear train, dry density, water content, and vertical stress. Moreover, a simplified evaluation procedure is proposed for the probable settlements of loess deposits subjected to earthquake shaking. [ABSTRACT FROM AUTHOR]

Many researchers have conducted a vast amount of research on water sensitivity and joint seepage in loess tunnels. However, studies on the mechanical effects of shallow buried loess tunnels under the influence of joint dominant seepage are still insufficient. In this study, we simulate the seepage range of loess tunnels with joints, and we use this seepage range to numerically simulate the mechanical properties of shallow buried loess tunnels. Studies have shown that the permeability coefficient and the number of joints are the key factors that affect the amount of surface water infiltration. An increase in the permeability coefficient on the order of 5 × 10−3 m/s will cause the surface water to pour into the ground and rapidly form a saturated soak zone on the liner, while an increase in the number of joints will increase the width and formation rate of the saturated zone. The results of the tunnel mechanics simulation show that the surface settlement and the surrounding rock displacement increase with the increase of the wetted area of the vault; the effect of arch footing water immersion on the surface settlement and the surrounding rock displacement is most significant for the same wetted area width. Compared with the three-stage method, the center cross diaphragm (CRD) method of excavation can better control the surface and the surrounding rock settlement under waterlogged conditions. In particular, the lining settlement on the waterlogged side can be effectively controlled, and the overall settlement of the tunnel is more uniform. [ABSTRACT FROM AUTHOR]

The shear strength (vertical pressure 50, 100, 200, and 300 kPa) and soil structure of the remolded loess with different proportions of the microbial cementing solution were measured and observed by the direct shear test and scanning electron microscopy (SEM).The results showed: (1) the increase in the microbial cementation solution led to the transformation of the stress-shear displacement curve from strain hardening to strain softening, with the transition interval of 26%∼34%. (2) With the increase in the microbial cementation solution, the change of cohesion showed a trend of "M," the inflection point of the microbial cementation solution was 14%, 22%, and 34%, while the internal friction angle showed a law of "W," where the inflection points of the internal friction angle were about 14%, 30%, 34%, and 38%, respectively. In the range of 18%∼30%, the cohesion and internal friction angle increase with an increase in the microbial cementation solution.(3) The change of cohesion is affected by the microbial mineralization saturation and the joint action of the occurrence state of Ca2+ and HCO3- in the microbial cementation solution. The change of internal friction angle is affected by the soil particle contact, the existing form of calcium carbonate formed by microbial precipitation, pore morphology, yield, and other forces. (4) By means of SEM, the distribution morphology of the soil particles and the contact microscopic images of the loess samples modified by microorganisms with different proportions were further verified to further verify the macroscopic changes of the shear strength of the modified loess samples cemented by microorganisms with different proportions. It provides a theoretical basis for the improvement of loess soil structure characteristics by microorganism calcium carbonate precipitation technology. [ABSTRACT FROM AUTHOR]

Aiming at the influence of water immersion in the loess stratum on the bearing capacity of the pipe gallery foundation, taking the prefabricated pipe gallery demonstration project in Xiongan New District as the research object, the static penetration test and indoor triaxial test of the foundation bearing layer of the pipe gallery before and after water immersion were carried out. In the experiment, the loss law of loess foundation bearing capacity before and after water immersion was analyzed by using the obtained soil physical index changes. The research results show that the foundation bearing capacity calculated using the static penetration test data before and after immersion are in good agreement with the field plate load test results, and the eigenvalues of the foundation bearing capacity calculated using the shear strength index are higher than the field measured results. According to the results of the static penetration test, the bearing capacity of the foundation after immersion will decrease, but the reduction range is within 4%, and there is a certain loss; the bearing capacity of the foundation before and after immersion calculated according to the shear strength index is reduced by 2.5%. It shows that the bearing capacity of the foundation will be lost to a certain extent after immersion in water, but the loss is small. It is recommended to strengthen drainage measures in the later stage to prevent the foundation from being soaked by rainwater. [ABSTRACT FROM AUTHOR]

The instability disaster of slope engineering is the third largest geological disaster after earthquake and debris flow, and rainfall is an important factor affecting the stability of slopes. In this study, the stability of a loess slope affected by rainfall infiltration was investigated using engineering examples, an in-depth site investigation, geotechnical physical and mechanical tests, and numerical simulations. The finite element software ABAQUS was used to establish a model of the rainfall infiltration slope to analyze the slope stability after rainfall infiltration while considering the influence of the groundwater level on slope stability and evaluated the reinforcement performance of retaining wall and biological measures on loess slope under rainfall conditions. The results showed that the displacement and pore pressure at the bottom of the slope increased after a strong rainfall. The rapid rise of the water level caused increases in the displacement and pore pressure of the slope, but the slope stability improved when the water table reached 0.8 m. Further analysis shows that under the joint action of rainfall and groundwater, the slope stability can be effectively improved by using retaining wall to strengthen the slope. Under the action of rainfall, the increase of vegetation depth and density has a certain effect on the improvement of slope stability. The ryegrass and alfalfa were the most effective slope stabilization approach if the vegetation parameters and economic factors were considered. [ABSTRACT FROM AUTHOR]

Due to unique landform and lithological features of the loess gully area, the geological disasters caused by coal mining have become more complex. Moreover, the advancing direction of the working face has an important influence on the deformation of the slope on both sides of the gully. In this paper, combined with the specific conditions of a coal mine working face in western China, we use the method of particle flow numerical simulation and theoretical analysis to examine and observe the deformation, as well as characteristics of failures in the loess gully area under different mining directions of the working face. The deformation process of the gully area can be obtained by combining the numerical simulation results. According to different mining directions of the working face, the failure mode of mining in the loess gully area was divided into the back slope and along slope advancing failure modes. Through empirical evaluation, our investigation demonstrates that the bottom of the gully was damaged seriously by both along slope mining and back slope mining. Albeit the influence of coal seam excavation on the slope surface was relatively small; however, it is greater on the flat ground of the upper slope. Under the same mining conditions, the advancing direction of the working face affected the horizontal movement of the loess gully area but had less effect on the subsidence. Furthermore, we observed that the failure mode of the mining slope is highly correlated with the mining direction and relative position of the working face. The results obtained from this research can provide useful information for deformation and failure prediction, working face mining method, and geological disaster assessment in the loess gully area. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Freeze-thaw cycles caused by climate change can change the structure and strength of the soil. In seasonally frozen soil areas, the use of improved loess as a filling material must consider the effects of freeze-thaw cycles. With the increasingly severe global environmental problems, the search for suitable new environmental protection improvement materials has become one of the hotspots in soil performance improvement research. The purpose of this paper is to use lignin fiber to improve the engineering performance and freeze-thaw resistance of loess and to reduce the negative impact of engineering construction on the environment of the loess area. Based on a series of triaxial shear tests, the effects of freeze-thaw cycles on the stress-strain relationship, shear strength, and Mohr-Coulomb's strength parameters of loess reinforced with lignin fiber were analyzed. Combined with the volume change and microstructure characteristics of fiber-reinforced loess before and after the freeze-thaw cycles, the reasons for the effects of the freeze-thaw cycles on the shear strength characteristics of fiber-reinforced loess are discussed. The research results showed that after 15 freeze-thaw cycles, the shear strength of loess reinforced with 1% fiber increased by 0.15%, 2.05%, and 1.35% at 80, 140, and 200 kPa, respectively. The shear strength of the reinforced loess with other fiber contents decreases to different degrees, and the maximum reduction ratio can reach 9.54%. Freeze-thaw cycles changed the variation of shear strength and strength parameters with fiber content. When the fiber content is less than 1%, the shear strength, cohesion, and friction angle of fiber-reinforced loess increase the fastest after freeze-thaw cycles. When the fiber content is 1%, the overall destruction effect of freeze-thaw cycles on fiber-reinforced loess is inhibited, and the soil has the best freeze-thaw resistance. [ABSTRACT FROM AUTHOR]

When loess is contaminated by oil, different modes of contamination will have a certain impact on the mechanical behaviors of loess in the oil-production areas. This study is aimed at evaluating the effect of diesel oil contamination on mechanical behaviors of loess through extensive laboratory tests. Two different modes of oil contamination were proposed and applied in compression tests, direct shear tests, and unconfined compressive strength tests to study the compressibility and strength characteristics of diesel-contaminated loess. Results show that two different modes of oil contamination have significant effects on the mechanical behaviors of loess. In comparison with clean loess, the compressibility of contaminated loess increases, and its unconfined compressive strength and shear strength all decrease under the first mode of oil contamination. The compression modulus, friction angle, and unconfined compressive strength of diesel oil-contaminated loess using the second mode of oil contamination are larger than those in the first mode of oil contamination at the same oil content and dry density. Understanding these effects on mechanical behaviors of loess under different modes of oil contamination can significantly guide soil and environment-remediation activities in loess oil-production areas. [ABSTRACT FROM AUTHOR]

The difference of sample preparation methods will directly lead to the difference of the structure of the sample, and the mechanical response characteristics under external load will be different. Based on the conventional tests such as particle analysis, specific gravity, critical moisture content, and consolidation test of undisturbed loess from Xi'an, this paper focuses on the static triaxial consolidated drained shear and consolidated undrained shear tests on undisturbed and remolded loess. The results show that: (1) the structure has an obvious influence on the stress-strain relationship of the sample, the stress-strain curve of the undisturbed sample is a strain softening curve, and the stress-strain curve of the remolded sample is a weak strain hardening curve; (2) under the same loading conditions, the triaxial shear strength of the undisturbed soil is significantly higher than that of the remolded soil; (3) the change rule of the consolidation ratio of undisturbed and remolded samples is basically the same, but the final consolidation ratio of remolded soil is greater than that of undisturbed soil. [ABSTRACT FROM AUTHOR]

Taking the Xi'an rail transit line orthogonal crossing the ground fissure section as the engineering background, the structural properties and adaptability of the shield tunnel lining segment under the ground fissure environment are carried out. Using the finite element numerical simulation method, combined with the relevant model tests that have been carried out by predecessors, the damage characteristics of shield tunnel lining segment structure under the action of the ground fissure are revealed. The results show that shield tunnel structure is mainly shear deformation, with local bending deformation. Secondly, the principal tensile stress and principal tensile strain at the arch waist and arch foot of segment ring intersecting with ground fissure section are the largest. The relative dislocation between the rings of the shield tunnel near the ground fissure in the hanging wall is the largest, and the opening of arch top ring seam near the ground fissure in the footwall is the largest. Finally, the conventional shield tunnel lining segment structure cannot be directly used in the ground fissure site with a vertical dislocation of 200 mm. [ABSTRACT FROM AUTHOR]

In order to study the effects of dry density and initial water content on the permeability characteristics of loess, the saturated permeability tests of undisturbed loess and remolded loess were carried out in Lanzhou New Area. The anisotropy of undisturbed loess under two factors is analyzed and compared, and the permeability difference between undisturbed loess and remolded loess is also analyzed. The results show that the structure of loess has an important impact on the permeability characteristics, and the dry density is an important structural parameter. The natural structure of undisturbed loess has obvious anisotropy. Under different pore conditions and water contents, it will show the anisotropic behavior of permeability. The influence of water content on permeability is mainly reflected in the influence on the properties of cement between soil particle skeletons. Under different dry density conditions, the influence of water content on permeability is different. When the pore structure is relatively compact, the influence of water content is more obvious. In practical engineering, these two factors need to be comprehensively considered in order to accurately describe the permeability distribution of loess. This study can provide some theoretical guidance for the construction of relevant loess projects. [ABSTRACT FROM AUTHOR]

As a large human gathering place, the city directly causes huge property losses and casualties due to the ground collapse accident every year. In order to explore the causes and damage characteristics of urban ground collapse, taking Xining City, Qinghai Province, as an example, this paper analyzes the accidental factors causing ground collapse through on-site geological exploration and then expounds the deformation characteristics of urban ground collapse by using the numerical simulation method of Fluent and PFC. Using the collapsible balance method, it is obtained that the insufficient thickness of the top plate of the Loess Soil Subgrade in the collapse pit area is the inevitable factor causing the final collapse. The results show the following: (1) The failure of water supply pipe is an accidental factor leading to the formation of underground cavity, and the ground collapse accident is hidden, sudden, and of high risk, and its deformation and failure development process is slow. (2) The critical thickness of the roof above the air raid shelter is about 22 m, and the actual thickness is about 11.6 m, which is prone to deformation and uneven settlement, which is also an inevitable factor causing ground collapse. (3) The numerical simulation study shows that the final formation deformation range is about 16 m. At this time, the height of the formed cavity is about 9 m. The plastic zone tends to develop around, and the maximum settlement position appears above the air raid shelter. (4) The collapse process includes four stages: water supply pipeline leakage, underground cavity formation, collapse critical state, and ground collapse. This research and analysis provides theoretical guidance and scientific basis for the investigation, measurement, detection, and monitoring of various underground pipe networks of urban roads. [ABSTRACT FROM AUTHOR]

Saturated soft loess has a large pore structure, high compressibility, low strength, fluid plastic state, and poor engineering properties. It is still one of the key problems that engineering needs to solve. In order to study the influence of deep foundation pit excavation in the saturated soft loess area on the deformation of foundation pits, the deformation laws of ground settlement, enclosure structure, and supporting axial force were clarified based on the field tests on the deformation characteristics of deep foundation pits in areas with the high-water level in saturated soft loess, combined with geological conditions and on-site construction procedures. The results indicate the following: water supply and construction process were found to be the main factor in changing the surface settlement curve of deep foundation pits in saturated soft loess; increasing the construction speed of the pit bottom floor, inverted braces, floor frame beams, and sidewall frame beams to close the structure, which is conducive to restraining the deformation of the continuous underground wall and foundation pits in similar areas. In the initial stage of support layout, the axial force of steel support tends to increase too fast or even exceed the standard control value. Therefore, a reasonable preadding axial force is an effective means to control the deformation of the continuous underground wall and the axial force of the steel support. The current research results may provide a reference for constructing deep foundation pits in similar areas. [ABSTRACT FROM AUTHOR]

This study investigates the permeability characteristics of unsaturated compacted loess by focusing on the anisotropy parallel and perpendicular to the compaction. Three tests are conducted on compacted loess: permeability test, soil-water characteristic curve test, and scanning electron microscope (SEM) test. Samples are maintained and tested at different dry densities under optimal moisture conditions. Test results showed that the horizontal saturated permeability coefficient of compacted loess was larger than in the vertical direction, showing obvious anisotropy. Based on the saturated permeability coefficient, the permeability coefficient of unsaturated compacted loess can be predicted according to the soil-water characteristic curve fitted using the van Genuchten model combined with the Childs and Collis-Geroge model. The relational formula was established between the unsaturated permeability anisotropy ratio and the matrix suction through introducing a pore connectivity-tortuosity parameter, which represents the relationship between unsaturated permeability anisotropy ratio and matrix suction. The difference of pore characteristics between horizontal and vertical directions of compacted loess is the main reason for the permeability anisotropy. [ABSTRACT FROM AUTHOR]

Frame anchor supporting structure is widely accepted in the loess area, especially in the multistage slopes along the road. However, the development between theory and practice is not synchronous, and there exist experience and uncertainty in the design process to a great extent. In response to this problem, the multistage loess slope supported by frame structure with anchors is taken as the research object, and the model test is carried out with the help of the large-scale geotechnical centrifuge under the condition of using prototype materials as much as possible. Then, according to the test data and phenomena, the structural stress and slope deformation concerned in the project are analyzed. The main differences in stress and deformation between the multistage loess slope supported by frame structure with anchors and other types of slopes are pointed out, and some targeted suggestions are given. The research results show that the frame anchor supporting structure and soil can always maintain coordinated deformation during centrifugal loading, and the failure process has certain toughness characteristics. The stress characteristics of the anchor rod and the sliding deformation range of the slope in the multistage loess slope supported by frame structure with anchors are different from the slope with structural surface and single-stage slope. The length of the anchoring section cannot be completely determined based on the potential sliding surface in the design. It is reasonable to strictly control the end position of the anchoring section from the perspective of changing the position of the potential sliding surface. The results obtained can enrich and improve the stability theory of multistage loess slope and provide guidance for engineering practice. [ABSTRACT FROM AUTHOR]

The similar material of collapsible loess is the basis and premise of the experimental study on the surface movement and deformation law of coal seam mining in collapsible loess-covered areas. The orthogonal experiment is used to make up similar material with different proportions using river sand and barite powder as aggregate, clay and gypsum as cementing material, and diatomite as adjusting material. The reasonable proportion of similar material in collapsible loess is studied by using range analysis, similar simulation, and field measurement. The results show that the content of diatomite plays a leading role in the collapsibility coefficient of similar material, and the collapsibility coefficient is positively correlated with the content of diatomite; moisture content is the main control of the cohesion of the material, and cohesion is negatively correlated with the moisture content; the ratio of bone-to-glue has the most significant effect on the internal friction angle, and the internal friction angle is positively correlated with the ratio of bone-to-glue. The reasonable ratio of the similar material in collapsible loess is 8 : 2 of the ratio of bone-to-glue, the ratio of clay-to-gypsum is 9 : 1, the barite powder content is 6%, the diatomite content is 23%, and the moisture content is 13%, and the mechanical parameters of the collapsible loess are 5.3%–6.3% different from the target value of similar material through laboratory tests, which can meet the experimental requirements. It is verified by a similar simulation experiment that the maximum surface subsidence value and the surface fracture width in the simulation results are 6.9% and 7.8% different from the field measured results, indicating a high degree of agreement. The results of the study have important references and guiding significance for the preparation of similar material with similar models. [ABSTRACT FROM AUTHOR]

Oil leakage will not only pollute the soil but also change its physical-mechanical behavior, and loess has complex properties in the environment of oil presence. Artificial loess contaminated by diesel oil is collected as the research object. The physical and mechanical properties of clean loess and contaminated loess, including liquid and plastic limits, permeability, compression properties, and compressive strength characteristics, are estimated through a series of laboratory tests under different oil contents, water contents, and dry densities. Results show that liquid limits, plastic limits, and permeability coefficient of diesel-contaminated loess decrease with the increase of oil content. The compression modulus and compressive strength of diesel-contaminated loess increase with the increase of dry density at the same oil content. Adding diesel oil, the change law of the unconfined compressive strength of contaminated loess is opposite at the two different water contents. The variation of the compression modulus and unconfined compressive strength of diesel oil-contaminated loess is basically identical at the same condition. The findings of this study would be expected to bridge the gap between theory and practice in treatment and remediation of contaminated soil in the region of oil production. [ABSTRACT FROM AUTHOR]

Malan loess in the middle and lower reaches of the Yellow River after mixed with different bentonite and HDTMA ratios was selected to carry out shear and consolidation tests for discussing the influence of bentonite-HDTMA on the mechanical property of loess lining material in landfills. Studies have shown that, after mixing 6% to 14% bentonite, the cohesive force of modified loess is significantly increased, the friction angle is reduced and remains stable, and the shear strength is improved. The compressibility of materials slightly decreases with the increase of bentonite ratio. The addition of 2%–4% HDTMA weakens the increment for the cohesive force caused by bentonite, but the friction angle is effectively recovered. The compressibility of materials increases with the increase of HDTMA ratio. As the dry density increases, the shear strength of the modified loess increases, and the compressibility coefficient decreases. The mechanical property of loess lining material can be optimized by adding 6%–14% bentonite. The incorporation of 2%–4% HDTMA does not have a significant negative impact on the optimization effect of bentonite. From the perspective of the mechanical property, it is recommended that the dry density of modified loess is 1.70 g/cm3, the bentonite ratio is 10%–14%, and the HDTMA ratio is 2%–4%. [ABSTRACT FROM AUTHOR]

Metastable loess soils can deform, inducing geological and engineering disasters. Therefore, the behavior of the loess under dynamic load is gaining massive attention from researchers to improve the strength of the soils. Fly ash mixed with loess can improve strength and reduce construction costs and environmental pollution. Moreover, it has strong economic and social benefits. This paper investigates the influence of fly ash on the dynamic properties of the modified loess through a series of dynamic triaxial tests of the fly ash modified loess with different fly ash contents. The treated soil samples were prepared using a static compaction method in both ends and cured for 28 days. The dynamic shear modulus ratio, the damping ratio, and the dynamic residual strain of the modified loess were analyzed. The variation characteristics of the dynamic shear modulus ratio and damping ratio with the dynamic shear strain of the fly ash modified loess were obtained. The effect of fly ash content on the dynamic nonlinear parameters of the modified loess was also investigated. In addition, the relationship between the dynamic residual strain and the fly ash content was discussed. The results show that the dynamic shear modulus ratio of fly ash modified loess decreases nonlinearly with the increase in the dynamic shear strain. However, the attenuation rate difference of the curves is small. The damping ratio increases gradually with increasing dynamic shear strain. Under a certain dynamic shear strain level, the damping ratio decreases with the increase in the fly ash content. The dynamic residual strain increases with the increase in the dynamic stress. However, it decreases with the increase in the fly ash content. When the fly ash content is between 10% and 20%, the dynamic residual strain of fly ash modified loess is reduced rapidly. However, when the fly ash content exceeds 20%, the dynamic residual strain decreases slowly. The fly ash content of 20% could be suggested as an optimal content for seismic resistance of the loess foundation. [ABSTRACT FROM AUTHOR]

To study the effect of the lime-soil compaction pile on the collapsible loess foundation and the influence of water-heat-force characteristics on the soil during compaction, the field test of lime-soil compaction pile treatment of collapsible loess foundation is carried out. This paper monitors the process of soil temperature, water, and soil pressure change in the process of pile forming. According to the macroprocess and micromechanism of lime-soil compaction pile treatment of collapsible loess foundation, the qualitative law of water-heat-force change in the process of pile forming is obtained. Meanwhile, the influence of soil damage is introduced in the process of compaction expansion. The model of pore expansion under linear damage conditions in the plastic zone is established. The formula of radial stress distribution under damage conditions is given. The influence of different damage factors on the expansion process is analyzed. [ABSTRACT FROM AUTHOR]

Dynamic loads such as earthquakes and traffic will simultaneously generate vertical dynamic stress and horizontal shear stress in the foundation soil. When the vertical dynamic stress amplitude is twice the horizontal shear dynamic stress amplitude, and the phase difference between them is 90°, a circular dynamic stress path is formed in the τ z θ d ∼ σ zd − σ θ d / 2 stress coordinate system. To simulate the stress state of soil in the area of the circular dynamic stress path caused by bidirectional dynamic stress coupling, a series of tests of compacted loess under the action of a circular dynamic stress path were carried out using a hollow cylindrical torsion shear apparatus. The effects of the mean principal stress, dry density, and deviatoric stress ratio (the ratio of deviator stress to average principal stress) on the dynamic modulus and damping ratio of compacted loess were mainly studied. The test results show that, under the action of the circular dynamic stress path, the larger the mean principal stress is, the larger the dynamic compression modulus and dynamic shear modulus are. The dynamic compression modulus increases obviously with increasing dry density, but the dynamic shear modulus increases only slightly. When the deviator stress ratio increases from 0 to 0.4, the dynamic compression modulus and dynamic shear modulus increase to a certain extent. In addition, the greater the dry density and deviatoric stress ratio are, the greater the initial dynamic compression modulus and initial dynamic shear modulus of the compacted loess. The dynamic compression damping ratio of compacted loess increases with increasing mean principal stress, but the dynamic shear damping ratio decreases with increasing mean principal stress. Dry density basically has no effect on the dynamic compression damping ratio and dynamic shear damping ratio of compacted loess. When the dynamic strain exceeds 1%, the greater the deviatoric stress ratio is, the smaller the dynamic compression damping ratio and the dynamic shear damping ratio are. The research results can provide reference for the study of dynamic modulus and damping ratio of loess under special stress paths. [ABSTRACT FROM AUTHOR]

When the traditional one-dimensional consolidator is used to study the creep characteristics of loess, due to the hoop effect of the ring cutter, only the attenuation creep stage and stable creep stage of loess can be studied, but the accelerated creep stage cannot be presented. In order to avoid the influence of drilling on the creep characteristics of loess, the paper improves the consolidation instrument by drilling holes along the diameter direction in the center of the sample to provide artificial space for soil failure. At the same time, the sample size is increased to ensure that the diameter of the sample is greater than five times of the diameter of the borehole, so as to avoid the influence of drilling on the creep characteristics of loess. The creep characteristics of loess are studied by step loading (vertical pressure at all levels is 125 kPa, 175 Pa, 225 kPa, and 275 kPa), and the whole creep process characteristic curves of loess under different stress conditions are obtained. An endoscope was placed in the hole to observe the deformation and failure characteristics of loess in different stages of creep. This method makes up for the defect that the traditional one-dimensional consolidator cannot obtain the whole process characteristics of loess creep. At the same time, it has the advantages of simple operation, less external influence factors, stronger data reliability, and can directly observe the changes of loess creep soil. It has a beneficial role in promoting the experimental research of loess creep characteristics. [ABSTRACT FROM AUTHOR]

To study the dynamic characteristics and mechanism of saturated loess after freeze-thaw cycles, a series of laboratory tests including freeze-thaw cycle tests, dynamic triaxial tests, and scanning electron microscope tests of the saturated remolded loess was conducted. The characteristics of the dynamic parameters of the saturated loess after different freeze-thaw cycles were discussed. The characteristics of the microstructure parameters changes were analyzed. The evolution process and mechanism of the microstructure of the remolded loess under freeze-thaw cycles were proposed. The results show that after different freeze-thaw cycles, the dynamic stress-dynamic strain curves of the saturated remolded loess conform to the hyperbolic model; however, the freeze-thaw cycle has a significant effect on the model parameter b. With the increase of freeze-thaw cycles, the dynamic shear modulus of saturated remolded loess first decreases and then increases, while the damping ratio is opposite. When saturated remolded loess experiences freeze-thaw cycles greater than four, its dynamic stability is better than that of saturated soil without freeze-thaw cycles. The dynamic stability reaches its peak after seven freeze-thaw cycles and is equivalent to that of saturated soil without freeze-thaw cycles after forty cycles. Combined with the results of the quantitative analysis of microstructure images, with the increase of the freeze-thaw cycles, the number of large and medium particles in the soil reduces, and the number of micros and small particles increases. The particle size tends to be uniform. The apparent porosity increases rapidly and then decreases sharply and tends to be stable after 4 freeze-thaw cycles. The pore and particle fractal dimensions continue to decrease. The probability of entropy increases first and then decreases. It is illustrated that the saturated loess has mainly experienced three steps under freeze-thaw cycles: (1) fracture and expansion of original skeleton cementation, (2) damage, crushing and aggregation of the particle, and (3) compaction and reorganization of soil structure. Besides, the saturation condition significantly accelerates the evolution process of the internal structure of the soil under freeze-thaw cycles. These lead to the strengthening effect of soil dynamic stiffness under long-term freeze-thaw cycles. [ABSTRACT FROM AUTHOR]

Loess cave dwellings are the most typical style of regional architecture in northwest China; now, there are still tens of millions of people living in them. The northwest is an earthquake-prone area, and cave dwellings have suffered a lot of damage in previous moderate and strong earthquakes, so their earthquake resistance has attracted people's attention. At present, the seismic analysis of aboveground building structures is relatively mature, while the seismic analysis of loess cave dwellings is less researched. To study the seismic response of loess cave dwellings, a single-hole cliff-side loess cave dwelling located in Yangjialing revolution former sites of Yan'an City of northwest China was investigated and surveyed; the three-dimensional numerical model was established by MIDAS/GTS NX. Combining the historic earthquake damage investigation, dynamic time-history analysis of the single-hole loess cliff-side cave dwelling subject to four horizontal earthquake actions was conducted to determine the weak positions, failure characteristics, and the corresponding displacement and stress of the loess cave dwelling under earthquake load. The results show that the loess has an amplification effect on the seismic waves, the arch vault is a key factor to the stability of the loess cave dwellings, the cliff-side loess cave dwellings in an 8-degree area cannot be used to continue living, and the entrance to loess cave dwellings is the most dangerous place when the earthquake happens. [ABSTRACT FROM AUTHOR]

The shear behavior of saturated loess was examined by performing a series of ring shear tests with different shearing rates. The effects of shearing rates on the shear behavior of saturated loess with different normal stress are presented and discussed. The results showed that peak shear strength and steady-state shear strength were greater when the shearing rate was low and vice versa. Compared with high and low shearing rates, the maximum strength reduction ratios of peak shear strength and steady-state shear strength were 34.2% and 37.2%, respectively. The axial displacement during shearing was measured and was found to increase with increasing shear displacement in all tests. A comparison of sample height reduction (when the shear rate was stopped) found that the low shearing rate test sample underwent a much greater reduction than the high shearing rate test sample; however, the variation reduction range was within 4 mm. Monitoring the pore-water pressure during the shearing process revealed that it increased with shear displacement, and a higher excess pore-water pressure was generated within the shear zone during the fast-shearing process. Comparing the particle size distribution of the samples after the test and the original sample showed that the particles were crushed during the shearing process. The percentage that was finer than 0.005 mm increased with shearing rates and normal stress, and the soil structure implosion became more pronounced with increasing normal stress. [ABSTRACT FROM AUTHOR]

In the Loess Plateau, seasonal freeze and thaw cause great damage to the mechanical behavior and microstructure of soil, which leads to frequent geological disasters during winter and spring. To investigate the influence of freeze-thaw (FT) cycling (FTC) on the shear strength and microstructure of intact loess, triaxial shear, nuclear magnetic resonance, and scanning electron microscope tests were carried out on soil samples after target FT cycles. The results indicate that the FTC has limited changes to the soil stress-strain curve, but has a significant attenuation effect on the peak deviatoric stress. The peak deviatoric stress was attenuated by FTC but changed insignificantly after ten cycles. The cohesive force decays exponentially with the number of FT cycles, while the internal friction angle increases slightly. Moreover, under FTC, the T2 hydrogen spectra of soil samples showed a multimodal distribution, with the main peak appearing to have two obvious upward shifts that occurred at 6 and 10 FT cycles. Indeed, a depolarization phenomenon related to the directional frequency of soil particles was observed, and the mass fractal dimension of the pore network increased slightly. In an FT environment, the shear strength declines due to accumulated internal microstructural damage. These findings contribute to a better understanding of the response of loess to FTC and provide novel ideas for the prevention of frost damage in loess areas. [ABSTRACT FROM AUTHOR]

The reliability of loess cave dwellings based on fuzzy failure criterion is researched to analyze the influence of fuzzy set, membership function, and different combinations of random variables. Due to the strength reduction method, the basic failure criterion is established, the fuzzy property of failure criterion is characterized by fuzzy set and membership function, and the performance function of loess cave dwellings is expressed by a quadratic polynomial without cross terms. Reliability is analyzed with different random variable combinations. The reliability research of loess cave dwellings in Shan Plateau, Henan province, China, shows that the loess property in this area is suitable for loess cave dwelling construction and the reliability index of loess cave dwellings is high, which will be decreased when considering the fuzzy failure criterion and will be increasing sharply when the small cave leg width is improved to average value. [ABSTRACT FROM AUTHOR]

As a national development plan, ecological protection of the Yellow River Basin has attracted extensive social attention in recent years. Considering the influence of acid rain on the engineering characteristics of loess in this area, we investigated changes in the physical and mechanical characteristics of remolded loess under the combined action of acid rain and dry-wet cycles by means of triaxial tests, nuclear magnetic resonance spectroscopy, and scanning electron microscopy. The results are as follows: in the acidic environment, the stress-strain relationship of remolded loess undergoes stress hardening after dry-wet cycling. The cohesion and internal friction angle of remolded loess are negatively correlated with the number of cycles. From the multiscale analysis of the dry-wet cycle process under acid rain condition, the T2 spectrum of the test soil has three peaks at the micropore level. With the increasing number of cycles, the spectral area increases gradually, and the sample transitions from small pores to large- and medium-size pores. At the microscopic level, the clay mineral particles among soil particles decrease in size, the contact mode between soil particles develops from stable to unstable, the particles are gradually rounded, and the fractal dimension decreases. Chemical erosion and physical erosion are special features of this experiment. Physical erosion causes particle erosion and pore growth, while chemical erosion includes reactions by feldspar. Together, physical and chemical reactions aggravate the soil deterioration process. These research results have laid a good experimental foundation for the ecological protection of the Yellow River Basin. [ABSTRACT FROM AUTHOR]

To analyze the antierosion performance of cement-improved loess (CIL), several influencing factors have been investigated based on two different compaction methods, which include the quasi-static compaction method (QSCM) and the vertical vibration compaction method (VVCM). Then, a prediction model for the cumulative erosion mass loss (CEML) has been established. The effects of erosion on the strength deterioration of CIL were also studied. The results show that, compared with QSCM, specimens compacted using the VVCM have better antierosion performance. As the cement content and the compaction coefficient are increased by 1%, the antierosion performance is increased by 16% and 6.2%, respectively. The eroding time has a significant effect on the antierosion performance of CIL, and the CEML increases linearly with an increase in the eroding time. The compressive strength of CIL decreases significantly due to erosion, and based on the average deterioration degree of the specimens, the design criteria for strength of CIL are proposed, which can provide reference for the design of CIL. [ABSTRACT FROM AUTHOR]