Intact Loess Research Articles

The unsaturated characteristics of natural loess in slopes, New Zealand

Loess and loess-derived soils in the Akaroa harbour area of New Zealand are typically unsaturated and vulnerable to shallow landsliding during rainfall events. In this paper, the water retention properties and unsaturated shear strength of these materials are characterised using laboratory testing and long-term field instrumentation. Because the pore size distribution of the loess is fractal, the laboratory-derived soil-water characteristic curve is described mathematically using a series of power-law relationships. The same soil-water characteristic curve applies to both recompacted and intact loess when suction is normalised by the air entry value. Stress–strain behaviours of the recompacted and intact loess exhibit significant differences, however. The unique microstructure of the intact loess contributes to its larger shear strength. Long-term field instrumentation data, including suction and volumetric water content, indicate that the hydraulic state of in situ loess remained on a scanning curve for the duration of the monitoring period, despite the occurrence of significant wetting events. This, in combination with triaxial test results, allows the contribution of suction to unsaturated shear strength to be quantified. Temporal variations in suction's contribution to strength confirms seasonal variability in unsaturated shear strength and thus slope stability.

Characterizing undrained shear behavior of loess subjected to K0 loading condition

Undrained shear behavior of loess after the K0 consolidation was investigated through laboratory experiments. Both the monotonic and cyclic triaxial tests were performed on the intact loess samples retrieved from a construction site in Yan'an city, China. It was found that the coefficient of lateral earth pressure at rest (K0) of the intact loess is stress-dependent, and it varies between 0.46 and 0.85. Under the monotonic loading condition, the peak q in the undrained shear test is higher for the samples under the K0 consolidation than those under the isotropic consolidation. Besides, the cyclic loading tests were conducted on the loess samples subjected to different magnitudes of the initial deviatoric stress, including the K0 stress state. The test results revealed that different initial stress conditions cause distinctly different failure patterns. It includes the cyclic mobility with progressive development of the axial strain, and limited deformation followed by an abrupt gain of the axial strain with no precursor. In accordance to the build-up of the excess pore water pressure during the tests, each failure pattern of loess was interpreted in the q-p' diagram alongside the undrained instability state (UIS) line. The outcomes of this paper suggest a pressing need to thoroughly consider the K0 values of the intact loess with variations of the initial stress state.

Micro-structural characteristics deterioration of intact loess under acid and saline solutions and resultant macro-mechanical properties

When exposed to chemicals, the micro-structural evolution of the loess and the impacts on the macro-mechanical properties are considered crucial for contaminated land reclamation. In this study, the micro-structural characteristics of the loess specimens that are exposed to acetic acid or sodium sulfate, are studied using scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy analyses. Further, their macro-mechanical properties are determined by direct shear tests. The corrosion of the cement between particles under acetic acid environments and the salt-induced swelling under saline conditions play an important role in the micro-structural deterioration. The cohesion and friction angle for the wetted loess are about 16 kPa and 19° respectively, while for the acetic acid-contamianted loess, they reduce to 10 kPa and 15° respectively. They, for the sodium sulfate-contaminated loess, reduce further to below 10 kPa and 13° respectively. The macro-mechanical properties show good correspondence with the micro-structural deterioration.

Characteristics of Microstructural Changes of Malan Loess in Yan’an Area during Creep Test

The shortage of land resources on the Loess Plateau has long been a thorny problem. Many high-fill projects are carried out, accompanied by a series of geological hazards, which threaten the ecological environment and personal safety. The creep characteristics of loess are an essential part of loess landslide research. The microstructural changes are closely related to creep behavior. By conducting triaxial creep experiments on Malan loess from the Yan’an area in China, scanning electron microscope (SEM) experiments on loess in different creep stages were carried out. Using qualitative and quantitative analyses of the microstructural characteristics of intact loess and remodeled loess during creep, the microstructural parameters were compared, and finally, the microscopic mechanisms during creep were analyzed. The qualitative analysis of remodeled loess during creep returned significantly higher results than it did for intact loess. During the creep process, among the microstructural parameters of loess change, the change in pore and particle size and shape were the most obvious, while the complexity of other microstructural parameters and orientation changed little. During the creep process of loess, the soil skeleton changed continuously, but the loess microstructure featured different changes at different levels of deviator stress.

Experimental Study on Seismic Compression of Chinese Loess under Cyclic Direct Simple Shear Testing

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.

Effects of in situ wetting–drying cycles on the mechanical behaviour of an intact loess

In situ wetting–drying cycles significantly and repeatedly influence the soil water content in the active zone but not in the steady zone. To investigate effects of in situ wetting–drying cycles on the mechanical behaviour of intact loess, a series of pressure plate tests was carried out with an intact loess sampled from the active and steady zones. Results show that the specimens from the steady zone have higher yield stresses at given suctions and a larger inclination of normalized loading collapse (LC) curve than the specimens from the active zone. As evidenced by the scanning electron microscopy results, the clay particles accumulating around the contacts between silt particles are widely observed in the former specimens but not in the latter specimens. The distribution of clay particles in the former specimens results in a stabilized skeleton with mechanical properties sensitive to suction variations. On the other hand, the maximum collapse potential of the specimen from the steady zone is 37% larger than that of the specimen from the active zone, mainly because of the larger inclination of normalized LC curve owned by the former specimen.

Creep Behavior of Intact Loess Followed Unloading Paths

Several high-fill projects are carried out on the Loess Plateau, China, accompanying the progressive failure of slopes due to excavation. The compelling need requires a deep understanding of variation in the creeping behaviors of intact loess exposed to unloading. A series of creep tests of intact loess were performed under two separated unloading paths: decrease in confining pressure at constant deviator stress and decrease in confining pressure at axial stress. The results demonstrated that axial deformation followed the first unloading path always appears as compression while the three forms of axial deformation followed the second path, depending on the applied axial stress level. At a low unloading stress level, the elongation of axial deformation was observed. At a relatively unloading stress level, the axial deformation of the soil experienced the first elongation and then compression. At a high unloading stress level, the axial deformation appeared as compression, and finally, failure occurred with the increase of the unloading stress level. The failure approach index was introduced to use as the criterion for the loess to transform from stable to accelerated creeping. Finally, a modified Burgers model was proposed to characterize the creeping behavior of intact loess followed unloading paths. There was a good comparison between the calculated and measured data of the soil that establishes the rationality and validity of the proposed model.

Effects of chemical contamination on microscale structural characteristics of intact loess and resultant macroscale mechanical properties

Soil contamination not only can cause environmental problems but also lead to a notable change in the mechanical properties of soil. Loess widely distributed over North-West (NW) China is featured with the metastable structure, and chemical contaminants produced especially during the rapid development of NW China in recent years seriously threaten the fragile loess environments. When exposed to chemical contaminants, the impacts on the microstructural characteristics of the loess and the resultant mechanical properties are deemed critical for land reclamation in NW China. In light of this, the microscale structural characteristics of the loess exposed to acetic acid, phosphoric acid, sodium hydroxide, and sodium sulfate respectively are studied using scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy tests. Additionally, their resultant macroscale mechanical properties are determined by direct shear tests. The deterioration mechanism regarding the microscale structural characteristics when exposed to the contaminants is revealed, and the resultant macroscale mechanical properties present a good correspondence with the deteriorated microscale structural characteristics. The findings of this work provide some guideposts for contaminated land reclamation in NW China.

Vibration-induced acceleration of infiltration in loess

Coupled effects of railway vibration, rainfall, and drying-wetting cycles contribute to new types of loess disasters such as the collapse and vibration-induced settlement of the railway subgrade, loess mass disintegration, and slope sliding, during the large-scale construction and operation of high-speed or heavy-haul railways in China. This has significant consequences for railways that require millimetre-level deformation control, seriously threatening railway operation and people’s lives. Therefore, for the first time, “five vibration-induced effects” on loess subjected to vibration, water immersion, and dry-wetting cycles are discussed including the vibration-induced acceleration of cracking, infiltration, disintegration, sliding, and subsidence. In this paper, the vibration-induced acceleration of infiltration is discussed mainly. The response of loess to locomotive vibration along railways was determined using field sampling and data processing. The dominant frequency of locomotive vibration are 18−22 Hz and the maximum amplitude is 0.4 mm. Tests regarding the vibration-induced acceleration of the permeability were conducted on intact loess using a newly developed vibration permeability apparatus. The results reveal that the vibration accelerates the water permeability and increases the water saturation of loess. The permeability coefficient of saturated loess under vibration is ~1.5−20 times of that without vibration, reaching a maximum at a vibration frequency of 20 Hz. Vibration-induced infiltration tests were carried out on a loess column and the vibration-induced acceleration of water infiltration, wetting front evolution, and air outflow were analysed. The vibration-induced acceleration of infiltration strongly depends on the dry density, vibration frequency, and vibration amplitude. This pioneering work explores the mechnism of loess disasters triggered by locomotive vibration, rainfall, and drying-wetting cycles.

Permeability and Microstructure of a Saline Intact Loess after Dry-Wet Cycles

Influenced by both dry-wet cycles and salt weathering, the loess will exhibit significant changes in microstructure and permeability, which threatens the stability of loess slopes. Triaxial permeability tests and industrial computed tomography (CT) scans were carried out on saline intact loess with sodium sulfate. The relationship between permeability and pore structure of the loess after dry-wet cycles was discussed. Results show that the permeability coefficient of loess increases after dry-wet cycles, with the increment declining. After specified dry-wet cycles, the permeability coefficient increases approximately linearly with sodium sulfate content. However, the permeability coefficient significantly declines at higher confining pressures, while its attenuation rate decreases. An empirical relationship based on log 10 1 + e − log 10 k was proposed to estimate the permeability coefficient of saline intact loess considering dry-wet cycles and salt content. Comparisons of measured and calculated results proved its rationality. CT scan images imply the damage to soil microstructure induced by dry-wet cycles and salt weathering, corresponding to the decline of the mean CT value (ME) and the increase of both crack ratio and fractal dimension of crack network.

Pore-Size Distribution Evolution of Intact, Compacted, and Saturated Loess from China during Consolidation and Shearing

To examine the pore-size distribution (PSD) evolution of intact, compacted, and saturated loess during deformation associated with consolidation or shearing, nominally identical specimens were consolidated to different confining stresses or sheared to sequential axial strains under the same confining stress, and the PSD of each deformed specimen was characterized using the Mercury intrusion porosimetry (MIP) technique. The results show that the PSD evolution during consolidation is similar to that during shearing, suggesting that the PSD evolution depends mainly on whether the soil volume contracts or expands. The volumetric contraction results mainly from compression of interaggregate pores, and the intra-aggregate PSD or intra-aggregate pores are not affected. In compacted and saturated loess, interaggregate pores are compressed from the larger to the smaller, while in intact loess, the PSD evolution depends on whether the soil yields. This difference arises from different cementations that dominate particle associating in three soils. In intact loess, carbonate cementations that can be damaged by remolding and loading contribute greatly to particle associating. As a result, the stability of a pore is controlled not only by its size but also by carbonate cementations at surrounding particle contacts. Clay cementations that play the dominant role in particle aggregating in compacted loess are resistant to loading; thus, aggregates could not be destroyed by loading and the mechanical responses of compacted loess are in fact interactions among aggregates. Both carbonate and clay cementations can fail under the combined effect of loading and inundation, leading to disintegration of aggregates and turning of the loess structure from the double-structured to the uniform type.

An Experimental Study on the Deformation and Strength Characteristics of Q3 Loess under a Plain Strain Anisotropic Consolidation Condition

To study the strength, deformation characteristics, and intermediate principal stress of intact loess, vertical loading stress tests with different initial stress state k value were conducted under different confining pressures. Plane strain tests were carried out by the transformed true triaxial apparatus of Xi’an University of Technology. The study shows that loess tends to be in a plastic failure state in different confining pressures and k values, and the stress-strain relationship curve is of a hardening type. Results reveal that loess lateral and volume deformations have nonlinear relationships with its vertical deformation, and volume deformation shrinks in the process of shearing. The effect of confining pressure on soil deformation is greater than k value. The intermediate principal stress coefficient decreases with the increase of the confining pressure and transforms from increasing to decreasing with the increase of k value (ranging from 0.2 to 0.4). In brief, the loess failure strength is closely related to k value, confining stress, and spherical stress state. When k value increases, cohesion effect reduces, whereas internal friction angle increases linearly. The influence of k value on soil strength and deformation is closely related to confining pressure.

Shear Strength and Microstructure of Intact Loess Subjected to Freeze‐Thaw Cycling

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.

Experimental Study on Electrical Resistivity Characteristics of Remodeled Contaminated Q 3 Loess

This study focuses on determining the electrical resistivity under acid and alkali contaminated loess and establishing a correlation between electrical resistivity with water content, composition, and concentration of contamination. Then, acid- and alkali-polluted intact loess were taken as research objects for the experiment of electrical resistivity by using the self-developed voltammetry resistivity test device. Test results show that the influencing characteristics of voltage, water content, pollution composition, and concentration on the resistivity of remolded loess were analyzed. The results reveal the major influence of acid and alkali contamination on the electrical resistivity characteristics of compacted loess. The resistivity of compacted loess decreases with the increase in water content. The resistivity is significantly reduced after being contaminated with hydrochloric acid, nitric acid, and sodium hydroxide. The rate of the decrease reduces with the increase in the concentration of pollutants. After the sulfuric acid pollution, the resistivity test result of the compacted loess sample increases with the increase in pollution concentration. A good correlation can be observed between the resistivity and the pollution degree of the Tongchuan loess used in the experiment, which is helpful for the establishment of an engineering quality evaluation system under the acid- and alkali-contaminated loess.

Compression and Shear Strength Characteristics of Unsaturated Intact Loess in True Triaxial Tests

Abstract Previous studies of shear behavior from suction-controlled direct shear tests or triaxial tests focused mainly on remolded soils. Limited attention has been paid to suction measuring on unsaturated intact soil. In this paper, two series of saturated drained tests and unsaturated constant water content (CW) tests containing isotropic consolidation and shear tests with different intermediate principal stress parameters b were conducted to investigate deformation and shear strength characteristics of intact loess using unsaturated true triaxial apparatus. The initial suction decreases with an increase in initial degree of saturation. The void ratio decreases with an increase in suction and net mean stress. The isotropic compression yield stress increases as initial suction increases. The suction after isotropic consolidation is less than the initial suction. The shear strength increases with an increase in b-value, net confining pressure, and suction. The failure state line on the effective stress p’–q’ plane of Schrefler is approximately a straight line passing the origin at different intermediate principal stresses. The effective stress–strength failure surfaces on the π plane are similar in shape and can be approximated by the spatially mobilized plane (SMP) strength criterion. When the initial suction is constant, the cohesion increases with an increase in b-value and increases linearly with an increase in initial suction at different b-values. The influence of initial suction (i.e., water content) on the internal friction angle is very small, and the internal friction angle decreases linearly with an increase in b-value. There is a nonlinear phenomenon between shear strength and suction. The equation expressions of cohesion, initial suction, and b-value, and internal friction angle and b-value are obtained.

Influence of freeze-thaw cycles on microstructure and hydraulic conductivity of saline intact loess

In this paper, influence of freeze-thaw (F-T) cycles on the microstructure and hydraulic conductivity were investigated on saline intact loess specimens by infiltrating sodium sulfate. X-Ray computed micro tomography (Micro-CT), and triaxial permeability tests were carried out to understand the relationship between the soil microstructure and hydraulic conductivity. The CT scan images suggest tiny holes in the soil structure expand to form cracks due to F-T cycles; these cracks were more pronounced in specimens with higher salt contents. The soil morphology was reconstructed using three-dimensional (3D) reconstruction technology, which highlights the significant influence of salt weathering on the microstructure of saline intact loess. Triaxial permeability test results suggest that the void ratio and hydraulic conductivity of saline intact loess both increase after F-T cycles or at higher sodium sulfate contents, which are consistent with the changes of the porosity of the 3D reconstructed model. Finally, an empirical relationship between hydraulic conductivity and void ratio was proposed incorporating the influence of F-T cycles and salt content. There is a good comparison between the calculated and measured data that establishes the rationality and validity of the proposed equation.

Experimental study on dynamic characteristics of Qingyang loess under different water contents

The complex cyclic stress state such as traffic load and seismic load effects proposes a new challenge for seismic design in the loess area. This paper presents an analysis of the dynamic characteristics and physical and chemical properties of Q3 loess in Qingyang of Gansu Province, China. The dynamic modulus, damping ratio, dynamic strength, and residual strain characteristics of the samples of intact loess under different water content and the saturated compacted loess under different dry densities and different confining pressures were studied through the dynamic triaxial tests. The experimental results show that under the same conditions, the dynamic modulus and dynamic strength of the intact loess sample decrease with the increase of water content, and the residual strain increases with the increase of water content. The intact sample under the natural water content has a large maximum modulus (315.3 MPa) and dynamic strength (325 kPa, using yield criteria) and smaller residual strain. The dynamic modulus and dynamic strength of the compacted sample increase with the increase of dry density and confining pressure, and the residual strain decreases with the increase of dry density. Compared with other regions, the dynamic modulus and dynamic strength of Q3 loess in the Qingyang area are relatively large, indicating that the structure of Qingyang loess is very strong, and the sand content is higher than other areas. Particle analysis shows that Q3 loess in Qingyang belongs to sandy loess and sand content reached 51.4%. Under the action of dynamic stress, the Qingyang loess shows strong elasticity.

Microstructure and soil-water retention behavior of compacted and intact silt loess

There are an increasing number of mega engineering projects of removing the top of hills to fill in the valley for new land creation in the Loess Plateau of China, which significantly changes the hydrological and geological equilibrium state of the site. The formation of a new hydrological equilibrium state is associated with the water movement in the compacted and intact loess layers. Most disasters occurring in the loess area are related to water. The soil-water retention curve (SWRC) is the key to the implementation of unsaturated soil mechanics in engineering practice and is required to numerically model the movement of water in the soil. In this study, the SWRCs and microstructure of intact and compacted specimens from Lanzhou and Yan'an, two typical cities that have new land creation projects, are investigated. The SWRC is obtained using the filter paper method. The mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) techniques are used to explore the microstructure. The results highlight that the intact loess has a higher air occlusion value (AOV) and a similar slope of SWRC in the transition zone compared with the remolded specimens that compacted at the natural water content. The normalized SWRC (i.e., degree of saturation versus suction/AVO) of the intact specimen is approximately the same as the remolded specimens compacted at the natural water content. The difference in the microstructure between intact and compacted specimens originates from different existing states of the clay particles and further contributes to the variation of the SWRC.

Damage of saline intact loess after dry-wet and its interpretation based on SEM and NMR

In this paper, influence of dry-wet cycles on the damage characteristics was investigated on saline loess specimens by infiltrating sodium sulfate solution with different preset concentrations. Scanning electron microscope (SEM), triaxial shear tests, and nuclear magnetic resonance (NMR) were used to detect the changes in soil strength parameters and microstructure with dry-wet cycles and salt content. The SEM images show that the skeleton of saline loess at higher salt contents or after more dry-wet cycles tend to be looser, and more large and medium-sized pores appear. This implies the weakening of soil structure, as proved by the increase of both pore area ratio and fractal dimension. The cohesion and internal friction angle both decrease with dry-wet cycles, and tend to stabilize after five cycles, while the two indexes vary with salt content in a linear pattern. The cohesion-based damage ratios due to dry-wet cycles and salt weathering were discussed. The equation for NMR-based porosity was established considering both dry-wet cycles and salt content. Finally, the predictive model for damage degree based on the porosity equation was obtained, which can well predict the damage of saline intact loess under the coupled influence of dry-wet cycles and salt weathering.

Water-retention curves of loess under wetting−drying cycles

Water-retention curves (WRCs) of intact, compacted and reconstituted loess were investigated using a pressure plate extractor. All specimens were prepared having the same initial density and subjected to two cycles of drying and wetting in the suction range of 0−400 kPa. Furthermore, their pore-size distributions (PSDs) were measured through mercury intrusion porosimeter (MIP) for explaining the measured WRCs. The results show that the air-entry value of reconstituted loess is 505% and 169% larger than that of intact and compacted loess, respectively. This is because the reconstituted loess has more uniform PSD and lacks large-sized pores, as supported by the MIP measurements. On the other hand, the average degree of hysteresis of reconstituted loess during the second wetting−drying cycle decreases by 73% compared to that of the first cycle, which is attributed to the significant soil shrinkage (i.e. 28%) induced by the first wetting−drying cycle. On the contrary, the number of wetting−drying cycles shows negligible effects on the hysteresis of intact loess. In addition, the wetting−drying cycles influence the hysteresis of compacted loess only in a narrow suction range (i.e. 10–50 kPa).