On characteristics of K0 value and shear behaviour of loess using triaxial test.

Compared with conventional soils, such as sand and clay, little knowledge on the coefficient of lateral earth pressure at-rest (K0) has been established for loess in the current literature. This paper presents an experimental investigation on K0 of compacted loess and the associated impacts on undrained shear behaviour. By adopting a K0 consolidation module in the triaxial system, the K0 stress state for loess samples was achieved through a unique feedback control. During the K0 consolidation, the deviatoric stress (q) increases progressively with the premise that the volumetric strain (εv) of the sample equals to the axial strain (εa). The results show that the K0 value of compacted loess is in a range of 0.28 to 0.53, which is dependent on the packing density and the clay content. A distinguishable decrease of K0 was found in the course of K0 consolidation for the loosely compacted loess sample, whereas a similar trend was not observed in the dense sample. In the undrained shear stage, all loess specimens revealed contractive response in the stress path (q-p') diagram, which can be quantified by a modified collapsibility index (Ic). The index is consistently higher for the K0 consolidated loess samples than for the isotropic ones. The experimental results indicate a strong impact of the initial stress state on the shear behaviour of compacted loess.

Hydro-mechanical effects of vegetation on slope stability: A review.

This study explores the complex interplay between vegetation and soil stability on slopes to enhance soil-bioengineering and slope stabilization techniques. We assess the multifaceted role of vegetation in soil stabilization, examining processes such as canopy interception, stemflow, and the effects of hydrological and mechanical changes induced by root systems and above-ground plant structures. Key underlying mechanisms and their effects on stability are reported, along with the evaluation of significant plant indicators from historical research. Our review revealed that plant coverage and root architecture are critical in reducing soil erosion, with plant roots increasing soil cohesion and reducing soil detachability. Above-ground vegetation provides a protective layer that decreases the kinetic energy of raindrops and allows for higher infiltration. The importance of species-specific root traits is emphasized as pragmatic determinants of erosion prevention. Additionally, the effects of root reinforcement on shallow landslides are dissected to highlight their dualistic nature. While root-soil interactions typically increase soil shear strength and enhance slope stability, it is crucial to discriminate among vegetation types such as trees, shrubs, and grasses due to their distinct root morphology, tensile strength, root area ratio, and depth. These differences critically affect their impact on slope stability, where, for instance, robust shrub roots may fortify soil to greater depths, whereas grass roots contribute significantly to topsoil shear strength. Grasses and herbaceous plants effectively controlled surface erosion, whereas shrubs mainly controlled shallow landslides. Therefore, it is vital to conduct a study that combines shrubs with grasses or herbaceous plants. Both above-ground and below-ground plant indicators, including root and shoot indicators, were crucial for improving slope stability. To accurately evaluate the impact of plant species on slope stability reinforcement, it is necessary to study the combination of hydro-mechanical coupling with both ground plant indicators under specific conditions.

Multispectral versus texture features from ZiYuan-3 for recognizing on deciduous tree species with cloud and SVM models.

Tree species recognition accuracy greatly affects forest remote sensing mapping and forestry resource monitoring. The multispectral and texture features of the remote sensing images from the ZiYuan-3 (ZY-3) satellite at two phenological phases of autumn and winter (September 29th and December 7th) were selected for constructing and optimizing sensitive spectral indices and texture indices. Multidimensional cloud model and support vector machine (SVM) model were constructed by the screened spectral and texture indices for remote sensing recognition of Quercus acutissima (Q. acutissima) and Robinia pseudoacacia (R. pseudoacacia) on Mount Tai. The results showed that, the correlation intensities of the constructed spectral indices with tree species were preferable in winter than in autumn. The spectral indices constructed by band 4 showed the superior correlation compared with other bands, both in the autumn and winter time phases. The optimal sensitive texture indices for both phases were mean, homogeneity and contrast for Q. acutissima, and contrast, dissimilarity and second moment for R. pseudoacacia. Spectral features were found to have a higher recognition accuracy than textural features for recognizing on both Q. acutissima and R. pseudoacacia, and winter showing superior recognition accuracy than autumn, especially for Q. acutissima. The recognition accuracy of the multidimensional cloud model (89.98%) does not show a superior advantage over the one-dimensional cloud model (90.57%). The highest recognition accuracy derived from a three-dimensional SVM was 84.86%, which was lower than the cloud model (89.98%) in the same dimension. This study is expected to provide technical support for the precise recognition and forestry management on Mount Tai.

Experimental study on shear strength of saturated remolded loess.

Loess has the characteristics of large porosity, loose structure, uniform composition and strong collapsibility. When encountering heavy rainfall and irrigation prone to saturation, resulting in loess landslides, roadbed subsidence and dam instability. In order to study the effect of dry density and shear rate on the shear strength of saturated remolded loess, the consolidated undrained (CU) test was carried out in Yan'an City by using SLB-6A stress-strain controlled triaxial shear permeability test instrument. The shear rate, confining pressure and dry density were controlled during the test. The dry densities of the samples were 1.5 g / cm3, 1.6 g / cm3 and 1.7 g / cm3, respectively. CU tests of saturated remolded loess were carried out at different shear rates under the confining pressures of 100 kPa, 150 kPa and 200 kPa, respectively. It is found that the stress-strain curve of saturated remolded loess gradually moves upward with the increase of dry density. With the increase of dry density, the cohesion and internal friction angle of remolded saturated loess samples increase. At the same shear rate, with the increase of dry density, the deviatoric stress of the specimen increases significantly.

Spatiotemporal distribution characteristics, causes, and prevention advice of fatal geohazards in Jiangxi Province, China.

Fatal geohazards result in severe losses of life and property worldwide, thus urging many large-scale studies of such geohazards. Further research on hotspots prone to fatal geohazard identified in national-scale studies is critical for government geohazard prevention. It has been pointed out that more detailed small-scale (sub-national) studies are essential for the hotspots (e.g., Jiangxi Province) identified in national-scale studies. However, there are only a few small-scale studies of hotspots and earlier studies have rarely delved into a thorough and detailed analysis of hotspots. In addition, previous studies of fatal geohazards have failed to offer specific geohazard prevention advice, significant for geohazard control policies. To bridge these gaps, this study took advantage of the Jiangxi Inventory of Fatal Geohazards (JIFGH) and employed spatial analysis and the geographical detector to analyze the spatiotemporal characteristics and causes and present prevention advice on fatal geohazards in Jiangxi Province. The study also analyzes the importance of provincial-scale (first-level administrative scale) studies for hotspots identified in national-scale studies. JIFGH includes 386 non-seismically triggered fatal geohazards that caused a total of 979 fatalities in the 1960-2020 period. The temporal trend of fatal geohazards in Jiangxi Province is mainly affected by rainfall and the government geohazard prevention measures. The causes of most fatal geohazards in Jiangxi Province include (i) slope-cutting activities in house construction projects that create steep slopes prone to failure, which threaten the vulnerable residents and buildings nearby and (ii) rainfall that triggers failures of cut slopes. This study not only proposes geohazard prevention advice for Jiangxi Province and tectonically stable areas but also analyzes the significance of provincial-scale studies of hotspots identified in national-scale studies. Therefore, this study contributes to the prevention of fatal geohazards in Jiangxi Province and tectonically stable areas, while also providing an essential reference for other studies of fatal geohazards.

Research on crack evolution law and macroscopic failure mode of joint Phyllite under uniaxial compression.

In order to explore the fracture mechanism of jointed Phyllite, the TAJW-2000 rock mechanics test system is used to carry out uniaxial compression tests on different joint inclination Phyllites. The influence of joint inclination of Phyllite failure mode is discussed, and the progressive failure process of Phyllite is studied. The test results show that the uniaxial compressive strength anisotropy of jointed Phyllite is remarkable. As the inclination increases, it exhibits a U-shaped change; When 30° ≤ α ≤ 75°, the tensile and shear failures along the joint inclination mainly occurs. the joint inclination controls the failure surface form of the Phyllite; The crack initial stress level of the joint Phyllite is 0.30-0.59σf, the crack failure stress level is 0.44-0.86σf. When α = 90°, the σcd value is the largest, and σcd with α = 90° can be used as the maximum reliable value of uniaxial compressive strength of Phyllite. Using the theory of fracture mechanics, it is analyzed that under uniaxial compression of the rock, the crack does not break along the original crack direction, but extends along the direction at a certain angle to the original crack. The joint effect coefficient is proposed to show the influence of the joint inclination on the uniaxial compressive strength of the phyllite. Both the test and simulation results show that when the joint inclination is 60°, the joint effect coefficient is the largest. The compressive strength is the smallest. Numerical simulation analyses the crack evolution law of phyllite under different joint inclination under uniaxial compression, which verifies that there are different failure modes of joint phyllite under uniaxial compression.

The cyclic expansion and contraction characteristics of a loess slope and implications for slope stability.

Loess covers approximately 6.6% of China and forms thick extensive deposits in the northern and northwestern parts of the country. Natural erosional processes and human modification of thick loess deposits have produced abundant, potentially unstable steep slopes in this region. Slope deformation monitoring aimed at evaluating the mechanical behavior of a loess slope has shown a cyclic pattern of contraction and expansion. Such cyclic strain change on the slope materials can damage the loess and contribute to slope instability. The site showing this behavior is a 70-m high loess slope near Yan'an city in Shanxi Province, northwest China. A Ground-Based Synthetic Aperture Radar (GB-SAR) sensor and a displacement meter were used to monitor this cyclic deformation of the slope over a one-year period from September 2018 to August 2019. It is postulated that this cyclic behavior corresponds to thermal and moisture fluctuations, following energy conservation laws. To investigate the validity of this mechanism, physical models of soil temperature and moisture measured by hygrothermographs were used to simulate the observed cyclic deformations. We found good correlations between the models based on the proposed mechanism and the exhibited daily and annual cyclic contraction and expansion. The slope absorbed energy from the time of maximum contraction to the time of maximum expansion, and released energy from the time of maximum expansion to the time of maximum contraction. Recoverable cyclic deformations suggest stresses in the loess are within the elastic range, and non-recoverable cyclic deformations suggest damage of the loess material (breakage of bonds between soil grains), which could lead to instability. Based on these observations and the models, we developed a quantitative relationship between weather cycles and thermal deformation of the slope. Given the current climate change projections of temperature increases of up to 3.5 °C by 2100, the model estimates the loess slope to expand about 0.35 mm in average, which would be in addition to the current cyclic "breathing" behavior experienced by the slope.

Integration of Spatial Probability and Size in Slope-Unit-Based Landslide Susceptibility Assessment: A Case Study.

Landslide spatial probability and size are two essential components of landslide susceptibility. However, in existing slope-unit-based landslide susceptibility assessment methods, landslide size has not been explicitly considered. This paper developed a novel slope-unit based approach for landslide susceptibility assessment that explicitly incorporates landslide size. This novel approach integrates the predicted occurrence probability (spatial probability) of landslides and predicted size (area) of potential landslides for a slope-unit to obtain a landslide susceptibility value for that slope-unit. The results of a case study showed that, from a quantitative point of view, integrating spatial probability and size in slope-unit-based landslide susceptibility assessment can bring remarkable increases of AUC (Area under the ROC curve) values. For slope-unit-based scenarios using the logistic regression method and the neural network method, the average increase of AUC brought by incorporating landslide size is up to 0.0627 and 0.0606, respectively. Slope-unit-based landslide susceptibility models incorporating landslide size had utilized the spatial extent information of historical landslides, which was dropped in models not incorporating landslide size, and therefore can make potential improvements. Nevertheless, additional case studies are still needed to further evaluate the applicability of the proposed approach.

Experimental Investigation of the Compactability and Cracking Behavior of Polyacrylamide-Treated Saline Soil in Gansu Province, China.

Polyacrylamide (PAM) is a water-soluble polymer with the ability to enhance a soil's stability. PAM is currently being used to prevent irrigation-induced erosion and enhance the infiltration in farmland soil. To improve the compaction properties of the saline-soil-based filling material that is used in highway subgrade and the cracking resistance capacity of a saline soil's crust, the consistency limits, compactability, microstructure, and cracking morphology of untreated and PAM-treated saline soil were investigated. The saline soils were sampled from the soil crust and a depth of 2.0⁻3.0 m in Gansu Province, China. Two PAM concentrations (0.1% and 0.5% in mass ratio) were selected. The liquid limits and plastic limits of the saline soil samples from the surface (0⁻0.05 m) and a depth of 2.0⁻3.0 m noticeably increased as PAM concentration increased. The maximum dry densities decreased as PAM concentration and plasticity increased, and the optimum water contents of the two saline soil types did not significantly change. These results suggest that a higher shearing resistance between particles partially prevented compression from occurring during compaction. Mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) test results showed that the PAM agent dispersed the bulky pellets, and the soil's structure was formed by flaky and acicular platelets that filled the micropores. A quantitative analysis of crack patterns showed that the cross-points of the crack network and the crack length decreased as the PAM concentration increased. These results indicate that an increase in PAM reduces the shrinkage strain and the flaws or pores within saline soils. Therefore, PAM's stabilizing effect on saline soil under a wetting⁻drying cycle was proven.

Universal confined tensile strength of intact rock.

Strength criteria for intact rock are essential for the safe design of many engineering structures. These criteria have been derived mainly from tests in the compressive stress region. Very few results have been published for confined, direct tensile tests on intact rock. No appropriate criteria are available for addressing the issue on tensile strength of intact rock at current stage. We present the results of direct triaxial tensile tests on Longmaxi Shales under varying confining stresses. These and the results from previous tests in marble and sandstone prove that the phenomenon of "tension cut-off" at low confining stress and the positive correlations between confining stress and tensile strength above the confining stress threshold for brittle rocks occur also in more ductile rocks like shales. Such findings are consistent with the concept that tensile failure processes for intact rock are universal. Our results demonstrate that friction processes still have a significant role on intact rock strength in the tensile region which is leading to confined tensile failure and transitioning to a purely tensile mode. Further, strength criteria are presented which consider the frictional processes leading to failure under confined, direct tension tests and validated against published tensile strength data.