Landslides frequently occurred in Jurassic red strata in the Three Gorges Reservoir (TGR) region in China. The Jurassic strata consist of low mechanical strength and poor permeability of weak silty mudstone layer, which may cause slope instability during rainfall. In order to understand the strength behavior of Jurassic silty mudstone shear zone, the so-called Shizibao landslide located in Guojiaba Town, Zigui County, Three Gorges Reservoir (TGR) in China is selected as a case study. The shear strength of the silty mudstone shear zone is strongly influenced by both the water content and the normal stress. Therefore, a series of drained ring shear tests were carried out by varying the water contents (7%, 12%, 17%, and 20%, respectively) and normal stresses (200, 300, 400, and 500 kPa, respectively). The result revealed that the residual friction coefficient and residual friction angle were power function relationships with water content and normal stress. The peak cohesion of the silty mudstone slip zone increased with water content to a certain limit, above which the cohesion decreased. In contrast, the residual cohesion showed the opposite trend, indicating the cohesion recovery above a certain limit of water content. However, both the peak and residual friction angle of the silty mudstone slip zone were observed to decrease steadily with increased water content. Furthermore, the macroscopic morphological features of the shear surface showed that the sliding failure was developed under high normal stress at low water content, while discontinuous sliding surface and soil extrusion were occurred when the water content increased to a saturated degree. The localized liquefaction developed by excess pore water pressure reduced the frictional force within the shear zone. Finally, the combined effects of the slope excavation and precipitation ultimately lead to the failure of the silty mudstone slope; however, continuous rainfall is the main factor triggering sliding.

The residual shear strength (RSS) of unsaturated soils is a crucial parameter for the reliable analysis and design of geostructures constructed with or within unsaturated soils undergoing large shear deformation. For investigating the RSS of unsaturated soils, two sets of data are specifically generated on the poorly graded sand with silt and Indian Head till using suction-controlled ring shear tests and three more sets (i.e., silty sand (SM), silty clayey sand, and fat clay) are gathered from the literature. A model is proposed extending two approaches for predicting the RSS for both coarse- and fine-grained unsaturated soils. In this model, the suction contribution was calculated considering the loss of degree of saturation due to shearing, which was described as a nonlinear function of degree of saturation. The capability of the proposed model is validated with the five sets of data using two different approaches. The best-fitting approach that is based on three fitting parameters provides good predictions. The approximate approach performs well for four studied soils, except for SM soil; this approach is simple for use in engineering practice because no fitting parameters are required. The proposed model is valid for the suction range where degree of saturation is higher than the residual degree of saturation.

Soil constitutive models are widely investigated and applied in soil mechanical behaviors simulation; however, the damage evolution process of soil with various shear deformation behaviors was rarely studied. This study introduces a novel shear constitutive model for slip zone soil considering its damage evolution process. Firstly, an innovative method for determining the shear stiffness is proposed to assess the damage degree of slip zone soil during shear deformation. Further, a damage evolution model based on the log-logistic function is derived to characterize the damage evolution process of slip zone soil, and a new shear constitutive model based on the damage evolution process is subsequently proposed. Both the damage evolution model and the shear constitutive model are verified by the ring shear test data of the slip zone soil from the Outang landslide in the Three Gorges Reservoir area of China. Compared to the traditional peak-solving constitutive model based on the Weibull distribution, the proposed shear constitutive model has the distinct advantage of describing not only the brittle (strain softening) mechanical behavior but also the ductile and plastic hardening mechanical behavior of soil. In summary, this method offers a rapid determination of the damage evolution process and the shear behavior constitutive relationship of slip zone soil in landslides.

The reactivation mechanism of multi-slide landslides entails high complexity, and the shear mechanical properties of high groundwater-level landslides are crucial for analyzing the formation mechanism of reactivated landslides. Taking the K39 landslide of Wenma Expressway in Yunnan Province as the research object, we identified the geological and hydrogeological conditions of the landslide, the physical and mechanical properties of the slip zone soil, and the landslide deformation law using geological mapping, geotechnical engineering, indoor testing, and in situ monitoring. The results show the landslide exhibited alternating acceleration and deceleration movements under seasonal heavy rainfall and high groundwater levels. The shear strength of the soil in the deep sliding zone was greater than that of the soil in the shallow sliding zone. The deep and shallow sliding zone soils showed a decrease in shear strength with increased water content. Moreover, the residual strength of the deep sliding zone soil displayed a negative rate with an increased shear rate. In contrast, the residual strength of the shallow sliding zone soil exhibited a positive rate. Furthermore, under different shear rates, the residual internal friction angle and cohesion of the deep sliding zone soil decreased with increased water content, whereas only the residual internal friction angle of the shallow sliding zone soil followed this pattern. Finally, we performed a sensitivity analysis using the GA-BP neural network for the ring shear test parameters of the deep and shallow sliding zone soils, which included consolidation pressure, water content, and shear rate. Our analysis revealed that the residual strength of deep sliding zone soils is most affected by water content, whereas the residual strength of shallow sliding zone soils is most affected by consolidation pressure. Furthermore, it was found that the effect of water content on residual strength is much greater than the effect of shear rate on residual strength for both deep and shallow sliding zone soils. The study results contribute to a unified understanding of how shear rate affects residual strength mechanisms, support research on shear mechanical properties for multiple landslide revivals, and inform engineering practices and policies in landslide-prone areas.

Slope stability analysis is a classical mechanical problem in geotechnical engineering and engineering geology. It is of great significance to study the stability evolution of expansive soil slopes for engineering construction in expansive soil areas. Most of the existing studies evaluate the slope stability by analyzing the limit equilibrium state of the slope, and the analysis method for the stability evolution considering the damage softening of the shear zone is lacking. In this study, the large deformation shear mechanical behavior of expansive soil was investigated by ring shear test. The damage softening characteristic of expansive soil in the shear zone was analyzed, and a shear damage model reflecting the damage softening behavior of expansive soil was derived based on the damage theory. Finally, by skillfully combining the vector sum method and the shear damage model, an analysis method for the stability evolution of the expansive soil slope considering the shear zone damage softening was proposed. The results show that the shear zone subjected to large displacement shear deformation exhibits an obvious damage softening phenomenon. The damage variable equation based on the logistic function can be well used to describe the shear damage characteristics of expansive soil, and the proposed shear damage model is in good agreement with the ring shear test results. The vector sum method considering the damage softening behavior of the shear zone can be well applied to analyze the stability evolution characteristics of the expansive soil slope. The stability factor of the expansive soil slope decreases with the increase of shear displacement, showing an obvious progressive failure behavior. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V.

A substantial number of shallow landslides can still occur in areas with high vegetation cover under extreme rainfall. The cause and mechanism of this type of landslide remain unknown, and thus a case study is selected for study in this paper. The extreme rainfall from June 10-13, 2019, caused mass landslides throughout Mibei Village. Most landslides happened in well-vegetated areas, and some even transformed into debris flows. This paper presents detailed field investigations on 31 of them, the result of which were used to perform numerical simulations and ring shear tests. The objective of this study was to investigate the damage cause and mechanism of well-vegetated soil slopes under extreme rainfall. The results indicate that vegetation exerts a dual effect on slope stability. Within the rhizosphere zone, roots significantly enhance the soil shear strength, thereby reducing the likelihood of slope instability. Simultaneously, the rhizosphere zone exhibits enhanced permeability and acts as a relatively impermeable layer at the bottom, which makes the shallow layer of the slopes more easily saturated by rainfall infiltration. The slopes are prone to instability at the soil layer situated below the bottom of the rhizosphere zone, and mostly are translational slides with suddenness. The main triggering factor for the landslide is the presence of positive pore water pressure in the soil, which makes the matrix suction lost and the effective stress reduced. The slip zone soil exhibits negative dilatancy, while the saturated slip soil undergoes liquefaction, which can make landslides conversion into debris flows.

This article centers its investigation on high-moisture-content solidified lightweight soil in the Binhai New Area of Tianjin. It examines the shear mechanical properties under conditions of significant deformation, supplying pertinent reference values and theoretical foundations for engineering practice. The results demonstrate that the curve for solidified lightweight soil under substantial shear conditions manifests a strain-softening tendency. As normal stress escalates, both peak and residual strength increase while the degree of softening diminishes. The peak strength envelope at varying shear rates forms an upward zigzagging line, with the bending degree of the line augmenting as the rate increases. As the shear rate elevates, the cohesive strength within the peak strength parameters intensifies, the internal friction angle remains constant, and the residual strength parameters remain unaffected by the shear rate. This furnishes theoretical guidance for the applied engineering of blown-cured lightweight soil.