Flow instability impacts negatively on hydraulic structures. Changes in water pressure or the periodic impact of water flows cause structural damage to channels. The rapid increase in water depth leads to overflows or sprays, which erode soil adjacent to channels. In this study, flow instability was examined through the basis of theories and experiments. The theoretical discriminants for flow instability were inferred by Vedernikov number and the effect of slopes on the Froude number was considered. A rectangular cross- channel was selected for the experiments. The experimental results were compared with theories, it was shown that when the flow conditions were on the margin of instability, the discriminant established by this study is able to accurately determine the occurrence of instability. Through this new discriminant, the discrepancy which appears in traditional method can be avoided. The presented results are ideal for channel design and offer new approaches for flow instability prevention.

Calcareous sands provide the foundational support for various marine infrastructures. In the harsh marine environment, earthquake or wave loads apply multidirectional cyclic shear stresses to the foundation soil. To explore the undrained multidirectional cyclic response of sand, a series of simple shear tests were performed on reconstituted sand specimens considering the effect of phase difference (theta). By comparing the results with those of siliceous sand under similar conditions, the behavior of calcareous sand under multidirectional cyclic loading became clear. The results demonstrated that calcareous sand shows a lower degree of cyclic instability compared to siliceous sand, corresponding to the weaker strain-softening observed in calcareous sand during monotonic shear tests. The trend in normalized pore water pressure evolution in siliceous sand exceeds that in calcareous sand. Furthermore, under multidirectional cyclic shear conditions, the liquefaction resistance decreases by 30 % in extreme cases, irrespective of sand type. The liquefaction resistance of calcareous sand surpasses that of siliceous sand. However, as the cyclic stress ratio decreases, the reverse trend is observed, regardless of the impact of theta. Subsequently, the possible causes of the above experimental phenomena are explored from the perspectives of shear modulus and energy dissipation.

Soil-rock mixtures (S-RM) are prevalent in both nature and practice, and stability of S-RM slopes is one of the focuses for engineers. In addition to soil strength, seepage erosion is one of the main factors affecting the stability of S-RM slopes. As water infiltration complicates the multi-field coupling effects and micro-scale mechanical behaviors of S-RM, it is essential to investigate seepage-induced S-RM landslides from both macro and micro perspectives. This study proposed a CFD-DEM fluid-solid coupling method, and the method was validated with Darcy experiments and lab slope stability experiments. The method was then applied to analyze seepage-induced slope instability, focusing on the impact of rock content and rock shape. The results indicate that slope failure under seepage showed the same characteristics as debris flow, with instability features such as sliding surfaces, damage range, and particle motions varying according to rock content and shape. As rock content increased, the accumulation of slope transitions through three distinct modes. Slope was prone to failure along the soil-rock interface, and low rock content further impaired the stability. The slope deformation was primarily driven by changes in particles contact. Once slope instability occurred, the system tended to adjust particle contacts to achieve new state of equilibrium.

With the continued development of water resources in Southwest China, fluctuations in water levels and rainfall have triggered numerous landslides. The potential hazards posed by these events have garnered considerable attention from the academic community, making it imperative to elucidate the landslide mechanisms under the combined influence of multiple factors. This study integrates laboratory tests and numerical simulations to explore the instability mechanisms of landslides under the combined effects of rainfall and fluctuating water levels, as well as to compare the impacts of different factors. Results indicate that the sensitivity of landslide deformation decreases as the number of water level fluctuations increases, exhibiting a gradually stabilizing tendency. However, the occurrence of a heavy rainfall event can reactivate previously stabilized landslides by increasing pore water pressure and establishing a positive feedback loop with rainfall infiltration. This process reduces boundary constraints at the toe of the slope, promotes the development of an overhanging surface, and ultimately leads to overall instability and landslide disaster. Under the same rainfall intensities, the presence of water level fluctuations prior to rainfall significantly shortens the time for the landslide to reach a critical state. The key mechanisms contributing to landslide failure include terrain modification, fine particle erosion, and outward water pressure, all of which generates substantial destabilizing forces. This research offers valuable insights for the monitoring, early warning, and risk mitigation of landslides that have already experienced some degree of deformation in hydropower reservoir areas.

In urban regions with karst developments, grouting is commonly utilized to fill cavities. However, the extent and control standards of grouting reinforcement are primarily determined through experience and field testing, which poses challenges in ensuring its effectiveness. Based on the instability mechanism of surrounding rocks in underwater karst shield tunnels, this study develops a mechanical model for analyzing the grouting reinforcement extent of such tunnels using strength theory. The reinforcement range for karst formations at various tunnel locations is clarified, and corresponding grouting reinforcement control standards are proposed based on cusp catastrophe theory. The findings indicate the following: the primary cause of surrounding rock instability in underwater karst shield tunnels is that the reduction in surrounding rock thickness during shield tunneling modifies the original constraints and boundary conditions and disrupts the initial equilibrium state. These changes influence the water content of the surrounding rocks and disturb the surrounding rock and soil mass, leading to surrounding rock instability. When grouting causes damage to the surrounding rocks between the karst and tunnel, the system is simplified into cantilever beam and plate models for analysis. It is determined that the grouting reinforcement extent is primarily influenced by factors such as karst size, properties of the karst filling material, and tunnel span. The total potential energy of the rock mass between the karst and tunnel is calculated, leading to the development of an instability and catastrophe model for the surrounding rocks. The proposed grouting reinforcement control standards are mainly dependent on factors such as the distance of the karst, characteristics of the reinforced surrounding rocks, shield machine support force, material properties post-reinforcement, and karst size.

The restraining effect of soilbags inhibits soil dilatancy, enhancing the strength and stiffness of the wrapped soil. As a permanent slope protection structure (SSPS), the application of counterpressure enhances stability by improving slope surface stiffness and limiting deformation. While reinforced slopes have been extensively studied, mechanistic investigations into the stability and failure processes of SSPS remain limited. This study numerically investigated the macro-meso mechanisms of SSPS instability using the discrete element method. Macroscopically, rainfall infiltration increases water absorption, resulting in longitudinal settlement, deformation, and eventual instability. With a friction coefficient of 0.5, the lower soilbags resist sliding forces until the front soilbags are damaged. Inadequate sufficient friction causes the front soilbags to be displaced outward, leading to structural collapse as the lower soilbags bear the additional load. Microscopically, geosynthetic wrapping restrains soil dilatancy, promoting tighter particle arrangements and secondary reinforcement through soilbag expansion. During instability, primary contact forces concentrate on longitudinal settlement, vertical back pressure, and downslope sliding, with force chain evolution revealing slip band formation. Soilbags facilitate coordinated particle deformation and stress distribution, transitioning from anisotropic to isotropic states as instability progresses. These findings enhance the understanding of SSPS instability mechanisms, providing guidance for more reliable design and construction practices.

This study examines the stability of the Huangyukou landslide in Yanqing District, Beijing, under varying rainfall conditions, focusing on the effects of rainfall infiltration and surface runoff on slope stability. Using a combination of field surveys, geophysical methods, drone photogrammetry, and laboratory testing, a high-precision 2D and 3D numerical model was developed. A hydrological-soil-structure coupling model was employed to simulate rainfall-induced infiltration and runoff processes, revealing that increased saturation and pore water pressure significantly reduce shear strength, enhancing the risk of slope failure. Stability analysis, using a reduction factor method, yielded stability coefficients of 1.06 and 1.04 for 20-year and 100-year return period rainfall scenarios, respectively. The results highlight the critical role of rainfall in destabilizing the upper layers of dolomite and shale, with significant deformation observed in the middle and rear slope sections. This research provides a comprehensive framework for assessing landslide risk under extreme rainfall events, offering practical implications for risk mitigation in similar geological contexts.

Loess slopes are susceptible to rainfall due to the water sensitivity and collapsibility of loess. The aim of this study is to investigate the instability mode, failure mechanism and control effect of homogeneous loess landslide under rainfall by using physical model experiments and numerical simulation, combined with a new anchor cable with negative Poisson ratio (NPR) structural effect. The findings indicated that the loess slope's failure under heavy rainfall is characterized by progressive shallow flow-slip instability, encompassing three deformation modes and seven deformation characteristics. Water content, pore water pressure and earth pressure monitoring instruments capture the dynamic response of internal hydromechanical properties within the loess slope during intermittent heavy rainfall, clarifying its failure mechanism. Rainfall leads to soil softening and a reduction in strength. The effective stress of shallow soil and potential sliding surfaces diminishes due to decreased matrix suction and increased pore water pressure. The accumulation of internal and external deformation eventually leads to the disintegration of the shallow layer of the loess slope. Numerical simulation results indicated that rainfall significantly affects the shallow layer of the loess slope, with greater subsidence deformation observed at the slope's crest. Indoor and field monitoring findings revealed the pattern of Newton force on the loess slope in response to rainfall and demonstrated its seasonal dynamics, characterized by an increase during the thaw-collapse and flood periods, followed by a decrease in the frost-heave period.

The proportional strain loading test is a prevalent method for investigation diffuse instability. The majority of current research concentrates on narrowly graded materials, with relatively less focus on binary mixtures under proportional strain loading. Therefore, a series of numerical tests have been conducted using the discrete element method to study the influence of fine content and strain increment ratio on the binary mixtures. The test results show that the fine content of binary mixtures is intimately connected to the critical strain increment ratio which precipitate a transition from stability to instability. Binary mixtures characterized by a low stress ratio at the onset of instability also demonstrate a heightened sensitivity to shifts in strain increment ratio. The macroscopic responses, such as the stress ratio at the onset of instability, shear strength, and pore water pressure, exhibit different trends of variation with the fine content compared to microscopic responses, including coordination number, friction mobilization index, and the proportion of sliding contacts. Furthermore, the anisotropy coefficient is introduced to dissect the sources of anisotropy at onset of instability, revealing that strong contact fabric anisotropy can mirror the evolution of the stress ratio. The stress ratio at onset of instability is predominantly influenced by anisotropy in contact normal and normal contact force.

The direct simple shear (DSS) test is one of the most popular testing techniques for measuring the shear strength of soils and mine waste tailings. However, uncertainties remain regarding the suitable sample diameter and whether a DSS sample should be saturated or can be tested without flushing with water. Various designs and configurations of shearing caps are also incorporated in different DSS equipment with little information on their performance and comparison soil shearing behavior with different caps. This study examines the monotonic shearing behavior, static liquefaction and instability, and post-liquefaction strength of a coarse oil tailings sand in extensive series of monotonic DSS tests on two different specimen diameters of 50 mm and 70 mm. Moist-tamped samples are reconstituted with and without flushing with water and sheared using top and bottom caps with concentric wedges and projecting pins. These are examined across a wide range of consolidation vertical stress, and for three different stress paths corresponding to undrained (CV), drained (CVS), and constant-shear unloading (CSU) shearing paths. Static liquefaction and instability were triggered in the CV and the CSU tests at the emergences of undrained strength reduction and volumetric collapse, respectively. The results show little effects of sample flushing and diameter on the static liquefaction triggering and post-liquefaction shear strengths of the tailings sand. The effect of sample diameter was primarily observed on the one-dimensional compressibility and volumetric strain of samples. The smaller diameter specimens underwent smaller volume changes during one-dimensional compression and drained shearing compared with the larger D = 70 specimens.