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.

This paper presents an experimental investigation into the interaction mechanism between aqueous foam and unsaturated granite residual soil during conditioning. Contact filter paper tests and undrained shear tests were used to analyze foam's effects on soil water retention and shear behavior, while surface tension tests, capillary rise tests, and microscopic observations examined the role of soil particles in foam stability. The findings demonstrate that foam-conditioned granite residual soils exhibit three distinct saturation- dependent phases (soil-only, transition, and soil-foam mixture) governed by foam's gas-liquid biphasic nature, with foam injection effectively reducing matric suction in unsaturated conditions. Increasing foam injection ratio reduces shear stress while enhancing pore water pressure, with vertical displacement transitioning from contractive to expansive behavior at low shearing rate. Effective cohesion stress varies with gravimetric water content via a rational function, while other effective cohesion stress and friction angles with respect to foam injection ratio, shearing rate, and gravimetric water content obey exponential relationships. The probability distribution function, cumulative distribution function, and decay pattern of bubbles in foam-only systems and soil-foam mixtures all exhibit exponential relationships with elapsed time. Furthermore, a new water-meniscus interaction model was established to characterize rupture and stabilization mechanisms of foam in unsaturated granite residual soils, with particular emphasis on capillary-dominated behavior. Saturation-dependent particle contact modes were identified for foam-conditioned unsaturated granite residual soils, offering valuable guidance for enhancing soil conditioning protocols in earth pressure balance shield tunneling operations.

Significant movement of in-situ retaining walls is usually assumed to begin with bulk excavation. However, an increasing number of case studies show that lowering the pore water pressures inside a diaphragm wall-type basement enclosure prior to bulk excavation can cause wall movements in the order of some centimeters. This paper describes the results of a laboratory-scale experiment carried out to explore mechanisms of in situ retaining wall movement associated with dewatering inside the enclosure prior to bulk excavation. Dewatering reduces the pore water pressures inside the enclosure more than outside, resulting in the wall moving as an unpropped cantilever supported only by the soil. Lateral effective stresses in the shallow soil behind the wall are reduced, while lateral effective stresses in front of the wall increase. Although the associated lateral movement was small in the laboratory experiment, the movement could be proportionately larger in the field with a less stiff soil and a potentially greater dewatered depth. The implementation of a staged dewatering system, coupled with the potential for phased excavation and propping strategies, can effectively mitigate dewatering-induced wall and soil movements. This approach allows for enhanced stiffness of the wall support system, which can be dynamically adjusted based on real-time displacement monitoring data when necessary.

Triggered by continuous heavy rainfall, a catastrophic large-scale high-locality landslide occurred in Hengshanbei mountain slope of Shangxi Village, Longchuan County, Guangdong Province, China, on June 14, 2022, at 12:10 (UTC + 8). The landslide had an estimated volume of about 1.45 x 105 m3 and resulted in severe damage to the region. To investigate the causative mechanisms of this landslide, a comprehensive study was conducted, involving geological and hydrological surveys of the research area, combined with field investigations, satellite imagery, drone photography, data analysis of rainfall and landslide displacement monitoring, and laboratory experiments. The research focused on analyzing the process of landslide formation and development, trigger factors, destruction characteristics, and instability mechanisms. Additionally, the study employed the Mohr-Coulomb strength theory to explain stress variations during the landslide process. Findings indicated that: (1) the slope soil structure was loose with well-developed pores, mainly composed of kaolinite with strong water absorption properties, causing softening and disintegration of the soil when encountering water, resulting in reduced cohesion and internal friction angle, and overall poor soil properties; (2) continuous heavy rainfall infiltrated the slope through soil pores and eroded channels, increasing pore water pressure and reducing effective stress, subsequently reducing anti-sliding force and increasing sliding force; as well as (3) unfavorable terrain conditions, such as high landslide starting point and high-locality, significant height, and steep slope, lead to landslides running farther and being of larger scale. The study further highlighted that the intrinsic properties of the slope soil were the decisive internal cause of the landslide, while continuous heavy rainfall and adverse terrain were external triggering factors. These findings provide essential insights for understanding and preventing similar landslide disasters.

The role of structural planes in controlling mudstone landslides is a key issue in the study of geo-disasters in the Loess Plateau of China. In this study, the effects of sliding-control structures on the mechanisms of mudstone landslides are investigated via three model experiments with different slope structures. The results show that the hydrological response and failure mode of the experimental slope vary with the structural conditions. The vertical joints serve as preferential seepage paths, which accelerate rainfall infiltration, resulting in earlier responses of volumetric water content and pore water pressure. With the incorporation of vertical joints, the slope failure mode tends to transform from shallow failure to deep-seated failure. The presence of a weak interlayer leads to significant increases in the velocity and runout of the sliding mass. The variation in the slope failure extent and deformation characteristics with varying sliding-control structures further changes the temporal and spatial distributions of volumetric water content and pore water pressure. The different slope failure modes correspond to different sliding-control mechanisms, which are dominated by the types of structural planes and their interactions with hydrological responses. In the action of these mechanisms, pore water pressure and seepage force play significant roles in the reduction of effective stress and shear strength.

Fulfilling the role of a soil conditioner, foam plays a pivotal role in Earth Pressure Balance (EPB) shield tunnelling by enhancing soil properties such as lowering permeability and increasing flowability. This study introduces a macro-model designed to quantify foam penetration behaviour in saturated sand, utilising rheological properties. To validate this model, experiments were conducted to replicate the foam penetration behaviour. Six sand beds characterised by varying particle sizes, along with foam having an expansion ratio of fifteen, were employed for penetration tests under different hydraulic conditions utilising a sand column device. The rheological profile of the foam is described by the power-law model, as also found by rheometer tests, although with different parameters. The flow behaviour of foam within the sand column conforms to the flow equation that governs powerlaw fluids in porous media. The developed model effectively predicts the foam penetration process under varying hydraulic conditions compared with the experimental results. Furthermore, the fitting results of the experimental data indicate that the flow behaviour index of the foam remains approximately 0.09 across all tests, regardless of the type of sand used. In contrast, the model-derived generalised permeability coefficient strongly correlates with the effective particle size (d10) of the sand bed. Overall, the model effectively quantifies the foam penetration behaviour, accounting for changes in infiltration velocity and pore water pressure, which is essential for understanding the transfer of support pressure in EPB shield tunnelling.

Cavity formations by soil dissolution or underground collapses are at the origin of large surface subsidence that constitutes a risk of damage or failure for infrastructures. Soil reinforcement with geosynthetics positioned at shallow depth is an economical and functional solution to reduce the induced surface settlements. Previous research has mainly focused on the load transfer mechanism and the arching effect in cohesionless reinforced backfills when the cavity opens. Experimental and numerical studies dealing with cohesive soils are very rare, although this situation is commonly found in practice. To overcome this lack of knowledge, a numerical study based on Discrete Element Modelling is carried out to better understand the load transfer mechanisms that are mobilized in cohesive embankments prone to underground cavity opening. The results are compared with experimental data obtained on a small-scale laboratory model in terms of vertical and horizontal displacements of both soil and geosynthetics. The numerical results focus on the collapse mechanisms of the cohesive embankment, the load transfer mechanisms, the shape of the vertical load distribution acting on the geosynthetic layer, the strain and traction forces within the geosynthetic sheet.

Channel retreat can be responsible for the significant loss of banks, farmland, and wetlands, leading to drastic changes in fluvial sediment and local river regimes. Although current studies focus on the erosion process of natural river channels, the mechanism by which revetments, such as the flexible mattress, influence bank evolution is still unclear. Hence, by conducting a generalized model experiment, this study investigates the point bar failure process under the mattress protection, i.e., the episodic event when the soil reaches the static equilibrium state. Specifically, a scour hole develops at the junction between the soft and hard materials, causing mattress suspension on the bank toe's side wall and resulting in a reduction coefficient for transverse scouring rate ranging from 0.08 to 0.15. Based on the theories of soil mechanics and river dynamics, the critical conditions for point bar instability were deduced, and a mechanical model describing its erosion process under the mattress protection was established. Furthermore, our model calculated the bank morphology and total erosion volume at different periods in the flume experiment, demonstrating a good agreement with the measured data. Additionally, variations in stability coefficient and forces exerted on soil (including shear strength, gravity, and fluid pressure) of the typical sections during point bar retreat were analyzed. Sensitivity analysis of the bank toe stability emphasized the controlling effect of soil mechanical properties and the negative feedback of mattress weight. The results reveal the interaction mechanism between the mattress protection and point bar failure, theoretically guiding the bank erosion strengthening and river management planning.

Advective heat transported by water percolating into discontinuities in frozen ground can rapidly increase temperatures at depth because it provides a thermal shortcut between the atmosphere and the subsurface. Here, we develop a conceptual model that incorporates the main heat-exchange processes in a rock cleft. Laboratory experiments and numerical simulations based on the model indicate that latent heat release due to initial ice aggradation can rapidly warm cold bedrock and precondition it for later thermal erosion of cleft ice by advected sensible heat. The timing and duration of water percolation both affect the ice-level change if initial aggradation and subsequent erosion are of the same order of magnitude. The surplus advected heat is absorbed by cleft ice loss and runoff from the cleft so that this energy is not directly detectable in ground temperature records. Our findings suggest that thawing-related rockfall is possible even in cold permafrost if meltwater production and flow characteristics change significantly. Advective warming could rapidly affect failure planes beneath large rock masses and failure events could therefore differ greatly from common magnitude reaction-time relations. Copyright (C) 2011 John Wiley & Sons, Ltd.