Bed Deformation Research Articles

Road Site Inspection and Measures to Avoid Further Deformation

Strength and stability of the roadbed and engineering structures in permafrost conditions and deep seasonal freezing of soils always remain complex scientific, technical and technological problem in construction and reconstruction of roads in northern regions. Particularly important is the spread of subgrade subsidence during construction and further operation and maintenance of roads in these regions. The road destruction is mostly caused by both global warming and geological and hydrogeological conditions.To prevent the deformation development, it is necessary to provide a number of special engineering measures to prevent thawing of permafrost soils or strengthen supra-permafrost layers of foundations. The purpose of the work is to study methods for eliminating deformation of the road bed in permafrost and choosing the main method for eliminating subsidence of the road.The article discusses several ways to solve the problem of eliminating subsidence on of the federal highway. Options for reconstruction of the roadbed are considered. These are different methods of thermal stabilization, i.e., horizontal (sloping) thermal stabilization system with inclined thermal stabilizers, temporary vertical thermal stabilizers for pre-construction freezing of foundation soils, and hybrid thermal stabilization system (horizontal and vertical thermal stabilizer in one product). A method is proposed for stabilizing the road base by installing soil-cement piles. The main advantages of and disadvantages the methods are noted and cost assessment is made. A conclusion is drawn on the effectiveness of different methods to restore the subgrade structure in order to ensure the road base stability. The main method is elimination of subsidence by stabilizing the road base with installation of soil-cement piles.

Series solution and its extension for the nonlinear flow response of soft hair beds

In nature and engineering applications, flexible fiber beds covering biological surfaces can play a role in reducing resistance. These fibers deform under the action of fluids, and this deformation affects the fluid flow state, forming a complex fluid–solid interaction phenomenon. To quantitatively analyze these issues, the physical model is simplified. We focus on the deformation of a soft hair bed caused by Stokes flow. Additionally, we study the deformation of a single hair under Stokes flow in greater detail. The deformation problem of an elastic single fiber in a channel caused by Stokes flow can be described by a nonlinear integral equation. We have obtained a new series solution, which has been compared with the previous perturbation method to verify the accuracy and effectiveness of the series solution. Meanwhile, we have further provided an extended form of flexible fiber deformation through experimental fitting. This fluid–solid interaction problem involves multiple fields and is very important in many natural and engineering systems. The research in this paper can not only help us better understand complex phenomena in nature but also delve into the interaction mechanism between fluids and solids, providing a theoretical basis for future scientific research and engineering applications.

Morphodynamics of sine-generated meandering streams under variations of the width-to-depth ratio: A numerical investigation

Meandering is one of the most common planforms of rivers, however, the ecological and fluvial integrity of meandering rivers are under challenges, resulting from increased anthropogenic pressure and extreme climate events. The present study elucidates the morphodynamics of meandering streams under variations of width-to-depth ratio (B/h) by delving into the two dynamic processes underlying within the flow, i.e. cross-circulatory motion and convective redistribution of the longitudinal flow velocity. Series of numerical simulations are performed in 70° sine-generated meandering streams under increasing flow depth that necessitates a wide range of B/h, characterizing flume experiments to large scale natural rivers. The radial flow structures and the early bed deformation patterns are computed and elucidated. Among the classical cross-circulatory flow patterns, outer bank cell occurs for flows having B/h < 9. Next, the different bed deformation patterns reveal the shift of significance in the driving forces of the bed deformation regarding different B/h ratio. At B/h = 100, bed deformation is dominated by the convective redistribution of the downstream velocity that causes longitudinal adjacent erosion–deposition zones. This implies downstream migration tendency of meander loops. At B/h = 12, the intensified cross-circulatory flow deflects the sediment trajectory, causing extra laterally adjacent erosion–deposition zones that necessitate the lateral expansion tendency of the loop. Decrement of B/h ratio (from 100 on ward) critically enhances the non-equilibrium transport of sediment that can lead to active fluvial evolution of meandering rivers. Overall, the results can be used to improve river management and restoration.

Advances in research on the ballast bed stability on railway bridges

The rapid development of high-speed railway bridges in China has brought enormous challenges to the ballast bed stability on the bridge. With the increase in the bridge span, the bridge deformation under the temperature load increases gradually, as well as the stability difference of ballast bed between different regions. Moreover, the track vibration response under high-speed driving conditions is larger, which can easily cause differential settlement, flying ballast, and deterioration of the ballast bed, thereby affecting the bearing capacity and stability of the ballast bed. In view of this, this article summarizes relevant research work on a ballast bed on the bridge from the perspectives of the resistance, deformation, and acceleration response of the ballast bed. In addition, this study discusses the existing measures for improving the ballast bed stability on the bridge, including the optimization of the sleeper and ballast bed structures. Finally, the main issues that need to be addressed to improve the ballast bed stability on the bridge are pointed out, and ideas for future research on ballast beds on bridges with different spans and structures are proposed.

Mechanisms and Morphological Time Scales of Avulsed Channel Process on the Modern Yellow River Delta

AbstractThe modern Yellow River Delta (YRD) has witnessed frequent channel avulsions followed by the morphological processes of “wandering‐ short‐lived braiding ‐merging” in history. However, process‐based investigation and relevant physics behind these processes remain poorly constrained. The present study complements the understanding for this evolution through numerical experiments. This is achieved by applying a two‐dimensional (2‐D) depth‐averaged fully coupled morphodynamic model that considers the feedbacks of sediment‐laden flow and bed deformation to a schematized fan‐shaped YRD. Under an uneven bed of uniform sediment, the avulsed processes of “wandering‐ short‐lived braiding ‐merging” during new channel routing on the YRD are satisfactorily reproduced by the present modelling in terms of the time evolution of planar channel pattern and channel length. Moreover, the mechanisms of the avulsed evolution have been elucidated through factor analysis on delta slope, incoming flow‐sediment conditions and artificial trenching, etc. It is suggested that morphological stability could be reached when a single meandering channel is finally sustained, of which the time scale is 4–8 years echoing the documented data of abandoned channels on the YRD. In addition, quantitative criteria have been advocated to help distinguishing the sub‐stages of the avulsed evolution together with the characteristic pattern judgement. Based on the above, implications for the YRD management are discussed.

Numerical Simulation of River Channel Change in the Suspended Sediment-Dominated Downstream Reach of the Sangu River

This study aims to clarify the characteristics of the riverbed deformation, bank erosion, and channel changes in the lower Sangu River basin using a depth-averaged 2-D flow model where suspended sediment transport is dominant and its flow characteristics are influenced by active tides. A 45 km long area including the river mouth is computed using the 2-D model, and the results are compared with the observed channel changes. The computation results show that bed deformation and channel change are mainly caused during the ebb tide period of the spring tide particularly in an area of about 10 km from the river mouth. The detail study domain calculation results show that the flow concentration at the bend area causes the bed erosion there, and the eddy separated from the main stream causes the sediment deposition at the inner bank, while this eddy is not developed at the outer bank, resulting in the bank erosion there. Through these investigations, the characteristics and mechanisms of the morphodynamics in the lower Sangu River reach, particularly the potential of the combination of tides and floods to enhance riverbed deformations and the associated bank shifts, have been clarified.

Experimental study on the dynamic characteristics of bi-block ballastless track under the combined action of vehicle load and temperature gradient

The unit ballastless track structure, persistently exposed to environmental conditions, experiences significant vertical temperature gradients, leading to warping deformation of the track bed and impacting track smoothness. This study investigates the relationship between temperature gradients and warping deformation of the track bed through the analysis of on-site measurement data combined with a dynamic simulation model. It also examines how temperature gradients influence the dynamic response and the interlayer contact area of the ballastless track. Findings reveal that vertical displacement of the track bed follows a diurnal cycle and shows a positive correlation with temperature gradients, with pronounced differences between the slab's corners and center. Under a positive temperature gradient, the vertical displacement of the track bed is high in the middle and low at the ends, with the vehicle load causing longitudinal tensile strain in the slab’s side surface center. Conversely, under a negative temperature gradient, the vertical displacement is low in the middle and high at the ends, with the vehicle load inducing longitudinal compressive strain in the slab’s side surface center. Additionally, warping deformation of the track bed leads to incomplete contact with the base plate, creating a local support state and reducing interlayer contact area as the gradient increases. Positive temperature gradients result in an "oval" distribution of interlayer delamination areas, while negative gradients mainly affect the sides and ends. Dynamic loads from trains temporarily counteract warping deformation, briefly increasing the interlayer contact area, but their effectiveness diminishes as temperature gradients rise.

Impacts of packed bed polydispersity and deformation on fine particle transport

AbstractStatic granular packings play a central role in numerous industrial applications and natural settings. In these situations, fluid or fine particle flow through a bed of static particles is heavily influenced by the narrowest passage connecting the pores of the packing, commonly referred to as pore throats, or constrictions. Existing studies predominantly assume monodisperse rigid particles, but this is an oversimplification of the problem. In this work, we illustrate the connection between pore throat size, polydispersity, and particle deformation in a packed bed of spherical particles. Simple analytical expressions are provided to link these properties of the packing, followed by examples from Discrete Element Method (DEM) simulations of fine particle percolation demonstrating the impact of polydispersity and particle deformation. Our intent is to emphasize the substantial impact of polydispersity and particle deformation on constriction size, underscoring the importance of accounting for these effects in particle transport in granular packings.

The dynamical response of turbulent flow to bed deformation under submerged discharge and plane vertical impinging jets

Abstract On the conditions of discharge under sluice gate and vertical jets, the research on the dynamical response of hydraulic elements to bed deformation is much concerned in the field of high-speed flow and hydraulic dredging but not be solved yet. Research achievement from numerical simulation are drawn as follows: (1) Under sluice gate discharge condition, the response of hydraulic elements to bed deformation is that the bed is deforming in the direction of the mechanical energy loss, the rate of the bed deformation and the mechanical energy loss near the local scour hole and the horizontal distance from the reattachment point to the maximum scour depth are all continuously varying with the scouring of time and synchronously approaching to its own certain value. (2) Under vertical jets discharge condition, the response of hydraulic elements to bed deformation is that the rate of bed deformation and the pressure of the maximum scour depth is monotonous continuously decreasing with the scouring of time and synchronously approaching to its own certain value.

Flow resistance of emergent rigid vegetation in steady flow

Abstract Enhanced understanding of flow resistance in open channels with emergent vegetation is essential for flood management and river ecosystem restoration. The presence of vegetation can significantly alter bed resistance, leading to a challenge in accurately predicting flow discharge, water levels, sediment transport, and bed deformation. Previous studies on vegetated flows have focused on vegetation resistance, on which the impact of vegetation has been ignored or poorly estimated. This study proposes a new analytical model, built upon the momentum conservation law, to predict flow resistance to vegetated zones in a plain bed without bed forms, explicitly quantifying bed resistance and vegetation resistance in a corollary manner. The proposed model is benchmarked against five typical sets of laboratory experiments. It is demonstrated that the present model using a modified logarithmic velocity distribution performs best, whereas that assuming a uniform velocity profile considerably overestimates the vegetation resistance and neglects the effect of vegetation on bed resistance. The ratio of bed resistance to the total resistance is shown to range between 5% and 40%, and it decreases with increasing vegetation density and decreases with water depth. Therefore, bed resistance cannot be ignored when modelling shallow water flow with sparsely distributed vegetation. It is also revealed that vegetation arrangements significantly affect flow resistance, and therefore a model incorporating the effect of vegetation arrangement performs better. Overall, the present model facilitates a viable and promising tool for quantifying flow resistance in emergent vegetated channels.

Deformation of vegetated channel bed under ice-covered flow conditions

A common feature of cold regions is the presence of ice cover on water surface. In the presence of vegetation in the channel bed which is normally leafless in winter, interaction between river ice and vegetated channel bed becomes very complex. In the present study, the impacts of ice cover and submerged leafless vegetation on channel bed deformation and flow resistance have been investigated based on 144 experiments conducted in a large-scale outdoor flume. The independent variables associated with the maximum scour depth around vegetation elements have been assessed and equations have been developed. Results indicate that the most important variable affecting the maximum scour depth around vegetation under ice-covered flow conditions is the ratio of the ice cover roughness to the bed roughness (ni/nb). However, under open channel flow conditions, the flow Froude number is the most important variable influencing the maximum scour depth. The methods based on the law of the wall provide reliable Manning's coefficient for both smooth and rough ice covers in the presence of submerged vegetation in the channel bed. As the vegetation density increases, the size of scour holes becomes smaller. With the increase in vegetation density, the median grain size of the armour layer in scour holes decreases correspondingly. When vegetation elements are placed in a staggered configuration, the median grain size of the armour layer in scour holes is less than that of squared configuration of vegetation elements. Regardless of the roughness of ice cover on water surface and the grain size of sediment in the bed, the maximum depth of scour holes always occurs at the upstream front face of each vegetation element. Under the rough ice-covered flow condition, the maximum depth of scour holes is more than that under the smooth ice-covered and open flow conditions.

Simulation analysis of the effect of a small amount of fine particle added to a slurry on the liquid behavior and forces acting on the main particles

The viscosity of highly concentrated slurries is reduced by adding a small amount of fine particle with the same material of main particles (high concentration ratio particles per total concentration) in the slurries. We investigated how added fine particles affect the liquid behavior and forces acting on the main particles in the slurry by using direct numerical simulations, and we examined one factor that contributes to the viscosity reduction mechanism in highly concentrated slurries containing a small amount of fine particle. The results indicated that whether fine particles are coated (they contact) on the surface or not have little influence on the change in forces acting on the main particles. In highly concentrated slurries containing a small amount of fine particle, there was a large change in the liquid velocity between and around the main particles. From these results, we proposed a possible mechanism to assist in reducing the viscosity of highly concentrated slurries containing a small amount of fine particle. The magnitude and direction of the forces acting on each main particle were non-uniform by adding fine particles, resulting in non-uniform strength within the main particle bed in the slurry, which could facilitate particle bed deformation and assist in reducing the viscosity.

A damage constitutive model of layered slate under the action of triaxial compression and water environment erosion

Based on the macroscopic structure control theory, The slate with a significant bedding plane is a composite rock mass composed of rock blocks containing microscopic defects, joint surface closure elements, and shear deformation elements. Considering the coupling damage effect of water erosion and triaxial compressive load on bedding structure plane, the transversely isotropic damage constitutive model of slate under triaxial compressive load is derived with the dip angle of bedding and confining pressure as the variable. Firstly, based on the statistical theory of continuous damage mechanics and the maximum tensile strain criterion, the transversely isotropic deformation constitutive model of rock block with micro-defects is given; Secondly, based on the phenomenological theory of closed deformation and shear-slip deformation mechanism of layered structural plane under the coupling action of water erosion and triaxial compression load, the calculation formula of axial deformation of layered structural plane under the coupling action is given; Finally, to verify the accuracy of the established constitutive model, triaxial compression tests are carried out to study the influence of dip angle and confining pressure on the macroscopic mechanical properties and mechanism of slate. The results show that: the established triaxial compression damage constitutive model of bedding slate can accurately describe the stress–strain relationship of bedding slate after water environment erosion. With the increase of bedding dip angle, the strength and deformation capacity of the bedding slate first decreases and then increases, showing a U-shaped distribution as a whole. There are three main types of failure: tension shear composite failure, shear slip failure, and splitting tension failure.

Coupled two-dimensional model for heavily sediment-laden floods and channel deformation in a braided reach of the Lower Yellow River

It is difficult to predict the detailed morphodynamic processes induced by heavily sediment-laden floods in alluvial rives, due to the complexity in topography and the significant channel evolution rates, especially in a braided reach of the Lower Yellow River (LYR). A two-dimensional morphodynamic model using a coupled solution approach was firstly developed, with the effects of sediment concentration and bed deformation being directly considered in the hydrodynamic governing equations. The finite volume method with the HLL-MUSCL scheme was adopted in the numerical solution, which can obtain second-order accuracy in space and time. In addition, unstructured meshes were used to accommodate the irregular boundaries. The proposed model was validated using measurements in the braided reach of the LYR during the flood events of 2020 and 2004, with general agreement obtained between the calculated and measured hydrographs of flow and sediment concentration. The highest accuracy was achieved in the simulation of discharge, with a Nash-Sutcliffe efficiency coefficient (NSE) of 0.92. The simulation accuracy in sediment concentration was improved by the coupled approach, with the relative error of the calculated peak value decreasing from 17 % to 9 %. In addition, transport characteristics of graded suspended sediments were well simulated, with the fine fraction being transported downstream and the medium fraction depositing in the study reach. Influences of channel adjustments on the sediment transport processes were investigated during a flood event. The results demonstrated that the incised channel in 2020 was characterized by a larger water depth and smaller velocity, as compared with the shallow channel in 2004. Consequently, the peak discharge decreased and the flood propagation time was extended under the 2020 topography, associated with the lower sediment discharge at the outlet due to the reduced sediment transport capacity.

Impact of speed and axle load on the static and dynamic mechanical behavior of the ballast bed laying under sleeper pads

With the evolution of railway infrastructure towards accommodating higher speeds and heavier loads, the under sleeper pad (USP) has emerged as a critical component in mitigating the deterioration of the ballast bed and reducing maintenance requirements. This study aims to enhance the understanding of the suitability of the USP for railway lines and to investigate the static and dynamic mechanical behaviors of the ballast bed (with USP) under the train load. To achieve this, the study conducts on-site tests and employs the discrete element method-multi-flexible body dynamics (DEM-MFBD) coupling analysis to develop a three-dimensional high-fidelity model of the ballast bed (with USP). This comprehensive analysis assesses the impact of varying train speeds and axle loads on the mechanical properties of the ballast bed with the USP, integrating both macroscopic and microscopic perspectives. The results show that there is an 8.54% reduction in longitudinal resistance of the ballast bed (with USP) compared to the ballast bed (without USP), yet it still fulfils the requirements of heavy railway lines. The ballast bed (with USP) exhibits significant improvements in vibration-damping performance, with the wheel-rail vertical force showing a reduction of 8.71% relative to the ballast bed (without USP). This suggests a potential extension of the track structure's lifespan. An increase in train speed markedly influences the vibration levels of the ballast bed (with USP), leading to an increase in ballast bed acceleration by 119.39%. The increase in train axle load demonstrates a minor effect on the plastic deformation of the ballast bed, indicating the ballast bed (with USP)'s capability to accommodate heavier axle load trains.

A study of the effect of local scour on the flow field near the spur dike

The flow field near a spur dike such as down flow and horseshoe vortex system (HVS) are susceptible to the topographic changes in the local scouring process, resulting in variation of the sediment transport with time. In this study, large eddy simulations with fixed-bed at different scouring stages were conducted to investigate the changes in flow field. The results imply that the bed deformation leads to an increase in flow rate per unit area, which represent the capability of sediment transportation by water, in the scour hole. Moreover, the intensity of turbulent kinetic energy and bimodal motion near the sand bed induced by the HVS were also varied. However, the peak moments between the two sediment transport mechanisms were different. Hence, understanding the complex feedback mechanism between topography and flow field is essential for the local scour problem.

Integrated modelling of flow-sediment transport and power generation in the Three Gorges Reservoir

Reservoirs play a crucial role in regulating runoff and generating energy. However, they also lead to significant sedimentation in the reservoir area. In this study, we propose an integrated model that combines a 1-D hydro- and sediment dynamic module with a power generation module. The model considers both suspended and bed load transports. This model is applied to the Three Gorges Reservoir (TGR) and evaluate its performance against corresponding measurements. The results demonstrate that:① the proposed model accurately reproduces the processes of flow and sediment transport, bed deformation, and power generation during the hydrological years of 2019 and 2020. The relative errors for average discharge and bed deformation volume are <6 % and 10 %, respectively. Moreover, the calculated total power (982 × 108–1115 × 108 kW·h) closely agree with the measured values (969 × 108–1118 × 108 kW·h); ② the inflows of small tributaries have a noticeable impact on the calculated water discharge in the TGR. This impact will lead to a 16 % increase in average discharge and alter the magnitudes and occurrence times of flood peaks; ③ the flocculation of fine sediment particles significantly affects sediment transport, particularly in the sub-reach close to the dam. This flocculation will result in a 37 %–57 % reduction in average suspended sediment discharge and a 63 %–93 % reduction in peak sediment discharge. This research provides a comprehensive tool for simulating flow and sediment transport as well as power generation, which can support the optimal regulation of the TGR.

Velocity Field due to a Vertical Deformation of the Bottom of a Laminar Free-Surface Fluid Flow

This article investigates the velocity field of a free-surface flow subjected to harmonic deformation of the channel bottom, progressing asymptotically from a flat initial state to a maximum amplitude. Assuming a uniform main flow with the primary velocity component transverse to the bed undulation, analytical solutions are obtained for the three velocity components and free surface distortion using the method of perturbations. The perturbation components of the velocity field, streamlines, and surface deformation depend on a dimensionless parameter that reflects the fluid inertia induced by bed deformation relative to viscous resistance. When viscous effects dominate, a monotonic decay of the perturbations from the bed to the free surface is observed. In contrast, when inertia dominates, the perturbations can exhibit an oscillatory behavior and introduce circulation cells in the plane normal to the main flow. The interplay between inertia and viscosity reveals scenarios where surface and bed deformations are either in or out of phase, influencing vertical velocity components. Figures illustrate these phenomena, providing insights into the complex dynamics of free-surface flows with harmonic bed deformation in the direction normal to the main flow, and amplitude growing with time. The results are limited to small deformations of the channel bottom, as imposed by the linearization of the momentum equations. Even so, to the best of the authors’ knowledge, this problem has not been addressed before.

3D SIMULATION OF THE EFFECT OF SPUR DIKES SPACING ON BED DEFORMATION, FLOW AND PERFORMANCE EVALUATION IN MEANDERING CHANNELS

3D SIMULATION OF THE EFFECT OF SPUR DIKES SPACING ON BED DEFORMATION, FLOW AND PERFORMANCE EVALUATION IN MEANDERING CHANNELS

Dynamic mechanical performance of geogrid–waste tyre-reinforced railway ballast under cyclic loading

The accumulation of permanent deformation under cyclic loading in the granular bed is the main source of ballast track settlement. Therefore, studying the dynamic mechanical behaviour of reinforced railway track beds under cyclic loading is crucial. To reduce the accumulated of permanent deformation of railway track bed, geogrids–waste-tyre reinforced railway track bed was used to study in order to effectively control the permanent deformation of ballasted railway track bed. The test was conducted to investigate the mechanism of permanent deformation of geogrid–waste tyre-reinforced railway track bed through the evolution of cumulative plastic strain, permanent strain rate and dynamic elastic modulus of railway track bed under different confining pressures, dynamic stress amplitude, and amount of reinforcement. Results show that geogrid and waste tyre reinforcement have significant effects on the accumulated permanent deformation of the railway ballast. Geogrid reinforcement is better than waste tyres in improving the performance of the ballast fill. Reinforcement with geogrid significantly reduces the permanent strain rate, although the impact on this rate varies depending on the position and amount of reinforcement used. Notably, reinforcing the middle three layers of the track bed sample with waste tyres substantially decreases the permanent deformation rate. In contrast, the application of reinforcement across all five layers with waste tyres increases the permanent deformation rate. The dynamic elastic modulus of the track bed sample not only increases significantly but also rises with the dynamic stress amplitude due to the reinforcement of the geogrid and the waste tyre. The dynamic elastic modulus gradually stabilises with the increase in cyclic loading, indicating a stable accumulated deformation in the railway track bed.