Weathered granite soil (WGS) is highly water-sensitive and widely distributed across southern China, where the region's rainy climate contributes to geological hazards such as collapsing erosion, landslides, and ground subsidence. This study aims to elucidate the impact of this rainy climate on the deterioration of WGS by investigating the suffosion characteristics of granite residual soil (GRS) and completely weathered granite (CWG) at various stages of weathering. The research explores how suffosion affects their mechanical properties and microstructural features. A series of suffosion tests were conducted under controlled water pressure, followed by one-dimensional consolidation tests, cyclic triaxial tests, scanning electron microscopy, X-ray diffraction, and X-ray fluorescence analyses to analyze the deterioration mechanisms at both macro- and micro-scales. The results show that suffosion leads to the loss of fine particles and overall settlement of the soil samples. Microscopically, Mica is almost entirely lost, iron cementation is disrupted, and clay minerals, along with quartz and feldspar debris, are eroded, causing microstructural damage. The loss of minerals at the micro-scale exacerbates the formation of pores and cracks, increasing WGS porosity and promoting the progression of suffosion. On the macro-scale, suffosion alters the physical properties of WGS, with fine particle migration and loss leading to soil skeleton deformation, reduced stiffness, and decreased compressibility. Furthermore, a suffosion index is proposed, correlating microstructural changes with macroscopic mechanical parameters. This study has practical and theoretical significance for slope stability, collapsing erosion prevention, and surface subsidence mitigation in WGS in southern China.
Suffusion, a process whereby water gradually carries away fine particles from soil, is thought to be one of the possible reasons for the settlement or inclination of bridge piers after a major flood (delayed displacement). The aim of this study is to offer fresh insights into suffusion and its mechanical impact on the affected soil, with a specific focus on how it relates to bridge pier failures. Riverbed material replicated with relatively larger fine particles than those used in past studies which focused on soil in embankments or dikes. Through both monotonic and cyclic loading tests on soil samples with varying initial fines contents, while maintaining a constant relative density of 79%, several important discoveries are made. The small strain stiffness of suffused soil fluctuates as erosion occurs, along with a decrease in shear strength and an increase in soil contraction under monotonic stress. Furthermore, the research simulates the train loading exerted on the base soil of bridge piers susceptible to suffusion by subjecting the soil samples to cyclic loading both before and after erosion, mirroring practical conditions. The key findings of this study reveal that the stiffness of soil drops during erosion with no significant deformation of the soil. This leads to a large strain accumulation in the soil specimens under subsequent cyclic traffic loading. These findings highlight that the delayed settlement or inclination of bridge piers under cyclic or train loading after major flood is possibly due to suffusion in the base soil of the piers. (c) 2024 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society. This is an open access article under the CC BY NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
In recent years, local scour has occurred on the pier foundations of river bridges during heavy rain and river flooding, often resulting in bridge collapse and outflow. This study focused on the characteristic displacement, called delayed displacement, of the river bridge pier. The critical displacement of the piers was first observed several days after the flood when the train passed and not immediately after the flood. The authors hypothesized that one of the possible reasons for the delayed displacement is the suffusion of the supporting ground beneath the pier foundation during the flood, followed by a compressive behavior due to the collapse of the soil skeleton under repeated traffic loads. Accordingly, this study performed erosion tests simulating flood and cyclic loading tests simulating train passage using a triaxial test apparatus to check the validity of this hypothesis. In some test cases, suffusion without any deformation occurred in the erosion test but deformed in the cyclic loading test just after the erosion test. This behavior matches the behavior of delayed displacement. It was also suggested that the risk of the delayed displacement becomes high when the soil skeleton was assumed to primarily comprise fine particles, and the void ratio and hydraulic gradient were high. By contrast, when the soil skeleton was assumed to primarily comprise coarse particles, suffusion occurred in the erosion test, but did not deform in the subsequent cyclic loading test. Thus, the risk of delayed displacement is considered to be low when coarse particles are dominant. Furthermore, clear relationship between suffusion and the consequent reduction in soil stiffness cannot be observed. This result indicates that no significant change in the stiffness occur in the supporting ground of the pier foundation at the stage immediately before the delayed displacement. Thus, identifying the deterioration in the stability of the piers through impact loading test, which is based on the concept that local scour reduces the natural frequency of the bridge pier, is difficult.
Due to rainfall, the soil-rock differential weathering interface of spherical weathered granite soil slopes is prone to evolve into a dominant seepage channel and undergo seepage suffosion, which accelerates the deformation and instability of these slopes. However, little research has been carried out on the characteristics of seepage suffosion and the migration of fine particles. Based on the unsaturated seepage theory of porous media, a numerical calculation framework is established to accurately describe the seepage suffosion process at the soil-rock interface, considering the coupling relationship between the fine particle migration, suffosion initiation response and unsaturated seepage. The finite element method is used to construct a seepage suffosion model for unsaturated granite residual soil under the effect of dominant flow. Based on the seepage suffosion process of homogeneous soil columns, the suffosion characteristics of dominant flow under three typical soil-rock interface burial states are systematically investigated. The results show that the soil-rock interface and the matrix permeability of spherical weathered granite soil slopes are highly variable, with the wetting front forming a downward depression infiltration funnel, and the degree of depression of the wetting front becomes more pronounced as rainfall continues. The degree of fine particle loss is related to the burial state of the soil-rock interface, in which the dominant flow potential suffosion of the under-filled soil condition is the most significant, and even excess pore water pressure occurs at the interface, which is the most unfavorable to the stability of this type of slope. The research results can provide a scientific basis for accurately evaluating the stability of spherical weathered granite soil slopes under rainfall conditions.
Bei dem Beitrag handelt es sich um die erweiterte Fassung der gleichnamigen Keynote Lecture auf der 4. Bodenmechanik Tagung im Rahmen der Fachsektionstage Geotechnik, die auf Anregung der Fachsektionsleitung auch in der Zeitschrift geotechnik veroffentlicht werden soll. Bei den Phanomenen der inneren Erosion in durchstromtem Boden und in Erdbauwerken geht es um das Losen, den Transport und die Ablagerung bevorzugter Fraktionen mit der Folge einer anderung der Bodeneigenschaften. Die Phanomene der inneren Erosion werden als Kontakterosion, Suffosion, Kolmation und ruckschreitende Erosion charakterisiert. Die Kinematik dieser physikalischen Prozesse ergibt sich mit der Energie einer Sickerstromung aus der Bewegung des Einzelkorns im Porenraum, den moglichen Freiheitsgraden beim Transport. Der Artikel gibt einen uberblick uber die Art und Bedingungen der verschiedenen Phanomene sowie uber deren spezifische Kinematik innerhalb der Bodenstruktur. Die relevanten international verwendeten Nachweismethoden und Kriterien werden aufgefuhrt und in ihrer Aussagekraft bewertet. Die kennzeichnenden Einflussparameter werden aufgezeigt. Fur die einzelnen Phanomene der inneren Erosion werden Strategien zur Bewertung und Beherrschung des Erosionsrisikos diskutiert. Phenomena, kinematics and risk assessment strategies of internal erosionInternal soil erosion due to seeping water in natural sediments as well as in earthworks can lead to a significant change in soil properties and could even destroy the structural integrity. The physical process of erosion always is induced by loosening, migration, and deposition of predominant fine particles within the soil structure. Depending on the kinematics, the phenomena are divided into contact erosion, suffusion, colmation and backward erosion piping. The kinematics is controlled by the energy of a seepage on the one hand, on the other by the degrees of freedom and the boundary conditions of an individual grain movement within the pore space. This article provides an overview of the characteristic, specifics, and conditions of the different phenomena considering their kinematics within the soil structure. Internationally used approaches and methods of assessment are listed, their significance and their limitations will be evaluated. The impact parameters that control the different processes are shown. Strategies for assessing and controlling the risk of structural damage are discussed for the different phenomena of internal soil erosion.
Internal erosion can induce significant changes in the mechanical properties of soils, posing various hazards to dam and dike structures. Despite its importance, our current understanding of this phenomenon remains incomplete. The influence of pre-shearing stress conditions on the mechanical behaviours of soils during the internal erosion process is particularly challenging, as existing experiments have not been able to maintain the constant pre-shearing stress ratios. To bridge this gap in knowledge, this paper presents a series of discrete element method (DEM) simulations focused on gap-graded cohesionless soil. The primary objective of these simulations is to investigate two specific cases of internal erosion: suffusion and suffosion processes. Soil specimens are subjected to different pre-shearing stress ratios in the standard triaxial tests before being submitted to different levels of erosion to study their constitutive responses. The results show that erosion-induced deformation (i.e. suffosion) only starts after a specific amount of mass loss. This mass loss and the pre-shearing stress ratio form a well-defined criterion for triggering suffosion, which is named suffosion surface. The volumetric strain is shown to be a better indicator to describe the suffosion process than the commonly used void ratio. The pre-shearing stress ratio significantly influences the suffosion response of the soil sample, with a higher preshearing stress ratio facilitating soil failure. Furthermore, soil specimens undergo both deviatoric and volumetric responses during the suffosion process. To this end, new DEM-based statistical equations were proposed to describe the observed mechanisms, which are helpful for the future development of constitutive models to describe internal soil erosion.
For the problem of suffosion in gap-graded sand with initial anisotropy, the Ganser drag force model, which can take into account the effect of the projected area of particles, is introduced to achieve a two-phase coupling of computational fluid dynamics (CFD) and discrete elements method (DEM) for non-spherical particles. The applicability of the numerical method in solving the interaction between the non-spherical particles and fluid is verified by comparing with single particle settlement tests. On this basis, specimens with different bedding orientations and fine contents are further generated to simulate upward seepage suffosion tests, during which both macroscopic and microscopic properties, such as the fine loss, composition of strong and weak force chains, and changes in grain fabric, are monitored to explore the seepage suffosion characteristics of anisotropic soils with various fabrics under different filling states (underfilled and overfilled). Drained triaxial tests are carried out on specimens before and after erosion to investigate the effect of seepage on the weakening of soil strength. The results show that the mass loss of the overfilled specimens increases with increasing deposition angle, while the mass loss of the underfilled specimens firstly increases and then decreases with the deposition angle. The loss of fines in the underfilled specimens is mainly due to the low connectivity fines, whereas for the overfilled specimens, suffosion leads to a simultaneous reduction in the number of both low and high connectivity fines. In addition, the triaxial tests show that suffusion causes a significant weakening of the peak strength of the soil, and the change in peak strength with the deposition angle is also influenced by the soil filling state.