Internal erosion, which involves the detachment and migration of soil particles from the soil matrix driven by seepage flow, occurs frequently in natural slopes, dikes and many other geotechnical and hydraulic structures. Previous studies primarily focused on soil internal erosion under the isotropic stress state and monotonic hydraulic loadings. However, the soil in engineering practices is under more complicated hydro-mechanical conditions, i.e. anisotropic stress states, and subjected to large and cyclically unsteady hydraulic loadings due to water level fluctuations. Under such conditions, the soil internal erosion process differs significantly from that under the monotonic seepage and isotropic stress states. Therefore, in this study, extensive laboratory tests were carried out to investigate the soil hydro-mechanical behavior subject to high cyclic hydraulic gradients and various stress states. Results show that the soil experienced a gradual internal erosion process under an isotropic or low shear stress state, whereas it experienced rapid erosion followed by a complete failure when the stress ratio (eta) was high. The cyclic hydrodynamic loading accelerated the occurrence of internal erosion due to strong disturbances to the soil structure. The soil pores became continuously connected under high cyclic hydraulic gradients, leading to significant soil deformations due to the collapse of soil force chains by massive particle loss. Additionally, the peak and critical friction angles for all the post-erosion soils decreased considerably and the soil tended to exhibit strain softening behavior after erosion at large cyclic hydraulic gradients.

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.

This study investigated the influence of sample preparation methods, moist tamping and wet pluviation, on the erodibility and mechanical behaviour of gap-graded soils with three gradations: fully stable, unstable, and on the borderline of stability. Drained triaxial tests were performed using a modified erosion-triaxial apparatus, followed by micro-CT scanning to assess pore network properties. The results indicated that for fully stable and fully unstable samples, the preparation method had minimal impact on both erosion and mechanical behaviour. However, for the samples on the borderline of stability, wet pluviation method resulted in fine particle segregation, creating a heterogeneous structure with reduced pore connectivity. This led to lower erosion rates (0.4 gr/min reduction compared to the moist tamping technique), but mechanical properties remained largely unaffected, as confirmed by similar intergranular void ratios and stress-strain responses. Micro-CT scanning quantified differences in pore structure, showing that wet pluviation samples exhibit lower connected porosity compared to those prepared by moist tamping. These findings highlight the critical role of specimen preparation in assessing suffusion susceptibility and erosion behaviour, particularly for soils near the threshold of instability.

Levees and the beneath foundation ground sometimes cause consolidation settlements. When the consolidation estimation is underestimated in advance, the structures and residents are damaged by consolidation. In addition, levees are subjected to piping and boiling with heavy rainfalls, which often have coarse permeable layers. The seepage is concentrated through coarse permeable layers. Owing to the repetition of regular rainfall and the changes in water levels, internal erosion occurs between a permeable and a finer soil layer. This study aims to reveal the effect of internal erosion on consolidation. Clay, including granular soil, is used to simulate both internal erosion and consolidation. First, long-term seepage is imposed on a soil specimen with a coarse layer. Then, clayey soil samples are obtained from the specimen subjected to seepage, and a consolidation test is conducted. The soil properties related to consolidation are revealed in several experimental cases. A numerical analysis is conducted using the experimentally obtained parameters to simulate the consolidation for embankment construction. Finally, the influence of internal erosion on consolidation is discussed. The soil consolidation behavior is similar to that of the clayey material when the specimen is loosely compacted. However, the behavior of the dense material appears to be sandy; consolidation is rapidly completed. The loosening caused by internal erosion is dissipated, and the consolidation properties approach those of the soil sample with a similar initial compaction degree. Finally, a practical application method for the internal erosion effect is proposed.

Erosion is regarded as a significant global concern caused by the inferior engineering characteristics of soils, encompassing their susceptibility to collapse and high porosity as well as their limited strength. These attributes render soils prone to the adverse consequences of erosion, which can have far-reaching implications for the environment, infrastructure, and human settlements. In order to counteract the detrimental effects of erosion, a range of engineering strategies have been devised, with the most innovative and sustainable approach being the microbially induced calcite precipitation (MICP) technique. This review article investigates past studies on the MICP technique as a potential sustainable stabilization method for controlling different kinds of erosion, including wind erosion, rainfall erosion, coastal erosion, internal erosion, and jet erosion. This comprehensive review first examines the various factors that affect the MICP method in controlling soil erosion, underscoring the significance of integrating supplementary chemical, mechanical, and ecological approaches with this biomediated approach. The study then delves into the effectiveness of the MICP method for erosion mitigation through an in-depth discussion of real-world applications and the durability of MICP-treated soils under harsh climatic conditions, such as freeze-thaw and wet-dry cycles. In addition, the study also highlights the existing limitations and challenges associated with stabilizing erosive soils using the MICP technique, including environmental and economic aspects. By identifying these limitations, the review sets the stage for future research opportunities to overcome the current barriers and further advance the application of MICP for effective soil erosion mitigation. The current research contributes to advancing MICP as an effective and sustainable solution for soil erosion control.

Internal erosion is one of the most important factors that cause earth structures that retain water, such as embankment dams, to collapse. Concentrated leak erosion, one of the forms of internal erosion, occurs in cracked fine-grained soils and pressurized flow conditions. To evaluate the concentrated leak erosion risk of cracks/voids, it is necessary to ascertain the erosion resistance of these materials. The erosion rate and critical shear stresses determine internal erosion resistance in concentrated leak erosion. This study determined soil's concentrated leak erosion resistance using test equipment that allowed the flow to pass through a hole with stress-free (no loading), anisotropic-compression stress, anisotropic-expansion stress, and isotropic stress conditions. The stresses that developed in the samples' hole wall where erosion occurred were determined with numerical modeling as pre-experimental stress conditions. The experiments were performed under a single hydraulic head on four selected cohesive soils with different erosion sensitivity. Time-dependent flow rates obtained from the test system can be used to determine hydraulic parameters, such as energy grade lines, with the help of basic theorems of pipe hydraulics in theoretical hydraulic models. Moreover, the erosion rates were quantitatively determined using the continuity equation, while critical shear stresses were qualitatively compared for concentrated leak erosion developed by the dispersion mechanism. As a result of the experiments, stress conditions influence the concentrated leak erosion resistance in the soil samples with dispersive erosion. Moreover, the shear strength in the Mohr-Coulomb hypothesis can explain the erosion resistance in these soils under stress conditions depending on the sand/clay ratio.

Dispersivity is a severe pathology that occurs mainly in clay soils and is usually catastrophic in geotechnical structures susceptible to this damage. Hundreds of dams worldwide have failed due to quality problems, mainly by piping in the body, foundation, spillway, culvert, and other peripheral structures. The pinhole test is currently considered the most accurate test for detecting the dispersivity of clay soils. However, it presents problems when objectively evaluating the dispersivity of a material due to the qualitative nature of the estimation of results. In particular, the methodology for determining turbidity has been identified. This document studies different piping paths in the sample, which a priori may be more realistic than the single path in the current test. A kaolinitic clay, widely studied through index and mineralogical tests, is used as the base material. Regarding the detection of dispersivity, a specialized test package was used to reduce the uncertainty of the results. Natural samples were analyzed using ASTM D4647-13. A modification of the pinhole test was proposed based on the imposition of additional artificial channels. The results revealed that this modification can make the test more realistic because when the dispersive front advances in the soil, it does not travel along a single path but instead looks for different erosive paths. The details of this assertion are discussed throughout the paper.

Water retaining structures are critical elements of civil infrastructure. Internal erosion of soils forming the containment structures may occur progressively and lead to expensive maintenance costs or failures. The strength, stress-strain behavior and critical state of soils which have eroded, as well as the characteristics of the erosion, may be affected by hydraulic gradient, confining stress and relative density of the soil at the start of the erosion. Here, erosion and triaxial tests have been conducted on gap-graded soil samples. The tests and results are novel as the samples were prepared to be homogenous post-erosion and prior to triaxial testing by adopting a new sample formation procedure. The post-erosion homogeneity was evaluated in terms of particle size distribution and void ratio along a sample's length. The erosion-induced mechanical property changes can then be linked to a measure of initial state, more reliably than when erosion causes samples to be heterogeneous. The results show that erosion causes the critical state line in the compression plane to move upwards. The movement is lesser than the increase in void ratio caused by erosion. The state parameter is therefore reduced, consistent with the soil's reduced peak strength and its less dilative response. Regarding the erosion characteristics, the flow rate decreases with the increase in initial relative density or effective stress, but increases with the increase in the hydraulic gradient being applied. The cumulative eroded soil mass increases with the increase in hydraulic gradient and decreases with the increase in initial density and effective confining stress.

Natural volcanic soils containing pumice particles are commonly found in Hokkaido, Japan, and this type of soil is prone to landslides, internal erosion, and liquefaction. Therefore, the purpose of this paper is to summarise the hydro-mechanical response of volcanic ash soil subjected to internal erosion using the modified erosion triaxial apparatus, based on the literature and additional investigations. The results of the study show that the rate of erosion and shear strain during the erosion process are influenced by initial density, stress state, and hydraulic gradient. Notably, anisotropic consolidation is experienced by specimens under seepage flow. Additionally, the removal of fines leads to a slight decrease in the grading state index. Moreover, suffosion increases the maximum shear modulus and Poisson's ratio of the soil, while increasing seepage time stabilises the peak shear strength of eroded specimens. Furthermore, the critical state line does not change much with internal erosion. To sum up, this study offers valuable insights into the behaviour of volcanic ash soil subjected to internal erosion and provides an integrated interpretation of hydro-mechanical response of volcanic ash on removal of fines. (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