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.

Seepage deformation in sand results from complex water-soil interactions, which are the primary reasons of sand surface collapse, as well as instability and deformation in dam foundations, building foundations, and slopes. Frequent fluctuations in groundwater levels cause changes in the direction, velocity, and pore water pressure of groundwater within the sand. Further research is essential to fully understand the characteristics and mechanisms of sand seepage deformation under varying groundwater conditions. In this study, natural undisturbed sand samples were collected. Laboratory seepage deformation tests were conducted to simulate continuous rises and falls in groundwater levels, exploring the response characteristics of internal erosion and hydraulic behavior of the sand under varying groundwater flow rates and directions. The results show that: As groundwater flow rate increases, the sand undergoes multiple episodes of seepage deformation, which includes the processes of structural stability, seepage deformation, and seepage failure. Initially, the hydraulic gradient for seepage deformation is small, and the particles carried by seepage are small. With a further increase in groundwater flow velocity, the hydraulic gradient rises, larger sand particles are migrated by seepage, and seepage failure may eventually occur. When the karst groundwater level is lower than the elevation of the sand bottom (H-2 < z(2)) and the sand bottom is in a negative pressure state, the hydraulic gradient of seepage deformation is usually smaller than that observed in the other two states of positive pressure. In these cases, pore water pressure exerts an upward buoyant force, while in the negative pressure state, the pore water pressure transforms into downward suction. This downward suction aligns with the direction of gravitational forces and downward seepage force acting on the sand, making seepage deformation of the sand more likely. Sands with greater unevenness, finer particle, and lower density are more prone to seepage deformation but failure at different hydraulic gradients.

The underestimated risk of contact erosion failure in railway substructures poses a significant threat to railway safety, particularly at the interface between the ballast/subballast and subgrade. The larger constriction size at this interface exacerbates the potential for long-term erosion, necessitating attention to safeguard railway integrity. This study introduces a novel laboratory erosion testing apparatus to evaluate contact erosion at the subballast-subgrade interface under cyclic loading. Subgrade soils with varying fines contents are tested, and the effect of pressure head on erosion is investigated in detail. The results indicate that sandy soil with higher internal stability exhibits a higher critical pressure head for contact erosion. Cyclic loading induces oscillations in pore water pressure within the subballast layer, with higher pressure heads leading to larger amplitudes. Excess pore water pressure is generated in the sandy soil layer during cyclic loading and gradually dissipates over time. Fine eroded particles migrate into the subballast layer, forming mud, while coarse eroded particles accumulate at the base, creating low-permeability interlayers. Notably, the geometric conditions alone may not guarantee effective prevention of contact erosion in railway substructures. The hydraulic conditions for contact erosion are more easily achieved under cyclic loading compared to static loading. These distinctive features of contact erosion in railway substructures, different from those observed in hydraulic structures, provide some insights for the development of remediation strategies and improvements in railway substructure design.

Previous studies investigating dynamic responses of gravelly soil were limited to high-strain conditions, in which a high level of pore-water pressure is developed, leading to a significant reduction in shear strength and subsequent liquefaction. This paper presents a series of dynamic centrifuge modeling tests performed on loose gravel-sand mixtures to evaluate progressive response under various shear strains. The centrifuge models simulated a uniform soil profile of gravel-sand mixtures with gravel contents of 20%, 40%, 65%, 80%, and 100% that were subjected to incrementally increasing shaking amplitudes from 0.01 to 0.40 g. Due to the influence of composition on the void ratio of the specimens, the results were analyzed in terms of their dominant behaviors (i.e., sandlike, gravellike, or transition soil). Although the soils had comparable initial relative densities, the sandlike soils had the lowest void ratio, and the void ratio increased when the gravel content was greater than 65%. Resonant column testing results indicated that the soils had comparable dynamic properties because of their loose condition. The results showed that dynamic shaking generates comparable shear strains ranging from 0.03% to 3.8% in all models, but the accumulation of pore pressure leads to upward flow in sandlike soils, whereas transient pore-pressure behavior leads to oscillatory flow in gravellike soils. Differences in the stress-strain response and the effects of the number of shaking cycles were observed in different soil mixtures depending upon the level of excess pore pressure. At low shaking amplitude and low excess pore pressure, stiffness degradation was observed while the stress-strain loop was symmetric. At high shaking amplitude and high excess pore pressure, significant stiffness degradation was observed followed by shear-induced dilation resulting in an asymmetrical stress-strain loop. This study clarifies the differences in the dynamic responses and behaviors of sandlike, gravellike, and transition soil over a wide range of strains.

With the development and utilization of large-scale urban underground space, the interaction between groundwater and underground structures has become a research hotspot. To analyze the pore pressure transfer mechanism of granite weathered soil under high water head conditions, this article uses the consolidation differential equation of saturated soil to solve the pore pressure transfer model for specific boundary conditions. To verify the effectiveness of the theoretical model, a large-scale head loss test system was constructed to carry out seepage tests of groundwater in granite weathered soil under high water head conditions. Using the experimental results, an expression for the relationship between the permeability coefficient and hydraulic gradient of granite weathered soil was established. Finally, the influence factors in the process of pore water pressure transfer in soil were studied using the pore pressure transfer theory model. The research conclusions can be used for antifloating design and seepage field analysis of underground structures, providing a basic theoretical basis and analytical method for studying the seepage law of soil, which is of great significance for ensuring the safety of underground structures.

This research mainly sought to evaluate the sediments of the Urmia lake bed at three different locations on the lake. Identification tests, organic content, specific gravity tests, saturated hydraulic conductivity, and hydraulic gradient tests were performed to assess the geotechnical properties of soil at three different areas along Lake Urmia namely Hyderabad, Chichest, and Urmia-Tabriz Bridge. The results showed that the soil type varied from coarser to finer in sediment particle size from the estuarine zone of the rivers to the lake's interior. The type of soil was identified for three areas of Urmia-Tabriz Bridge, Chichest, and Hyderabad ML, SP-SM, and SP-ML, respectively. The shallow depth of lake leads to an almost uniform distribution oforganic materials in the lake sediment, with a slightly higher value along the shorelines. Compared to inorganic soils, the presence of organic material reduced the Gs value in the lake bottom to 17.82%. This increases the risk of bed mud creep due to the low hydraulic gradient beneath any structures intended to reduce the lake's size sustainably through river discharge and evaporation. In the southern parts, the average hydraulic conductivity for finer soils near the lake's shore was between 57 and 127 cm/day for coarse material. The critical hydraulic gradient in the southern area varied from 1.65 for the lake shore to 0.95 up to 3 km into the lake. In the northern area, far from the mouth of the main tributaries, the values varied between 1.9 and 0.75 for the same distances, giving a zonal map of Lake Urmia's bottom sediment properties. The results of this study allow a first estimation of the geotechnical characteristics of a hypersaline lake to choose an appropriate structure, such as sheet piles.

Dynamic loading-seepage causes the migration of railway subgrade filling particles, leading to frequent engineering problems such as ballast fouling, mud pumping, settlement, and erosion. However, few studies have focused on the permeation features and internal erosion characteristics of subgrade materials, making it difficult to uncover the evolution mechanism of service performance of subgrade under complex geo-environmental conditions. Therefore, the seepage characteristics and permeability stability of subgrade materials were investigated using self-developed equipment to reveal the seepage failure mechanism under dynamic loading. The main conclusions are as follows: (1) The internal stability of the soil is affected by fluctuations in pore water pressure and hydraulic gradients in graded aggregate and gravel-sand-silt mixtures caused by dynamic loading. (2) Critical hydraulic gradients leading to the migration of fine particles (J(cr)) and seepage failure (J(F)) in graded aggregate and gravel-sand-silt mixtures are determined as follows: J(cr) =1.30 and J(F) =6.88 for graded aggregate, and J(cr) =1.23 and J(F) =2.71 for gravel-sand-silt mixtures. (3) The seepage failure process of subgrade materials can be divided into three stages under coupled action of train loading and seepage: stable seepage, dominant flow development, and seepage failure. The relationship between flow velocity and hydraulic gradient follows the Darcy's law under the low hydraulic gradient. (4) The evolution process of subgrade performance was analyzed, and the mechanisms and types of railway flood hazard were summarized. The research provides theoretical support for the design and maintenance of railway disaster prevention, and has significant engineering implications.

In this paper, the research progress made in the methods used for assessing the internal stability of landslide dam soils was reviewed. Influence factors such as the gradation of soil and the stress state in the soil in different analysis methods were discussed, as these can provide a reference for the development of more accurate methods to analyze the internal stability of landslide dam soils. It focuses on the evaluation of internal stability based on the characteristic particle size and fine particle content, hydraulic conditions such as the critical hydraulic gradient and critical seepage velocity, and the stress state such as lateral confinement, isotropic compression, and triaxial compression. The characteristic particle size and fine particle content are parameters commonly used to distinguish the types of seepage failure. The critical hydraulic gradient or seepage failure velocity are necessary for a further assessment of the occurrence of seepage failure. The stress state in the soil is a significant influence factor for the internal stability of natural deposited soils. Although various analysis methods are available, the applicability of each method is limited and an analysis method for complex stress states is lacking. Therefore, the further validation and development of existing methods are necessary for landslide dam soils.

Seepage -induced suffusion involves the migration of fine particles within a soil matrix. Seepage flow is affected by the soil permeability anisotropy of anisotropic soil fabric; however, suffusion anisotropy is unclear because of the limited function of existing permeameters. In recent studies, the effect of seepage direction has been investigated under only low hydraulic gradients because the control of seepage direction relies merely on gravity. In this study, a new, large -sized permeameter is developed with which suffusion tests can be conducted along horizontal or vertical seepage directions under high hydraulic gradients. Correspondingly, the permeameter can accommodate a specimen of 540 x 500 x 470 or 540 x 540 x 440 mm3 (length x width x height). The seepage direction is switched by changing the boundary conditions of the specimen with detachable perforated plates that allow pressurized water originating from different inlets to flow along horizontal or vertical directions. Two repeated pairs of tests were performed on a gap -graded clayey gravel to investigate the suffusion anisotropy of saturated clayey gravel. The results show that the maximum relative deviations of measurements for initial hydraulic conductivity, initiation, and failure hydraulic gradients are less than 3.5 %, demonstrating satisfactory reliability. The ratio of the initial horizontal hydraulic conductivity to vertical hydraulic conductivity for the test soil is 13.87, indicating a significantly anisotropic fabric induced by compaction. The ratios of horizontal initiation and failure hydraulic gradients to vertical initiation and failure hydraulic gradients are 0.52 and 0.59, respectively. This implies that suffusion anisotropy should not be neglected for evaluating the internal instability of anisotropic soils.

Internal erosion refers to the movement of fine particles within soil framework due to subsurface water seepage. Existing criteria for assessing internal erosion usually are based on static loading, and the effect of cyclic load is not considered. Additionally, there are limited studies to examine the particle -size distribution and origin of eroded fine particles. This study presents an experimental investigation that examines the impact of cyclic loading on internal stability through a series of seepage tests. The composition and origin of lost particles are quantitatively studied using particle staining and image recognition techniques. With increasing hydraulic gradient, particle erosion progresses from top layer to bottom layer, with a gradual increase in the maximum particle size of eroded particles from each layer. After significant loss of particles, the specimens reach a state of transient equilibrium, resulting in a gradual slowdown of both particle loss rate and average flow velocity. The results indicate that cyclic loading promotes massive particle loss and causes erosion failure of specimens that are considered stable according to existing criteria. The reason is that under cyclic loading, local hydraulic gradients is oscillating, and a larger than average hydraulic gradient may occur, which is responsible for the internal instability. The analysis suggests that existing criteria can provide a reasonable assessment of the relative stabilities of specimens under static loads but fail to capture the stabilities under cyclic loading conditions.