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 failure is a common problem in engineering, and the calculation and analysis of critical hydraulic gradient are of great significance for the safety and protection of engineering. Based on the principle of discrete element method and computational fluid dynamics, the fluid-solid coupled models were established to study the critical hydraulic gradient and particle loss rate of granular soils at seepage failure. The evolution of seepage failure was divided into four stages: seepage development stage, local damage stage, volume expansion stage and overall damage stage. The validity of numerical simulation was demonstrated by comparing the critical hydraulic gradient obtained by numerical simulation and by Terzaghi's formula. According to the fabric damage and flow velocity variation of the models at seepage failure, the influences of model size and particle size on the critical hydraulic gradient and particle loss rate were analyzed. The results indicate that critical hydraulic gradient and particle loss rate were not sensitive to changes in model size. A wide particle size distribution range resulted in large critical hydraulic gradient and small particle loss rate at seepage failure. The discrete element numerical simulation can not only be used to determine the critical hydraulic gradient of geotechnical and hydraulic engineering, but also offer a visual portrayal of the evolution of seepage failure, serving as an important complement to comprehend the intricate microscopic mechanisms underlying soil seepage failure.