Preferential flow is widely developed in varieties of voids (such as macropores and fissures) in loess areas, affecting slope hydrology and stability and even leading to geological disasters. However, the model of seepage evolution with dynamic preferential flow is not clear, which obstructs the disclosure of the mechanism of landslides induced by the preferential flow. This study aimed to capture the seepage and occurrence status of water in loess voids, explain the variability characteristics of the loess pore structure, and reveal the seepage evolution model of dynamic preferential flow. Preferential infiltration experiments were conducted by combining X-ray computed tomography (X-ray CT) nondestructive detection with contrast techniques under dynamic seepage conditions. Three-dimensional (3D) visualized reconstruction, digital image correlation (DIC), image processing, and quantitative analyses were performed in AVIZO 2019.1, including two-dimensional (2D) and 3D characteristics of preferential flow distribution and macropore changing, dynamic variation of the porosity, pore number, volume, dip angle, and connectivity. Results showed that (1) preferential flow exists under saturated and unsaturated conditions in loess with strong uniformity and anisotropy; (2) preferential flow not only migrates into existing connected macropores, but also connects the original isolated pores into channels and forms larger percolation groups of contrast medium under the gradually increased high pressure; (3) the seepage develops with the evolution model of ‘preferential flow–piston flow–preferential piston mixture flow–piston flow’ in the dynamic process. The new insights into the characteristics of the seepage evolution in undisturbed loess under dynamic preferential flow will enrich the understanding of loess seepage and provided an important reference for future research on the slope instability of the loess induced by preferential flow.

This paper establishes the physical model of the prototype tailings pond using the similarity theory by taking a heightened and expanded project of tailings reservoir in Chengde, Hebei Province as an example and studies the process and mechanism of partial failure of the tailings reservoir when the drainage system fails by adopting the model test of the dam failure. The result shows that the dam damage can be divided into three categories: soil flow damage, gully damage and collapse damage. In addition, this paper analyzes the reasons for the three categories of damages.

The fly-ash dam is used to store the fly ash discharged from the thermal power plant. A fly-ash dam is a special slope built with fly ash, and rainfall infiltration is an important reason to induce the landslide of this kind of slope. In this paper, the laboratory tests of different slope ratios and initial seepage fields under rainfall were carried out, aimed at studying the failure mechanism, failure mode, triggering mechanism, and influence factors for the slope instability of the fly ash dam slope under rainfall infiltration. The results show that: (I) Three failure mechanisms were found in the tests: sliding failure, runoff erosion, and flow-slide failure. Due to the low density of fly ash, runoff erosion is more likely to occur under rainfall. Differently from clay slope, flow slide is an important failure mechanism of fly ash slope under rainfall. (II) Local erosion damages caused by runoff erosion and flow slide are the important triggering factors of the fly-ash dam slope failure under rainfall. (III) Three failure modes were observed in the test: the overall sliding failure of the slope, the retrogressive landslide caused by multi-stage local sliding, and the gradual erosion failure of the slope (caused by the combined action of runoff erosion and flow slide). (IV) The slope ratio has an important influence on the failure mode. With the decrease in slope ratio, the failure mode evolves from sliding failure to flow-slide failure and runoff erosion failure. The greater the slope ratio, the more obvious the sliding failure characteristics; the lower the slope rate, the greater the runoff erosion damage. The existence of an internal seepage field in the slope intensifies the occurrence of flow slide.

Based on the actual tailings reservoir project, a model test study on the evolution pattern of dam-breaking debris landslide is carried out. According to the similarity criterion of water flow and mud-sand movement, coal dust satisfying the similar relationship is selected as model sand. By analyzing the dam-breaking debris landslide’s range, propulsion speed, submerged depth distribution and particle deposition characteristics, the movement pattern of the landslide is obtained. In the short time after the dam break, the submergence range and advancing speed of the debris landslide increase rapidly. During the propulsion process, due to the increase of the debris landslide in the downstream of the valley, the debris landslide is diffused and the energy consumption during the flow is made. The advancing speed decrease until the landslide stops; The final submerged depth distribution of the dam breach has obvious randomness and non-uniformity, which is mainly related to the downstream terrain; According to the particle size grading analysis of the soil sample at the characteristic location, the further away from the dam toe, the finer particle content in the debris landslide.

Due to different soil strengths, drilled bored piles and slurry-supported bored piles affect the side friction. Through the application of post grouting technology, the friction resistance and bearing capacity of piles can be improved. In this paper, an analysis was conducted on the strength increasing rule of the side drilled bored hole and the side slurry-supported bored pile after post grouting at different ages, by means of chemical reinforcement. The results show that: the post grouting technology has better effect on the slurry-supported bored piles, especially in poor ground conditions, and that the strength on the 120^th day can be used as the design strength. Our research achievements can provide a theoretical basis for the post-grouting technology in engineering work.