Rock and soil masses in geotechnical engineering projects, such as tunnels, mines and slopes, undergo relative motion, exhibiting mechanical characteristics of solid–fluid transition under critical conditions. This work analyzes the characteristics of the solid–fluid transition interface and the mode of load transfer through biaxial compression particle flow photoelastic experiments on granular materials. The study documents that this interface forms an arch shape, marked by a force chain arch. The granular material exhibits two distinct states depending on its position: below the arch, the granular material is in a solid–fluid transitional state, with bearing capacity reduced, while above the arch, it is in a stable solid state, capable of bearing the overlying rock layer’s load. The presence of the force chain arch alters the direction of the originally downward-transferring load, redirecting it along the trajectory of the arch. Analysis of the force and stability of the force chain arch revealed that the arch shape parameters and boundary loads control the instability of the arch. Changes in the overlying and lateral loads lead to different types of instability of the force chain arch. The findings of the study are crucial for underground engineering construction and for the prevention of geological disasters related to granular material.

The physical arch and the arch effect are related to many geotechnical problems, such as the stability of mine openings, a tunnel’s shape, and disaster prevention. However, observing the arch in geo-material is challenging due to Earth’s invisible force network distribution. The arch shape is a critical parameter related to the arching effect, which is still an open question. Specifically, the force network evolution after local excavation in granular material is revealed using the photo-elastic technique, and the system pressure variation is calculated qualitatively in particle scale according to the G² algorithm. Besides, the force chain arch is formed by strong force chains, which are important in redistributing the stress field. The principal direction of the spatial correlation colormap represents the direction of the strong force chains in the network. Based on this preferred angle between the strong force chain and the horizontal direction, a simplified force chain arch trajectory is iteratively rebuilt. Then, the total force chain distribution is studied considering both G² and the total length of strong force chains, with the corresponding results demonstrating a relationship between the changes in these characteristics and the force chain arch formation.

The creep failure of open-pit mine slopes with weak interlayers is one of the main types of slope instability in open-pit mines. The scientific and reasonable treatment of this type of landslide is of great significance for improving the quality of open-pit mining. In this study, we study a gently inclined and creep-type slope with weak interlayers in an open-pit mine in Inner Mongolia, China, and conduct systematic on-site engineering geological investigations, laboratory tests, and numerical simulations. The particle swarm optimization algorithm is introduced, and the creep model combining Burgers and Mohr–Coulomb is selected. Combined with triaxial compression creep test data, the creep model parameters of the weak interlayer soil are intelligently inverted. A typical profile is selected to analyze the stability of the slope. The results show that the creep of the weak interlayer is the main controlling factor for the deformation and failure of the slope. Under natural conditions, a clear continuous plastic zone appears at the front edge of the weak interlayer and the rear edge of the sliding body, resulting in slope instability and large deformation. Our results are in good agreement with the reality of engineering. Furthermore, we study the effectiveness of the local reinforcement treatment method for the weak interlayer. This study shows that local reinforcement of the weak interlayer is one of the most economical and effective means of preventing and controlling landslides. After reinforcement, the plastic zone of the slope only appears near the rear edge of the sliding body and the reinforced rock mass, with a poor connection, and the stability of the slope is good. Our results provide effective technical support for the treatment of this slope and offer a reference for the disaster prevention and mitigation of gently inclined and creep-type open-pit mine slopes with weak interlayers.