# Interfacial Shear Properties of Red Clay and Polyurethane with Different Densities
## 1 Introduction  
Red clay, a soil type characterized by complex physical and mechanical properties, exhibits distinctive features including high plastic and liquid limits, elevated moisture content, significant void ratio, strong dispersibility, and low compressibility. This special soil is widely distributed in regions such as Guangxi and the Yunnan-Guizhou Plateau in China [1]. Consequently, it is highly sensitive to changes in moisture content. When moisture levels are excessively high, the soil becomes muddy, and when too low, it is prone to cracking. These conditions increase the likelihood of landslides, collapses, and other issues [2], potentially leading to casualties and property damage. Therefore, reinforcement measures are essential to improve the performance of red clay slopes and mitigate their negative effects on surrounding structures. Currently employed reinforcement methods primarily include external engineering stabilization techniques such as retaining walls and concrete slabs [3, 4, 5]. These approaches establish physical barriers to mitigate rainwater scouring and surface movements, thereby controlling soil erosion on slopes while ensuring safety and stability. However, such methods frequently induce ecological disturbances bydisrupting local ecosystems, suppressing vegetation growth, while their durability and structural integrity remain susceptible to environmental variations. Alternative strategies involve internal soil improvement through additives like cement, lime, fibers, and biopolymers [6; 7; 8; 9; 10; 11; 12]. These additives effectively enhance red clay's mechanical properties, permeability resistance, and erosion mitigation capabilities, making them applicable to slope stabilization and vegetation engineering. Nevertheless, despite achieving partial improvement in soil mechanics, these techniques exhibit inherent limitations. For instance, excessive lime content may reduce soil's peak strength [6] and compromise pore structure integrity [7]. However, these methods are often costly, and cementitious materials require a long curing period, resulting in slow construction speeds. These methods are not suitable for urban development, where rapid execution is necessary. In recent years, polymer organic materials have gained considerable attention. Several studies have shown that using these materials to improve soil can enhance the structural stability and erosion resistance of red clay slopes. Therefore, polymer organic materials have a good prospect in red clay slope engineering. Polyurethane is a polymer with dual properties of metal and rubber, specifically toughness and elasticity. It exhibits characteristics such as self-expanding properties, rapid gelation, high strength, corrosion resistance, and excellent impermeability. Due to its environmental safety and non-polluting nature, polyurethane is widely used in anti-seepage, repair, and foundation projects, such as roadbeds and underground pipelines [13; 14; 15; 16; 17]. To further assess the stability and safety of engineering structures repaired with polyurethane grouting, both domestic and international scholars have conducted extensive studies on its bonding performance with other matrix materials. [PERSON] [14] employed polyurethane to solidify sand slopes and, through strength and permeability tests, concluded that polyurethane significantly enhanced the cohesion of sand and soil, improved cohesion between sand particles, and increased the erosion resistance of slopes. Furthermore, polyurethane improved the water retention properties of soil, creating a favorable environment for vegetation growth. [PERSON] [18] used polyurethane prepolymers to enhance asphalt, demonstrating that polyurethane prepolymers effectively improved asphalt performance, especially at low temperatures. [PERSON] [19] used polyurethane to solidify calcareous sand, investigating its impact on the strength and stiffness of the improved soil through triaxial tests.