# Interfacial Shear Properties of Red Clay and Polyurethane with Different Densities
## 1 Introduction
The results indicated that polyurethane effectively enhanced the mechanical properties of calcareous sand and suggested optimal polyurethane dosages for different grades of calcareous sand. [PERSON] and [PERSON] [20; 21] investigated the effects of interfacial moisture content, polyurethane density, and concrete strength on the interfacial shear properties between polyurethane and concrete through scanning electron microscopy (SEM) and direct shear tests. [PERSON] [22] found that an appropriate amount of polyurethane in modified concrete improved the elastic modulus, compressive strength, and flexural strength. Regarding the interfacial shear characteristics between polyurethane and other engineering structural materials, [PERSON] [23] studied the interfacial shear behavior between polyurethane and expansive soil, proposing the concepts of equivalent internal friction angle and equivalent cohesive force at the interface. [PERSON] and [PERSON] [24; 25] investigated the shear behavior at the polyurethane-sand interface. [PERSON] [26] examined the factors influencing the shear strength at the polyurethane-soil interface and the polyurethane-steel plate interface. In current domestic and international studies on polyurethane-stabilized red clay, [PERSON] [27] demonstrated that the incorporation of polyurethane enhances the compressive strength of red clay through comparative analyses of its mechanical properties--including compressive strength, shrinkage behavior, and resistance to water erosion--before and after polyurethane treatment. [PERSON] [28; 29] examined the mechanism and effects of polyurethane on red clay solidification through a series of experiments, finding that polyurethane improved the strength, stiffness, and toughness of red clay, reduced the damping ratio, and prevented excessive settlement in red clay roadbeds. In summary, polyurethane materials have demonstrated considerable potential in geotechnical applications such as foundation treatment and slope reinforcement. Due to the diversity and complexity of polyurethane materials, the stabilization mechanisms and effects of different polyurethane types on soil exhibit significant variations. However, research on polyurethane-stabilized red clay remains relatively scarce, particularly regarding the interfacial shear characteristics of polyurethane-soil composites. Furthermore, existing studies have predominantly focused on comparing the strength and mechanical properties of red clay before and after polyurethane incorporation, while investigations into the influence of polyurethane density variations remain insufficient. Based on this foundation, this study conducted laboratory direct shear tests to investigate the effects of polyurethane density variations and normal stress levels on the interfacial shear characteristics of composite matrices. A finite element model of the composite system was subsequently developed, incorporating the influences of polyurethane density and normal stress based on experimental findings, and validated through comparative analysis with empirical test data.