# Hypoplastic Modeling of Soil-Structure Contact Surface Considering Initial Anisotropy and Roughness
## 1 Introduction
Based on that, [PERSON] [23] introduced the reduced component stress tensor and strain rate tensor into the von Wolffersdorff eight-parameter hypoplasticity model, establishing a one-dimensional contact surface hypoplasticity model with a predefined limit state surface. [PERSON] and [PERSON] [24] proposed a new transformation method, converting the von Wolffersdorff eight-parameter model into a two-dimensional contact surface hypoplasticity model. This model can be used to model line contact in a two-dimensional state and surface contact under three-dimensional conditions. Based on this model, [PERSON] et al. [25] introduced an in-plane stress and established an enhanced three-dimensional sandy soil-structure contact surface hypoplasticity constitutive model. Following that, [PERSON] and [PERSON] [26] applied the reduced component stress and strain rate tensors to the hypoplasticity Cambridge clay model by introducing a variable that reduces the strength and stiffness of the contact surface, thereby establishing a three-dimensional Cambridge clay contact surface hypoplasticity model. As a relatively recent constitutive theory, hypoplasticity has garnered significant applications in the development of contact surface constitutive models [27; 28; 29; 30]. Initial anisotropy, as an inherent property of soil, has a significant impact on soil-structure contact surfaces [31; 32; 33; 34]. [PERSON] [31] proposed a constitutive model grounded in a nonlinear tensor function to depict the three-dimensional deformation-strength behavior of naturally anisotropic sandy soils, achieving a comprehensive treatment of both deformation and strength anisotropy. [PERSON] et al. [32] introduced a fabric tensor into the Gudehus-Bauer hypoplasticity model and proposed a hypoplasticity constitutive model for granular soils based on anisotropic critical state theory. [PERSON] et al. [33] analyzed the formation of shear bands in cohesionless and initially transversely isotropic granular materials, presenting a constitutive equation for stress evolution. Surface roughness of structures is also an important factor influencing the mechanical properties of contact surfaces, which has been considered by researchers from different perspectives [24; 26; 35]. [PERSON] and [PERSON] [24] proposed a hypoplasticity model that uses a parameter, \(K_{r}\), to describe contact surface roughness. [PERSON] and [PERSON] [26] argued that parameter \(K_{r}\) depends on contact surface roughness, establishing an enhanced hypoplasticity contact surface model. [PERSON] et al. [35] conducted axisymmetric contact surface shear simulations using hypoplasticity contact and soil models, comparing the results with shear tests on contact surfaces of varying roughness. In many existing intrinsic models for hypoplasticity contact surfaces, the stress and strain rate components within the contact surface are often overlooked, and initial anisotropy is not considered. This paper introduces a hypoplasticity constitutive model for sand-structure contact surfaces, derived from the widely adopted Gudehus-Bauer low-plasticity model by simplifying the stress tensor and strain rate tensor components. The model incorporates the effects of initial anisotropy and surface roughness. The developed model is then applied to predict and compare with experimental data from sand-structure contact surfaces of varying materials and boundary conditions, thereby validating the model's rationality.