# Hypoplastic Modeling of Soil-Structure Contact Surface Considering Initial Anisotropy and Roughness
## 1 Introduction  
The interaction at the soil-structure contact surface is commonly found in engineering, occurring in scenarios such as the interface between soil and retaining walls, and between piles and the encircling soil. Serving as a critical conduit for stress and deformation transfer between soil and structures, the contact surface is often the weak link in the failure of soil-structure systems, with its mechanical properties exerting a substantial impact on the overall stress and deformation of both the soil and the structure. Consequently, the mechanical characteristics and the development of a constitutive model for soil-structure contact surfaces have consistently been a significant area of focus in the field of soil-structure interaction research. The constitutive model for soil-structure contact surfaces is a fundamental method for describing their stress-strain relationships. Currently, the constitutive models used in practical applications include elastic constitutive models, elastoplastic constitutive models, hypoplastic constitutive models, and damage constitutive models primarily developed for easily breakable particles [1, 2, 3, 4, 5, 6, 7]. However, the equations and theoretical underpinnings of these models are quite intricate, presenting numerous challenges for research endeavors. In contrast, hypoplasticity theory is more straightforward in form, with fewer assumptions, and has proven to be highly amenable to numerical implementation. This has led to itsbroad application in characterizing the mechanical properties of geotechnical engineering materials. The basic framework of hypoplasticity theory was first proposed by [PERSON] [8] in 1977, and [PERSON] [9] was the first to introduce the term \"hypoplasticity\". Based on the hypoplasticity framework, many scholars proposed constitutive models focusing on different soil types and aspects. [PERSON] [10] first proposed an isotropic nonlinear tensor function to describe the relationship between the stress rate and strain rate of elastoplastic materials. Based on this equation, [PERSON] and [PERSON] [11] introduced a simple four-parameter model known as the Wu-Bauer hypoplasticity model. This model is capable of describing the nonlinear stress-strain relationships of sandy soil under both drained and undrained conditions, as well as the contraction of loose sand and the dilation of dense sand. [PERSON] [12] and [PERSON] [13] introduced the concept of critical state into the Wu-Bauer model and proposed the well-known eight-parameter hypoplasticity model, i.e., the Gudehus-Bauer eight-parameter model. The model captures the strain softening behavior of dense sand and the mechanical responses of sandy soils under a variety of stress paths effectively, including triaxial extension, triaxial compression, laterally confined compression, and consolidated undrained conditions. [PERSON] et al. [14] addressed the issue of excessive lateral contraction of the Wu-Bauer model under large strains by establishing the von Wolffersdorff hypoplasticity basic model, where two nonlinear material parameters were assumed to be equal. [PERSON] et al. [15] introduced a new item that disappears at the critical state in the von Wolffersdorff hypoplasticity model, resulting in a wildly used hypoplasticity constitutive model. As hypoplasticity theory has evolved, researchers in recent years have concentrated on delineating the mechanical behavior of soil-structure contact surfaces, leading to the establishment of diverse forms of hypoplasticity constitutive models tailored for these interfaces [16; 17; 18; 19; 20; 21]. [PERSON] [22] pioneered the adaptation of the sandy soil hypoplasticity model to a sandy soil-structure contact surface hypoplasticity model. This was achieved by positing that the stress tensor and strain rate tensor at the contact surface are composed solely of normal and tangential components.