# Hypoplastic Modeling of Soil-Structure Contact Surface Considering Initial Anisotropy and Roughness
## 5 Conclusions  
Drawing from the foundational equations of the Gudehus-Bauer eight-parameter hypoplasticity model, this paper examines the influences of initial anisotropy and surface roughness on the mechanical behavior of sandy soils and develops a hypoplasticity constitutive model for the sandy soil-structure contact surface that incorporates these factors. The model is subsequently validated against results from various contact surface experiments. The principal findings are summarized as follows:  
1. A new hypoplasticity constitutive model for sandy soil-structure contact surfaces is proposed by incorporating the effects of initial anisotropy and surface roughness into the Gudehus-Bauer eight-parameter hypoplasticity model. This model consists of 13 fundamental parameters, which can be determined based on the particle size characteristics of granular materials.
2. The proposed model is validated by comparing its predictions with experimental results from various contact surface conditions, including Toyoura sand-steel plate, Changing composite gravel/uniform gravel-steel plate, and dense sand-concrete contact surfaces under different roughness conditions. The predicted results align well with experimental data, confirming that the model accurately captures the mechanical behavior of contact surfaces under varying roughness conditions.
3. A FRIC-UMAT subroutine was developed to implement the hypoplasticity model in finite element simulations. The modified subroutine was applied to typical direct shear tests on contact surfaces, and the simulation results were compared with experimental data. Additionally, the performance of the proposed model was compared with that of the Mohr-Coulomb contact surface model under varying initial deposition angles and surface roughness conditions. The results demonstrate the accuracy of the FRIC subroutine and validate the rationality and superiority of the proposed model.  
As far as the isotropic condition is considered, more influencing factors, such as stress-induced anisotropy and temperature, should be introduced into the proposed hypoplasticity contact surface model for the practical engineering. This will enhance the reliability of the model. At the same time, the hypoplasticity model of cohesive soil will be further adapted to the cohesive soil-structure contact surface. This innovation work will expand the applicability of the hypoplasticity model, enabling it to be used for both cohesive and non-cohesive soil contact surfaces. The development of this hypoplasticity contact surface model offers potential advantages for practical engineering applications, particularly in analyzing large deformations of soil foundations, such as the floating or settling of pile foundations.  
**Author Contributions:** Conceptualization, Z.C.; data curation, J.Y. and J.Z.; investigation, J.Y. and J.C.; methodology, Z.C.; software, Z.C. and Y.Z.; supervision, P.Y.; validation, Z.C.; visualization, J.Z.; writing--original draft, J.Y., J.C. and Z.C.; writing--review and editing, J.Z., Y.Z. and P.Y. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Details of the analytical model presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy.  
**Conflicts of Interest:** The authors declare no conflicts of interest.