## 5 Model Validation  
In this section, we use the UMAT subroutine of the MGCM to simulate drying-wetting cycle tests of unsaturated soil triaxial samples for the model verification. By comparing the pore water pressure and void ratio changes in the simulation process with the prediction results of the Fortran program, the practicability of the UMAT subroutine is verified. The sample is a triaxial sample with a diameter of 40 mm and a height of 80 mm. The cell type is selected as a three-dimensional pore water pressure element with 8 nodes (C3D8P). Figure 3 shows a schematic diagram of the triaxial sample model. The sample material parameters are as follows: \(\lambda=0.11\), \(\kappa=0.02\), \(\lambda_{\rm w}=0.125\), \(\kappa_{\rm w}=0.063\), \(k_{1}=0.012\), \(k_{2}=1.8\), \(M=1.2\). The initial hardening parameter of effective stress is \(p_{0}^{*}=200\) kPa, and the permeability coefficient of the sample model is \(3.6\times 10^{-5}\) m/s. In this experiment, we set the displacement boundary condition at the initial analysis step to constrain the axial degree of freedom of the bottom surface. The pore water pressure boundary condition is set to a uniform pore water pressure of \(-100\) kPa, and its corresponding \(S_{\rm r}\) is \(0.1\). The \(e_{0}\) is \(1.186\), the three initial normal stresses are \(-100\) kPa, and the three initial shear stresses are zero (tension is positive in ABAQUS). In the consolidation analysis step, an equivalent pressure load of 100 kPa is set to balance the initial stress. During the drying-wetting cycle, a negative pore water pressure (matric suction) is applied on the top of the sample by using the Amplitude function in ABAQUS. The distribution laws of the pore water pressure and void ratio in the initial equilibrium state, wetting process and drying process are shown in Figures 4-6. Figure 4: Distributions of the pore water pressure and void ratio in the initial equilibrium state: (**a**) Pore water pressure distribution; (**b**) Void ratio distribution. Figure 3: Triaxial sample model. The initial equilibrium process of the sample model is similar to the triaxial consolidation process of triaxial samples of the same size under 100 kPa of confining pressure. According to the pore water pressure distribution result and void ratio distribution obtained in the equilibrium analysis step, the applied pressure boundary, displacement boundary, pore water pressure boundary and initial stress are well balanced. During the wetting process, the pore water pressure gradually decreases from the bottom of the sample model upward, and the \(S_{\text{r}}\) also decreases. At the same time, due to wetting, the matric suction inside the sample model gradually decreases, which causes the effective stress to decrease, so the specific volume of the sample model also increases. During the drying process, the pore water pressure gradually increases from the bottom of the sample model upward, and the \(S_{\text{r}}\) also increases. At the same time, due to drying, the internal matric suction of the sample model gradually increases, which causes the effective stress to increase, so the specific volume of the sample model continues to decrease. The relationship between the void ratio and matric suction during the drying-wetting cycle is shown in Figure 7. A comparison between the UMAT subroutine and the Fortran program calculation results is also shown. The calculation results of the UMAT subroutine and Fortran program are basically the same: the sample model produces elastic expansion deformation during the wetting process, and drying process causes the plastic deformation. Therefore, the UMAT subroutine can accurately simulate the unsaturated soil hydro-mechanical coupling characteristics during a drying-wetting cycle. Figure 5: Distributions of the pore water pressure and void ratio during the wetting process: (**a**) Pore water pressure distribution; (**b**) Void ratio distribution.