# Mechanical Behavior of Weathered Granite Exposed to Water
## 4 Modeling  
### Model for Predicting Strain Softening  
Under the action of excavation and unloading, the settlement of the foundation caused by the strain and softening of weathered rock is an essential part of the post-construction of an immersed tube tunnel. Therefore, the establishment of a strain-softening model to describe the changes in stress and strain under the action of excavation and unloading can help more accurately predict the long-term settlement of the foundation, thereby providing corresponding guidance on the settlement monitoring during the operation of the immersed tube tunnel and ensuring the tunnel safety. [PERSON] (2003) [32] used the results of triaxial tests combined with the Dafalias bounding surface framework and the state-dependent dilatancy sand model proposed by [PERSON] (2000) [33] to obtain the constitutive model of fully weathered granite. However, there are many model parameters, and there are 17 model parameters without considering the structural influence of weathered rock and small strain characteristics. [PERSON] (2003) studied and compared the constitutive models of unsaturated, compacted, weathered granite soil and believed that the elastoplastic constitutive model of unsaturated soil proposed by [PERSON] et al. (1990) is more suitable for unsaturated, compacted, weathered granite soil after modification. On the basis of summarizing many research reports and experimental documents in Hong Kong, [PERSON] (2008) [34] used the unified model of clay and sand proposed by [PERSON] [25] to present a \begin{table}
\begin{tabular}{c c c c c} \hline
**Specimen Type** & **Consolidation Method** & \(\mathbf{q_{n,\text{max}}\) (kPa)} & \(\mathbf{q_{s,\text{max}}\) (kPa)} & \(\mathbf{\xi_{r}}\) \\ \hline \multirow{4}{*}{Fully weathered granite} & 0 & 292.63 & 267.03 & 0.91 \\  & 0.3 & 343.77 & 305.89 & 0.89 \\  & 0.6 & 460.75 & 375.81 & 0.82 \\  & 0 & 400.46 & 338.74 & 0.85 \\ Sandy weathered granite & 0.3 & 484.22 & 402.20 & 0.83 \\  & 0.6 & 575.96 & 469.73 & 0.81 \\ \multirow{4}{*}{Fragmented weathered granite} & 0 & 783.96 & 633.31 & 0.81 \\  & 0.3 & 1109.74 & 827.09 & 0.75 \\ \cline{1-1}  & 0.6 & 1249.13 & 901.40 & 0.72 \\ \hline \end{tabular} Note: In the Consolidation method column, 0, 0.3, and 0.6 indicate the isotropic consolidation, the deviator stress consolidation with 30% deviator stress, and the deviator stress consolidation with 60% deviator stress, respectively.  
\end{table} 
Table 3: The peak strength and degradation index of the weathered granite under the different consolidation conditions and in the various rock states. practical constitutive model for remodeling fully weathered rock. This model can reflect the critical state theory and the phase transformation of sand and can better represent the main mechanical properties of remolded, fully weathered granite. All the above models are based on the test results of weathered rocks in their natural state and do not take the strain-softening characteristics of weathered granites exposed to water into account. Therefore, it is necessary to develop a strain-softening model suitable for both natural and saturated weathered granite. For the convenience of studying the problem, this paper employs the following model to study the strain-softening characteristics of the rock mass. The strain-softening process of the rock mass is presented in Figure 8. In this model, the elastic modulus of the rock mass before the peak strength is regarded as a constant quantity. In the figure, AB represents the elastic stage before the peak strength of weathered rock, and BC is the softening stage. In addition, the straight line parallel to AB is the unloading path during triaxial compression, and the distance from the point of intersection with the abscissa to the origin is the plastic internal variable \(\epsilon_{1}^{p}\).