# Deformation Behavior of Saturated Soft Clay under Cyclic Loading with Principal Stress Rotation
## 5 Conclusions  
To study the deformation characteristics of soft clay under traffic loading with the continuous principal stress rotation, a series of cyclic HCA tests were conducted on Wenzhou soft clay under different torsional cyclic stress ratios and degrees of principal stress rotation. The conclusions are summarized as follows:  
1. The continuous rotation of the principal stress axis could be controlled by dynamic hollow cylinder torsional shear apparatus (DHCA) to simulate traffic loading. Compared with the traditional triaxial test without considering the principal stress rotation, the rotation of the principal stress axis would aggravate the axial deformation of soil samples. Therefore, the influence of the principal stress rotation must be considered in the prediction of settlement and stability for pavement engineering suffering traffic loading.
2. Changes in the vertical cyclic stress ratio VCSR and torsional cyclic stress ratio \(\eta\) had a more pronounced effect on the dynamic mechanical properties of soft clay. Under the traffic loading, the permanent strain and pore pressure increased with the increments in VCSR and \(\eta\), and there was stiffness degradation of the soil.
3. A modified pore pressure model considering the rotation of the principal stress was proposed, and the relationship between the growth of pore pressure and cycles considering the effect of the principal stress rotation was established. The reasonableness of the model was confirmed by predicting the growth of the accumulated pore pressure under different cycles.
4. The permanent strain and pore pressure showed an overall increasing trend with the number of cycles, but both tended to stabilize with cycles. At the same time, the slope of the backbone curve of saturated soft clay decreased, and the stiffness of the soil decreased.
5. The traditional H-D backbone curve model without considering the continuous rotation of the principal stress axis under traffic load was modified, and the modified H-D backbone curve model considering the continuous rotation of the principal stress axis was established. Compared with the traditional H-D backbone curve model, the prediction accuracy of the modified model is significantly improved, which can provide some guidance for pavement design.  
Figure 14: Comparison of predicted and measured axial dynamic strain.  
**Author Contributions:** Conceptualization, Z.F.; methodology, Z.F., G.W. and W.S.; software, G.W. and W.S.; validation, Z.F. and G.W.; formal analysis, W.S.; investigation, Z.F.; resources, Y.Y. and W.S.; data curation, Y.Y. and P.W.; writing--original draft preparation, Y.Y., T.W. and P.W.; writing--review and editing, Z.F., T.W. and G.W.; visualization, T.W. and P.W.; supervision, Z.F. and P.W.; project administration, Z.F. and T.W.; funding acquisition, Z.F. All authors have read and agreed to the published version of the manuscript.  
The work presented in this paper was financially supported by the Natural Science Foundation of Zhejiang Province (grant numbers LQ21E080015).  
Not applicable.  
The data used to support the findings of this study can be obtained from the corresponding author upon request.  
The authors declare no conflict of interest.