# Research on the Accumulated Plastic Strain of Expansive Soil under Subway Loading
## 3 Analysis of Test Results
### Analysis of Accumulated Plastic Strain Behavior of Expansive Soil under Intermittent Loading
#### 3.2.2 Analysis of the Influence Law of Static Deviatoric Stress on the Accumulated Plastic Strain  
When the cyclic stress ratio \(\eta_{d}\) is 0.2, the development law of static deviatoric stress on accumulated plastic strain under intermittent cyclic loading in subway tunnels with \(\sigma_{\rm s}=15\) kPa and \(\sigma_{\rm s}=30\) kPa is analyzed as shown in Figure 7. The following conclusions can be drawn from the graph:  
The accumulation of plastic strain development curves under two different static deviatoric stresses exhibited a similar pattern. Both displayed rapid initial growth followed by a deceleration in growth rate, ultimately reaching a stabilized state. Under the same cyclic loading ratio conditions, it was observed that static deviatoric stress notably influenced the accumulation of strain. The accumulated plastic strain increased with higher static deviatoric stress, leading to conclusions analogous to those from continuous loading scenarios. When the cyclic loading ratio \(\Delta T/T\) was 1, the accumulated plastic strain at the point of stabilization under \(\sigma_{\rm s}=30\) kPa conditions was approximately 150% higher compared to \(\sigma_{\rm s}=15\) kPa conditions. For cyclic loading ratios of \(\Delta T/T\) equal to 5, 10, and 50, the accumulated plastic strain increased by about 150%, 150%, and 149%, respectively. Consequently, doubling the static deviatoric stress resulted in an approximate 150% expan  
Figure 7: Translation: Development curve of accumulated plastic strain with the number of cycles under different levels of static deviatoric stress. (Sample numbers are S8, S11, S14, S17, S20, S23, S26, S29). (**a**) \(\Delta T/T=1\); (**b**) \(\Delta T/T=5\); (**c**) \(\Delta T/T=10\); (**d**) \(\Delta T/T=50\).  
sion in accumulated plastic strain under intermittent subway cyclic loading. Furthermore, the initial strain rate during cyclic loading was also influenced by the magnitude of static deviatoric stress. Larger static deviatoric stress led to higher initial strain rates.  
This behavior primarily stems from the structural characteristics of the soil, characterized by weak inter-particle connections and significant instability. In dynamic triaxial testing, the soil experiences the combined effects of cyclic dynamic loading and static deviatoric stress. The substantial external loads lead to instantaneous rupture of the inter-particle connections, causing significant deformation in the initial stages. Consequently, the strain development rate is highest during this phase. When static deviatoric stress remains constant, the strain development rate diminishes, eventually reaching a stable state with minimal fluctuation within a small strain range as the cyclic cycles progress. With increasing static deviatoric stress, the instantaneous deformation of the soil also becomes more substantial, leading to a higher initial strain development rate. In summary, excessive static deviatoric stress significantly accelerates the sustained growth of soil strain.