# Effect of Drying-Wetting Cycle and Vibration on Strength Properties of Granite Residual Soil
## 3 Test Results and Analysis
### Analysis of Direct Shear Test Results
#### 3.1.2 Variation in Shear Strength Parameters  
(1) Cohesion  
As the stress-strain curve exhibits strain hardening with no distinct peak, the shear stress corresponding to a shear displacement of 4 mm was taken as the shear strength according to the specification [32]. Based on the Mohr-Coulomb criterion, the shear strength parameters under different DW cycles and vibration times were calculated, as shown in Figure 6. The results show that the cohesion decreases nonlinearly with the increase in the number of DW cycles and vibration time. When the specimen undergoes only DW cycles or vibration, the cohesion decreases monotonically, which is in line with a study conducted  
Figure 5: Stress–strain curves of direct shear test. by [PERSON] et al. [3]. However, when the specimen undergoes both DW cycles and vibration, the cohesion exhibits fluctuating decreases in the early stages of DW cycles. For example, under the vibration durations of 30 and 45 min, the cohesion experiencing one DW cycle was 5.1% and 7.6% higher than that of the specimens without DW cycles, respectively. The degradation cohesion index \(\xi_{t,n}\) was used to quantify the effects of DW cycles and vibration on cohesion degradation, as defined in Equation (1), and the calculation results are presented in Table 3.  
\[\xi_{t,n}=\frac{c_{0,0}-c_{t,n}}{c_{0,0}}\times 100\% \tag{1}\]  
where \(\xi_{t,n}\) is the cohesion degradation index after \(t\) minutes of vibration and \(n\) times of DW cycles; \(c_{0,0}\) is the cohesion without DW cycles or vibration; and \(c_{t,n}\) is the cohesion after \(t\) minutes of vibration and \(n\) times of DW cycles. Table 3 shows that the cohesion undergoes significant degradation when specimens are subjected to either DW cycles or vibration alone. \(\xi_{t,n}\) reaches 41.4% and 35.9% when the specimens are vibrated for 45 min or undergo five DW cycles, respectively. The degradation effect of GRS under the combined action of DW cycles and vibration is not simply additive. Under the first three DW cycles, the degradation from 0 to 15 min of vibration is remarkably significant, with a difference in \(\xi_{t,n}\) exceeding 26%. Under the last two DW cycles, the difference in \(\xi_{t,n}\) between 0 and 15 min of vibration is less than 8.5%. Under the same vibration time (except for 0 min), there is no significant degradation in \(\xi_{t,n}\) with the increase in the number of DW cycles. The average difference in \(\xi_{t,n}\) between 5 and 0 DW cycles is approximately 15%. (2) Internal friction angle  
Figure 6: Cohesion change graph. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Vibration**} & \multicolumn{6}{c}{**Cohesion Degradation Index \(\xi_{t,n}\)/\%**} \\ \cline{2-7}  & \multicolumn{6}{c}{**DW Cycles/Times**} \\ \cline{2-7}  & **0** & **1** & **2** & **3** & **4** & **5** \\ \hline
0 & 0.0 & 4.5 & 10.5 & 14.8 & 30.3 & 35.9 \\
15 & 26.2 & 33.1 & 37.9 & 40.3 & 38.8 & 42.6 \\
30 & 39.7 & 36.6 & 40.6 & 44.8 & 44.8 & 43.1 \\
45 & 41.4 & 36.9 & 41.4 & 42.8 & 49.1 & 58.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: \(\xi_{t,n}\) under DW cycles and vibration. Figure 7 presents that the internal friction angle increases in a fluctuating manner with the increase in the number of DW cycles and vibration time. The research results regarding the influence of DW cycles on the internal friction angle are inconsistent. Some scholars believe that DW cycles mainly weaken interparticle bonding, but the main factor affecting the variation in the internal friction angle is the frictional characteristics between soil particles. Therefore, the impact of DW cycles on the internal friction angle is not significant [4,14]. However, some scholars argue that the matric suction generated by DW cycles can cause consolidation and volume shrinkage of the soil, enhancing the contact between soil particles, thereby leading to an increase in the internal friction angle [34,35]. The latter view is more in line with the results of this study.