## 4 Comparative Analysis of Test Results
### Comparative Analysis of Numerical Simulation and Triaxial Photogrammetry
#### 4.2.2 Comparative Analysis of Shear Strength Indicators  
Mohr's circle's radius and center were calculated using the bias stress at the time of damage. The Mohr's circles under different confining pressures of the three methods were obtained by editing the program and running it with MATLAB. The program automatically calculated and fitted the common tangent of the three Mohr's circles, i.e., the shear-strength envelopes. Figure 16 illustrates that the simulated values for cohesion, angle of internal friction, and conventional three-axis results are closely aligned. This consistency supports the reliability of the numerical simulation and demonstrates that it is a viable method for determining the shear strength of soft clay. The numerical simulation can effectively replicate the conventional triaxial test for this type of soil sample. It allows for the rapid determination of cohesion and internal friction angle, enhancing efficiency and lowering laboratory testing costs.  
Figure 16: Cont.  
The test results of conventional triaxial, numerical simulation, and photogrammetry were plotted as Mohr-Coulomb envelope plots (e.g., Figure 16). The shear strength index was obtained from Figure 16 as in Figure 17.  
From Figure 17, it can be seen that the cohesive forces obtained by conventional triaxial, numerical simulation, and photogrammetry are 0.50, 0.60, and 0.91 kPa, respectively, and the angles of internal friction are 11.54, 11.49, and 11.17 degrees, respectively. It can be seen that the conventional triaxial cohesion is the smallest, and the angle of internal friction is the largest. Taking the conventional triaxial as the true value, the numerical simulation of the cohesive force increases by 0.10 kPa, while the photogrammetric increase is 0.41 kPa; the numerical simulation of the angle of internal friction decreases by 0.05 degrees, while  
Figure 16: Mohr stress circle: (**a**) CTC, (**b**) FEM, and (**c**) photogrammetry.  
Figure 17: Comparison of shear strength parameters.  
the photogrammetric decrease is 0.37 degrees. The numerical simulation results align with the conventional triaxial test outcomes regarding the variations in cohesion and the internal friction angle. This suggests that numerical simulations can effectively replace traditional triaxial tests for determining the shear strength index. On the other hand, the photogrammetric method provides a more precise measurement of the deformation across the entire surface by monitoring the CT points, resulting in slightly larger increases and decreases.  
Figure 18 depicts the shear strength envelope on the axes. The figure illustrates that a confining pressure of 100 kPa serves as a critical threshold. Below this pressure, the shear strength remains relatively consistent across the three methods. However, when the confining pressure exceeds 100 kPa, the shear strength derived from photogrammetry is lower than that obtained from conventional triaxial testing and numerical simulations. Numerical simulations can efficiently derive the cohesion and internal friction angle for soft clay in areas with pressure less than or equal to 100 kPa. When the pressure exceeds 100 kPa, photogrammetry is advised to obtain more precise cohesion and internal friction angle values. The three curves remain closely aligned at pressures up to 100 kPa, but they begin to diverge as peripheral pressure gradually increases to 300 kPa. Consequently, 300 kPa is selected as the reference point for this study.