## 3 Mechanical Characterization of Tailing Sand
### Effect of Particle Size on Consolidation Undrained Shear Test  
The different sizes of the grain groups can make a big difference to the strength of the soil. In this section of the test, specimens under the same conditions are made at least five different grain groups (<0.075 mm, 0.075-0.1 mm, 0.1-0.25 mm, 0.25-0.5 mm, 0.5-1 mm) for triaxial shear tests. The extracted tailing sand was dried and sieved to prepare triaxial specimens, which were pumped and saturated for consolidation undrained triaxial tests, and the test data and results were compiled and analyzed to derive the stress-strain relationship curves under different grain groups. The specific results are shown in Figure 10.  
Figure 9: Schematic diagram of the stress and strain curve type.  
From the experimental results, it is concluded that:  
(1) The peak partial stress of the sample increased with the particle size at 50 kPa and 100 kPa circumference pressures. Coarse-grained specimens have a greater internal occlusion so that the overall strength is high for large-grained samples and a significant increase in strain occurs at the initial stage of the test.  
(2) Under the same grain group condition, the stress-strain showed a monotonic increasing trend in the initial stage of the test, and the stress appeared to be the maximum value when the strain increased to a certain value. As the test proceeds, the strain continues to increase. At this stage, the partial stress remains constant or gradually decreases to a certain value and then remains constant, so most of the specimens are weakly hardened or weakly softened.  
(3) The stress-strain curves of the specimens of each grain group differed greatly when the Confining pressure was low (50 kPa, 100 kPa). That is, when the enclosing pressure is the same and the strain is the same, the difference in the shear-generated partial stresses of specimens from different grain groups is larger, and the difference is more obvious at 50 kPa than at 100 kPa. The difference in the values of the defective stresses of the specimens from different grain groups was smaller at higher envelope pressures (200 kPa) and the same strains. This is because the pore volume inside the specimen is squeezed at high envelope  
Figure 10: Stress–strain curve.(under different grain groups).  
pressure, so the difference between them is not significant, although the partial stress of the specimen increases with increasing grain size.  
With the axial stress \(\sigma\) as the horizontal coordinate and the shear stress \(\tau\) as the vertical coordinate, the total stress circle of damage is drawn in the horizontal coordinate with \((\sigma_{1}+\sigma_{3})/2\) as the center and \((\sigma_{1}-\sigma_{3})/2\) as the radius, and the tangent line of the stress circle. The stress Mohr circle strength envelope is made. The inclination angle of this envelope is the internal friction angle \(\varphi\), and the intercept with the vertical coordinate is the cohesive force \(c\), as Figure 11.  
Based on the inclination and slope of each intensity envelope, the intensity parameter values for the above five different particle sizes are obtained, as shown in Figure 12. The left axis represents the cohesion, the right axis represents the internal friction Angle, and the horizontal axis represents the particle size range.  
What can be obtained from the test results is: In the triaxial test, the cohesive force \(c\) tends to decrease as the particle size increases, while the internal friction angle \(\varphi\) tends to increase, indicating that the strength of the specimen increases as the particle size increases.  
Figure 11: _Cont._