## 3 Test Results and Analysis  
### 3.1. Triaxial Shear Test Results and Analysis  
#### 3.1.1 Stress-Strain Relationships  
Figures 7-11 display the stress-strain curves of the modified soil and granite residual soil plain under various enclosing pressures. As shown in Figure 6, the bias stress exhibits a transient linear increase at axial strain \(\varepsilon\) < 2%, followed by a nonlinear increase, and the stress-strain curve of the plain soil is of the strain-hardening type. Figures 7-10 show that the curve of the enhanced soil with the fly ash admixture displays a strain-softening type, meaning that the specimen's stress first increases with strain and then stops increasing when the strain reaches a particular amount. Following an increase in the perimeter pressure, the axial stresses at the damage of the four doped fly ash-amended soils increased concurrently. The axial stresses of the four doped fly ash-amended soils simultaneously increased upon damage when the perimeter pressure increased from 100 kPa to 300 kPa. A comparison of the stress-strain curves of the two soil samples shows that the soil treated with fly ash had better structural characteristics than the plain soil. The peak strengths of the vegetative soil for each of the perimeter pressures are lower than the peak strengths of  
Figure 6: TST-55 penetrometer. The variation curves of the peak strengths of the soils modified with pulverized coal and five fly ash admixtures at varying enclosure pressures are displayed in Figure 12. The peak strength of each doped soil body is strongly associated with the perimeter pressure, as shown in Figure 12. At the perimeter pressures of 100, 200 and 300 kPa, the peak strengths of the fly ash-amended soil specimens modified with 5% fly ash increased by 50.21%, 36.17% and 28.56%, respectively, in comparison to those of the plain soil specimens. The peak strengths of the fly ash-modified soil specimens with 10% admixture increased by 42.90%, 34.54% and 59.41%, respectively, in comparison to those of the plain soil specimens; these increases were 75.54%, 54.59% and 43.30%, respectively, for the soil specimens modified with 15% fly ash and 61.88%, 45.30% and 37.29%, respectively, for the soil specimens modified with 20% fly ash. Compared with those of the plain soil specimen, the soil specimens modified with 20% fly ash showed peak strength increases of 61.88%, 45.30% and 37.29%. The specimens' peak stress first increases as the fly ash dosage increases from 0% to 15%, and then decreases as the fly ash dosage increases to 20%. This indicates that there is an optimal dosage value of 15% for enhancing the strength of the granite residual soil with fly ash. The significant internal porosity, relatively loose soil skeleton and poorly connected soil particles in the granite residual soil give rise to this trend. The tiny particles of the material fill the pores in the granite residual soil once the fly ash is added. The silica and alumina in the fly ash also experience a hard coagulation reaction with the granite residual soil, enhancing the coagulation ability of the soil. Furthermore, when fly ash and water come into contact, the fly ash can initiate a hydration reaction that results in the formation of a gelling material that reinforces the soil body and further bonds the soil particles. When the fly ash dose was increased from 0% to 15%, the fly ash particles adhered to the granite residual soil more firmly, and the strength of the granite residual soil increased concurrently. The reinforcing effect of fly ash on the soil is weakened when the dosage is increased to 20% because the fly ash particles essentially fill the pores in the granite residual soil, and the excess fly ash particles are not present in the soil, reducing  
Figure 11: Stress-strain curves of the modified soil with 20% fly ash. Figure 10: Stress-strain curves of the modified soil with 15% fly ash. the size of the soil skeleton.