# Experiments on the State Boundary Surface of Aeolian Sand for Road Building in the Tengger Desert
## 3 Test Results  
### Stress-strain Relationships  
Figure 4a,b shows the variation of the generalized shear stress and volumetric strain with axial strain under the drained conditions. Due to the complex and diverse in natural particle shapes of the aeolian sand (Figure 1), the particle internal friction is strong, resulting in obvious nonlinear characteristics in the stress-strain curves. At a low effective confining pressure (\(\sigma_{3}\leq 100\) kPa), the specimens exhibit strain hardening behaviour, and at a high effective confining pressure, they show strain-softening behaviours. The higher the effective confining pressure, the more obvious the softening characteristic, the greater the elastic modulus, the higher the peak strength, and the longer the elastic-plastic stage. At low effective confining pressure, the aeolian sand first undergoes contraction deformation and then dilates until failure is achieved. Meanwhile, the characteristics of contraction and dilatation are affected significantly by the effective confining pressure. With the increase of the effective confining pressure, the volumetric changes develop from dilatation to contraction. While the effective confining pressure reaches 800 kPa, only the contraction deformation occurs (compared to the initial volume of the specimen). Figure 5a,b show the variation of generalized shear stress and pore pressure with axial strain under undrained conditions. Figure 5a shows that the stress-strain relationships are softened only at relatively low effective confining pressures (e.g., 50, 100 and 200 kPa), and hardened at high effective confining pressures (>200 kPa). A greater effective confining pressure gives rise to a greater elastic modulus and a higher peak strength. Compared with the stress-strain relationships under the drained condition, the peak stress point of the undrained test is higher and the elastic-plastic stage is longer under the same load conditions. The results suggest that the undrained shear rate needs to be reduced to fully dissipate the excess pore pressure. Figure 5b shows the variation of pore pressure, implying that the dilatancy increases gradually with the increase of effective confining pressure. At low effective confining pressure, the negative pore pressure generally increases, while at high effective confining pressure, the pore pressure increases generally. It also shows that only dilation occurs at low effective confining pressure, while only contraction occurs at high effective confining pressure. This is consistent with the results obtained from drained tests. However, pore pressure has a greater influence on the stress-strain relationships under undrained conditions, and aeolian sand is more prone to dilatancy failure. Figure 4: Relationships between stress and volumetric strain with axial strain at drained condition. (**a**) Stress-strain relationships. (**b**) Volumetric strain-strain relationships. Figure 6 shows the variation of pore pressure coefficient \(A\) with axial strain. [PERSON] [37] reported that the specimen contracts for \(A>0\), and it dilates at \(A<0\). Therefore, in Figure 6, the pore pressure and strain relationships above auxiliary line \(A=0\) are contraction, and below auxiliary line \(A=0\) is dilation. The test results of the pore pressure coefficient \(A\) also show that the deformation and failure mode of the aeolian sand is dilatation under undrained conditions, and the contraction part only occurs within 2% of the axial strain. Figure 7a,b shows the relationships between axial strain and radial strain under drained and undrained conditions. The relationships between the axial strain and radial strain change linearly under drained condition. As the effective confining pressure increases, the slope of the linear relationship increases negatively, indicating that the aeolian sand has an initial anisotropy, and the initial anisotropy decreases with the increase of the effective confining pressure continuously.