# Shear Behaviour of Aeolian Sand with Different Density and Confining Pressure
## 3 Results and Discussion  
### Stress-Strain Behaviour  
Figure 5 shows the stress-strain volumetric behaviour of aeolian sand under drained condition. The stress of dense sand specimens shows a visible peak followed by strain softening accompanied by dilative volumetric strains. On the other hand, the \(q\)-\(\epsilon_{1}\) curves for the tests of loose sand exhibit strain hardening along with overall contractive volumetric strains. As can be seen from Figure 5a,c,e, the peak strength of aeolian sand increases with the increase in density. However, the strength of aeolian sand tends to have the same residual strength at the end of loading. At the same time, greater residual stress is developed at higher confining pressure for aeolian sand. Figure 5b,d, and f show the stress and volumetric strain of aeolian sand. For loose sand, the contraction occurs during the shear process, but for dense sand, the contraction occurs at the initial stage of loading and then dilatancy. The test results for the stress and volumetric strain of all the specimens reached constant values at the end of the tests, indicating that all the specimens reached the critical state. As can be seen from Figure 5b, the volume change of loose sand is larger than that of loose sand, while in Figure 5f, the volume change of loose sand is larger than that of dense sand.  
The undrained triaxial shear behaviour of aeolian sand with different densities is shown in Figure 6. The \(q\)-\(\epsilon_{1}\) curves indicate that the specimens with a \(D_{\rm r}\) of 90% mobilized the greatest deviatoric stresses while the specimens with a \(D_{\rm r}\) of 10% mobilized the smallest deviatoric stresses. The specimens with the smallest \(D_{\rm r}\) exhibited positive \(\Delta u\) that increased to almost confining pressure, while the specimens with a \(D_{\rm r}\) of 90% exhibited initial positive \(\Delta u\) and decreased to negative values by the end of shearing. As can be seen from Figure 6a, the deviatoric stress of loose sand (\(D_{\rm r}\) = 10% and 30%) reached the peak stress of about 20 kPa at an axial strain \(\epsilon_{1}\) = 1%, and then the stress gradually decreased to the end of shear, which the axial strain was about 2.5%, the specimen collapsed and lost its strength. The trends exhibited by loose sand with \(\sigma_{3}\) = 100 kPa was also under high confining pressure, which can be seen from Figure 6c,e. This phenomenon can be regarded as a kind of static liquefaction of sand. With the increase in strain, the excess pore water pressure rises sharply and the effective stress decreases sharply, so the sand is in a flow state, also known as \"flow slip\". In this phenomenon, the loose sand (\(D_{\rm r}\) = 10%) generated positive \(\Delta u\) which is close to confining pressure at a small axial strain and its deformation at the end of shear is shown in Figure 4a. The \(\Delta u\) of the dense sand specimens during the tests increased rapidly at axial strains smaller than 1%, after which the magnitudes tended to decrease slowly throughout the end of the tests. These trends are consistent with the rapid rate of contraction observed in the drained tests in Figure 5 at small axial strains followed by the dilation at larger axial strains. The \(q\)-\(\varepsilon_{1}\) curves for dense sand exhibit a peak followed by strong strain softening accompanied by visible shear band as shown in Figure 4c.  
Figure 5: Deviatoric stress-axial strain-volumetric strain behaviour of the aeolian sand under drained condition. (**a,b**) \(\sigma_{\mathcal{Y}}=100\) kPa; (**c,d**) \(\sigma_{\mathcal{Y}}=300\) kPa; and (**e,f**) \(\sigma_{\mathcal{Y}}=600\) kPa.  
the increase in confining pressure. The undrained test results indicate that medium-dense sand exhibits stress behaviour not in agreement with that of drained test results but the dilatancy behaviour in agreement with that of drained test results.