# Experiments on the State Boundary Surface of Aeolian Sand for Road Building in the Tengger Desert
## 4 Conclusions  
In this paper, the state boundary surface and deformation and failure characteristics of aeolian sand in the Tengger Desert, China are studied through a series of drained and undrained triaxial tests. The following conclusions can be drawn from this study:  
1. The generalized peak shear stress of aeolian sand increases linearly with the increase of effective confining pressure under drained and undrained conditions; the undrained strength is greater than the drained strength at low effective confining pressure, and the strength is close to the same with the increase of effective confining pressure. The peak friction angle decreases nonlinearly with the increase of effective confining pressure. The peak friction angle of the two test conditions is quite different at low effective confining pressure and is close to the same at high effective confining pressure.
2. At low and medium effective confining pressures, the dilatancy is obvious. With the increase of effective confining pressures, the dilatancy develops to contraction. At high effective confining pressures, it only contracts. The development of pore pressure under undrained conditions also reflects a similar law.
3. The medium-density specimen of aeolian sand obtained by the multiple sieving pluviation method has strong initial anisotropy. With the increase of effective confining pressure, the effect of initial anisotropy gradually weakens. While the effective confining pressure is 800 kPa, the initial anisotropy has almost no effect on the deformation characteristics. The initial anisotropy characteristics of the undrained test are not obvious, and the relationships between axial strain and radial strain always satisfy the relationship \(\varepsilon_{1}=-2\varepsilon_{3}\).
4. The three-dimensional state boundary surface of aeolian sand considering dilatancy is quite different from that of remoulded clay. The study of the state boundary surface and the determination of critical state line and phase transformation line equations in three-dimensional space describe the unique state relationship formed by the generalized normal stress, shear stress and void ratio accurately, which can predict the quantitative relationship between stress state and volumetric state reasonably. The state boundary parameters can provide the basis for the establishment of the constitutive model of aeolian sand and provide basic test support for the geotechnical design, construction and maintenance of foundations, roadbeds and another foundation engineering in desert areas.  
Conceptualization, Z.M. and X.L.; methodology, X.L.; validation, Z.M., X.L.; formal analysis, X.L.; investigation, Z.M.; resources, X.L.; data curation, Z.M.; writing--original draft preparation, Z.M.; writing--review and editing, Z.M. and X.L.; visualization, Z.M.; supervision, X.L.; funding acquisition, X.L. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Projects for Leading Talents of Science and Technology Innovation of Ningxia (No. KJT2019001), the National Natural Science Foundation of China (No. 12162028), and the innovation team for multi-scale mechanics and its engineering applications of Ningxia Hui Autonomous Region (2021), and these supports are gratefully acknowledged.  
Not applicable.  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The authors declare no conflict of interest.