# Experiments on the State Boundary Surface of Aeolian Sand for Road Building in the Tengger Desert
## 1 Introduction  
The state boundary surface is the unique physical state relationship formed by effective stress paths of soil in a three-dimensional space composed of generalized normal stress, shear stress and specific volume, and it provides an outer limit to the combinations of effective stress and specific volume which the soil can reach [1]. In road building, it can predict the limit state of aeolian sand subgrade failure. The state boundary surface is the basis of critical state soil mechanics and is crucial in studies of the soil mechanical properties, which have been widely used to study the mechanical behaviors of remodeled clay. Aeolian sand is a special material for roadbed filling, which is widely distributed in desert areas, while there are only a few basic experiments for engineering applications. Moreover, the basic experimental results related to critical states are rather scarce. In recent years, much infrastructure, such as highways, railways, transmission lines and other projects, has been built in desert areas. In addition, the numerical simulation of the dynamic response of multi-layer pavement under a moving load is gradually enriched [2, 3]. However, the lack of research on the mechanical properties of aeolian sand has seriously restricted the geotechnical application in desert areas. Therefore, experimental characteristics of the state boundary surface of aeolian sand is urgently required for a better understanding the mechanical behaviour of this material. Currently, most of the existing research on aeolian sands focuses on their engineering aspects, such as particle-size distribution, compaction characteristics, bearing capacity, shear resistance and proportioned concrete [4, 5, 6]. The particle-size distribution of aeoliansand in several regions was analysed by different researchers, for instance, [PERSON] et al. [7] for the Tengger Desert, [PERSON] et al. [8] for the Mu Us Desert, [PERSON] et al. [9], [PERSON] et al. [10] for the Badain Jaran Desert, [PERSON] et al. [11] for the Qinghai Lake East Sandy Land, etc. These studies provide rich references for the construction of a foundation treatment in desert areas. Other researchers, e.g., [PERSON] et al. [12], [PERSON] et al. [13], [PERSON] et al. [14], studied the compaction characteristics of aeolian sand. [PERSON] et al. [15], [PERSON] et al. [16], [PERSON] et al. [17] and [PERSON] et al. [18] studied the shear strength characteristics in different desert regions. [PERSON] et al. [19] and [PERSON] et al. [20] studied the bearing capacity characteristics of aeolian sand foundations. Although the existing research has accumulated valuable engineering experience for foundation construction in desert areas, a systematic and in-depth triaxial test study is needed to better obtain the unique characteristics of its physical state. So far aeolian sand has been rarely studied in triaxial tests. Some authors have studied the dynamic properties of aeolian sand with dynamic triaxial tests, e.g., [PERSON] [21], [PERSON] [22], [PERSON] [23], [PERSON] et al. [24], [PERSON] et al. [25], [PERSON] et al. [26] and [PERSON] et al. [27]. Other authors focus on the static strength of sand. For instance, [PERSON] et al. [28] studied the strength characteristics of cement-improved aeolian sand; [PERSON] [29] obtained the shear strength and stiffness of aeolian sand in the Sahara Desert, Libya, by a series of drained and undrained triaxial tests; [PERSON] [24] obtained the strength characteristics of the aeolian sand in the Mu Us Desert, China, at different stress paths, densities and moisture contents; [PERSON] et al. [30] obtained the strength and softening resistance of aeolian sand treated by biopolymer in Al-Sharqia desert; [PERSON] et al. [31] determined the critical state parameters of aeolian sand in Natal, Brazil by triaxial tests with different initial densities; [PERSON] et al. [32] obtained the mechanical properties of aeolian sand and fly ash at different proportions. In particular, [PERSON] et al.