## 1 Introduction  
Strength is one of the main indicators for evaluating the mechanical properties of pavement materials. The normal laboratory test methods to evaluate the strength of pavement materials include uniaxial compression test, uniaxial tensile test, Brazilian splitting test, bending test and the conventional triaxial test. However, any point inside a pavement is in a three-dimensional stress state. The conventional triaxial test is very popular because of its complex stress state. However, the intermediate principal stress is always equal to the minimum stress for the conventional triaxial test. It means that the intermediate principal stress effect can't be considered. Therefore, it is necessary to study how the pavement materials' strength can be influenced by intermediate principal stress. Many researchers have carried out true triaxial tests for materials such as rock, soil, and concrete, which obtained highly significant achievements. For example, [PERSON] carried out the constant water content triaxial tests on Yunnan red clay to study soil-water characteristics and shear strength [1]. The true triaxial test performed by [PERSON] for joint rock mass shows that the established strength criteria can precisely reflect the strength of the rock mass [2]. [PERSON] carried out biaxial and triaxial tests for mass concrete and put forward the failure criteria of mass concrete in principal stress space and octahedral stress space [3]. [PERSON] carried out a true triaxial test on cubical sample of plain cement concrete. The results show that the ratio of intermediate and minor principal stress had an incremental effect on the strength of the concrete [4]. [PERSON] studied the triaxial compressive strength and deformation of air-entrained concrete under freeze-thaw cycles [5]. [PERSON] carried out polyaxial tests on cubical specimens of sandstone. The results show that the intermediate principle stress effect can not be ignored, and modified Mohr-Coulomb criterion is suitable for sandstone [6]. [PERSON] conducted a true triaxial test on different materials in the context of new mixed-boundary conditions to identify their stress-strain and strength under triaxial stress [7]. [PERSON] conducted a true triaxial test on gravel and coarse-grained soil materials and discovered that the rules reflected by the Lade-Duncan criterion is the closest to test results [8; 9]. Undrained triaxial and simple shear tests were carried out by [PERSON] under the condition of different cement dosages and different stress paths [10]. Triaxial compression tests were carried out by [PERSON] on the high-performance concrete specimens with different mixing proportions [11]. Under conditions of different stress ratios, the triaxial tests on recycled aggregate concrete were carried out by [PERSON] [12]. By comparing the true triaxial test and the discrete element analysis method, [PERSON] studied the effect of different intermediate principal stress ratio on the mechanical properties of granular materials [13]. Triaxial compression tests were carried out by [PERSON] to study the influence of intermediate principal stress ratio b on the stress-dilatancy relational of coarse granular soil [14]. However, little has been reported regarding true triaxial test on asphalt mixture or cement-stabilized materials. [PERSON] conducted a triaxial compressive strength test on modified asphalt AC-13 mixture with an apparatus allowing triaxial independent loading [15]. [PERSON] qualitatively validated the applicability of twin-shear strength for asphalt mixture using a proprietary plane strain apparatus [16]. [PERSON] verified the anisotropy of asphalt mixture through modulus variation during true-triaxial independent loading test [17]. [PERSON] compared the respective stress-strain curves of maximum, intermediate and minor principal stresses for the true triaxial test on cemented soil [18]. [PERSON] carried out triaxial tests on hollow cylinder asphalt mixture specimen at tensile-compress and compress-compress state.