## 1 Introduction  
The development of hydro-mechanical models of unsaturated geotechnical materials plays an important role in solving the problems in geotechnical engineering [1; 2; 3]. Numbers of elastoplastic models were developed for unsaturated geotechnical materials based on critical state theory [4; 5; 6; 7; 8]. For example, [PERSON] et al. [4] proposed a Barcelona basic model (BBM) based on the modified Cam-clay model [9] and derived a loading collapse yield function (LC curve) to describe the collapse phenomenon when wetting. [PERSON] and [PERSON] [5] proposed an elastoplastic model similar to BBM while using a different LC yield function. As a classical model, BBM has been widely quoted and modified by researchers [10; 11; 12]. In addition, [PERSON] et al. [13] presented a volumetric model, called SFG (Sheng-Fredlund-Gens), for describing the volume change of unsaturated soils during the wetting and drying processes. These models adopt the double-stress variables, mean net stress, and suction, but ignore the effect of saturation on strength and deformation. Lots of experimental data shows that geotechnical materials with different saturation present different mechanical properties, even when the suction and mean net stress are identical. Some authors introduce saturation into the constitutive model to describe the effect of saturation on unsaturated geotechnical materials characteristics [14; 15; 16]. In addition, soil-water characteristic curves (SWCC) have obvious hysteresis characteristics, which hasbeen considered when establishing the hydro-mechanical coupling models of unsaturated geotechnical materials [13; 17; 18; 19]. Many existing coupling models of geotechnical materials are based on classical elastoplastic theory, which is also the most widely used constitutive theory. However, most of the constitutive equations used in these models are empirical equations obtained by fitting the experimental results [20], which lack a strict theoretical basis. With the help of thermodynamic theory, some authors established the corresponding hyper-plasticity constitutive models by defining the free energy function and the dissipative potential function to overcome the theoretical defects of the classical elastic-plastic model [21; 22; 23; 24]. Although these models still follow the theoretical framework of the classical elastoplastic model, they provide a clear physical meaning for the concepts of yield criteria and flow rules. Recently, based on the physical conservation laws and non-equilibrium thermodynamic theory, the granular solid hydrodynamics modeling method has been successfully applied to the constitutive model of geotechnical materials [25; 26; 27]. The constitutive relations of geotechnical materials are established by constructing thermodynamic identity and combining free energy function and energy dissipation law. The dissipative system, which causes the system to deviate from the equilibrium state, can be determined theoretically. Energy dissipation has a great influence on the mechanical behavior of geotechnical materials. It is most convenient and reliable to describe these contents with entropy and thermodynamic language, and it can also ensure that the model does not conflict with the basic physical principles. For multi-scale geotechnical materials, this method is simple and effective and it can grasp the essential characteristics of multi-scale behavior of materials from the perspective of energy dissipation. In this paper, a new hydro-mechanical coupling model for unsaturated geotechnical materials is established based on the non-equilibrium thermodynamic theory. The dissipative system and thermodynamic relationships are determined theoretically, and the constitutive relationships of non-elastic strain and granular temperature are obtained by constructing thermodynamic identity and dissipative relations. The relationship between dissipation mechanism and macro mechanical behavior is established by migration coefficients and energy functions.