## 1 Introduction
[PERSON] evaluated RAP mixtures at both the material level and the structural level using mechanistic models and observed that the performance of RAP mixtures depended on the mixture design and pavement structure [14]. Specifications for Design of Highway Asphalt Pavement (JTG D50-2006) [4] stipulate that the modulus ratio between the base course and the asphalt surface course of semi-rigid base asphalt pavement should be between 1.5 and 3, the modulus ratio between the base course and the sub-base should not be greater than 3.0, and the modulus ratio between the sub-base and the soil base should be between 2.5 and 12.5. [PERSON] measured the modulus ratio of key layers by falling weight deflectometer (FWD) through mechanical analysis and practical project tracking observation, and suggested that the modulus ratio of flexible base and semi-rigid base to surface layer should be 8-10 and 1.5-3, respectively [15; 16]. [PERSON] established a fatigue equation of asphalt mixtures through a fatigue test, which was characterized by the true stress ratio, and proposed a new idea of asphalt pavement structure design that would increase design life from top to bottom and a new idea of durable asphalt pavement structure design, and suggested that the design life of the middle and upper layer, the lower layer, and the base layer should be 10-15 years, 20-30 years, and 40-60 years, respectively [17]. According to the design index of California, the bending strain of asphalt mixture layers is less than 700 and the compressive strain of the top surface of soil foundation is less than 200 when the uniaxial load is 80 kN [18]. The design of Illinois transportation department requires that the flexural strain of the asphalt mixture base should be less than 600 \(\mu\epsilon\)[19]. [PERSON] suggested control indexes suitable for the structural design of durable pavement in China; the tensile strain of the bottom layer of asphalt was not more than \(120\times 10^{-6}\), and the compressive strain of the top surface of soil foundation was not more than \(280\times 10^{-6}\)[20]. [PERSON] proposed a reasonable combination form of asphalt pavement structure, and systematically studied the structural design method of asphalt pavement, design parameters of graded gravel material, and construction technology and construction quality detection methods of graded gravel [21]. In view of the defects of the current fatigue design method of asphalt pavement in China, [PERSON] found that semi-rigid base asphalt pavement can meet the requirements of durable pavement through the reasonable matching of the service life of each structural layer, and through the optimization of methods such as replacing materials, changing thickness, and changing layers [22]. [PERSON] proposed a pavement structure design method such that the deflection of the asphalt pavement structure layer matches the fatigue life of the tensile stress index of the bottom layer [23]. [PERSON] proposed a design process and steps of the flexible base asphalt pavement, and recommended the appropriate thickness of the structural layer of the granular base asphalt pavement under heavy load traffic [24]. Although scholars worldwide have done a lot of studies, there are still the following problems: (1) Through theoretical analysis or laboratory tests, the relationship model between different design indexes and the cumulative number of standard loads has been established, but the designed indexes mostly consider the failure behavior of each layer of materials separately, and do not evaluate the coordinated relationship between layers of a pavement structure. (2) The key steps of the existing structural design process are to calculate the cumulative number of standard loads by design indexes, and repeatedly adjust the material parameters until the cumulative standard loads meet the design requirements.