# Dynamic Responses of Semi-Flexible Pavements Used for the Autonomous Rail Rapid Transit
## 1 Introduction  
With accelerated urbanization, the urban population and motor vehicle ownership are increasing. To meet the growing demand for public transportation, all countries in the world are actively developing urban public transportation [1]. The autonomous rail rapid transit (ART) combines the advantages of rail transit and bus rapid transit (BRT) [2]. The vehicle identities the virtual track line marked on the road through the vehicle camera equipment and controls the vehicle to run along the virtual track through the vehicle control unit, which has the same exclusive right-of-way as the rail transit [3, 4]. Compared with the traditional rail transit, the ART system vehicles do not need to reconstruct the physical track but only need to mark the virtual track line on the existing road base; its road construction cycle is short and the cost is low [5]. As the vehicles of ART always travel along the route marked by the virtual track (see Figure 1), it makes the pavement show significant damage characteristics during operation, i.e., the vehicle load is concentrated in a specific area of the asphalt pavement, resulting in the accumulation of deformation in the wheel track zone of the pavement to producepermanent deformation [6; 7], which affects the roughness of the pavement, thus reducing the comfort of the ride and shortening the service life of the pavement. Semi-flexible pavement (SFP) material is a composite material that is composited of an open-graded asphalt mixture with a high air void filled with special cement mortar [8; 9; 10]. It combines the viscoelasticity of asphalt concrete and the stiffness of hardened cement paste. On the one hand, SFP has better load bearing and anti-rutting performance than conventional asphalt pavement [11; 12; 13]. On the other hand, the riding comfort and skid resistance of the SFP surface layer are much better than the cement concrete pavement [14; 15]. Compared with the dense-graded asphalt concrete that is commonly used in the conventional asphalt pavement, the SFP material has better durability and stability [15; 16; 17; 18; 19; 20; 21; 22]. In the previous studies, SFP materials showed great potential to be used as anti-rutting overlays and surfaces in road construction [23; 24; 25]. It has been widely used in road intersections, bus stops, bus lanes, long steep slopes, ports, and other road sections [26; 27; 28]. Observation results from existing SFP roads show that the SFP layer in the asphalt road structure can effectively improve the high temperature stability of asphalt pavement. The in-site fast falling weight deflectometer (FWD) test proved that the application of the SFP layer can improve pavement resistance to dynamic load repetitions [10; 14]. Numerical simulation also suggests that the application of an SFP surface layer can effectively reduce the shear stress and optimize the stress environment of pavement structure under vehicle load [29; 30]. Therefore, the semi-flexible pavement material has a good application prospect in the ART system. The previous studies on the response of pavement incorporating an SFP layer mainly focus on the general road use environment, namely passenger cars or buses. In the simulation models of previous studies, the tire-pavement contact area was regarded as a circle with a diameter of 150 mm and the contact pressure was standardized to 0.7 MPa [31]. However, the vehicle used in an ART system is different from the common passenger cars or buses, due to high capacity and special wheel set, its tire ground pressure is higher than 0.7 MPa [32]. Therefore, to simulate the unique load of ART vehicles on the pavement, it is necessary to build the tire-pavement contact model considering the practical tire conditions and pavement structure. In this way, the loading mechanism of the ART vehicle, as well as the dynamic response characteristics of the pavement that incorporates the SFP layer, could be investigated.