# Application of Mechanistic Empirical Pavement Design Guide Software in Saudi Arabia
## 1 Introduction
The prediction precision of the empirical models depends heavily on the hierarchical input levels of the design parameters and the calibration of transfer functions [20; 21]. In MEPDG (the latest production software version in AASHTOWare Pavement ME Design) [22], the performance models for rutting and alligator fatigue cracking with the calibration coefficients are explained briefly [21]. This main objective took into consideration the changes in the materials along with prevailing environmental and traffic loading in various regions of KSA. The primary  
Figure 1: The development of flexible pavement design methods [18]. Figure 2: Typical cross section of the layered system in conventional flexible pavement [19]. objective of the design of a hot mix asphalt (HMA) pavement is to resist the rutting of the subgrade and the bottom-up fatigue cracking. In a traditional approach while designing a pavement, the thickness of the pavement must increase with an increase in the design load, and this traditional approach is based on the concept that in the case of thicker pavements bottom-up cracking does not occur. Distress is the most important factor to consider in the pavement design as it is directly related to the pavement performance. Each of the failure criteria defined under distresses needs to be used in the mechanistic-empirical design approach. Among the various distress, some are caused due to the deficits in materials, construction, and proper maintenance and are not related to the pavement design directly. The distress evaluation in pavements is an important part of the pavement management system, if the pavements are evaluated properly and a strategy is developed in an effective method then proper maintenance and rehabilitation can be performed at the right time. Typical pattern observed in deterioration of HMA pavements is rutting, this develops rapidly in the initial years and then levels off to a much slower rate. The fatigue cracking usually does not occur until several repeated loadings, once it starts then it increases rapidly as the pavement is weakened. Generally, the main causes of the failure of HMA pavements are the defect in quality of material used along with the method of construction and quality control during construction. Moreover, four main causes of failures are the surface and subsurface drainage problem, increase in traffic volume and the magnitude of wheel load, deformation in foundation, and the environmental factors including heavy rainfall, snow, rising water table, frost action. The distresses in pavement are classified into three major groups named as cracking, deformation, and surface defects [19]. Although the concept of pavement structural design may vary from country to country, but the input is found to be similar. The damage models were also found to be common including the following:  
* Rutting (asphalt layer), Figure 3;
* Fatigue is parallel to the road (asphalt layer and stabilized soil), Figure 4;
* Thermal cracking is perpendicular to the road (asphalt layer), Figure 4.  
Figure 3: Example of rutting in flexible pavement.