# Application of Mechanistic Empirical Pavement Design Guide Software in Saudi Arabia
## 1 Introduction
Critical stresses and strains within the structure are estimated by structural analysis at each time step, followed by the use of distress models for estimating incremental distresses such as rut depth, cracks, and roughness to the previously calculated stresses and strains. The empirical design method for the pavement performance was developed using predictions using laboratory-developed models to observe the differences between field performances and the predicted one. In addition, they utilized a significant amount of pavement segments of the long-term pavement performance program (LTPP) throughout North America [12,13]. Calibration of these models is therefore essential in order to implement them, considering environmental conditions, local materials, and traffic information. Factually, locally based calibration consideration factors are included in the AASHTO MEPDG distress modelling only to consider the differences in maintenance policies, material specifications, and construction practices across the USA [14]. Confirmation of predicted performance models is a critical step in order to establish reliability prior to their adoption for designing practices [15]. A biased model with significant cost effects consistently produces over designed or poorly designed pavements. An inaccurate model leads to inefficiencies for design performance [16]. AASHTO MEPDG prediction model validation conducted for ensuring calibrating paving models yield accurate and robust paving failure predictions for different cases compared to the ones used in calibrating. Roods are an essential part of any country infrastructure, and researchers have devoted great effort over recent decades to improve the structure design of flexible pavements. Figure 1 presents the evolution of pavement design methods (only the well-known methods) through the previous decades. Currently, the US Departments of Transportation (DOTs) implement different pavement structure design methods, and most DOTs implement more than one method of design for the same pavement type. A recent survey, in 2014 revealed that the AASHTO empirical methods (1993 and earlier versions) are by far the most used design methods among the US transportation agencies [17]. The design of flexible pavements has progressively evolved from an art to a complicated science but the empirical approach still has an important role in the pavement design process. In earlier days up to 1920s, the thickness of the pavement was determined based on experience or certain thumb rules and the same thickness of the pavement was used for a particular section of the pavement irrespective of the varying underlying soil conditions. As time passed and more and more experience was gained, different agencies started to develop various methods to determine the thickness required for a particular pavement. The load applied on top of the flexible pavement causes different stresses and strains throughout the layers from top to the bottom decreasing in magnitude. The typical cross section of the layered system in a flexible pavement is shown in Figure 2. The mechanistic-empirical method is based on the mechanical properties of the materials that give a relation between the input, in the form of loading, and the output, in the form of stresses and strains in the pavement. The stress strain values, response values, are used to determine the distress from laboratory and field data. This approach is much better as the performance cannot be determined by theory alone. The main objective of this study was the implementation of the new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) for pavement design in KSA. The MEPDG procedures are intended for designing and analyzing new and rehabilitated pavement structures. Empirical transfer functions use the critical responses with material properties to estimate pavement performance throughout the pavement design life. Pavement performance is expressed in the form of rutting, longitudinal and alligator fatigue cracking, thermal cracking, and pavement roughness.