The Mechanistic Empirical Pavement Design Guide (MEPDG) developed by the National Cooperative Highway Research Program (NCHRP) project 1-37A, is a very powerful tool for the design and analysis of pavements. The designer utilizes an iterative process to select design parameters and predict performance, if the performance is not acceptable they must change design parameters until an acceptable design is achieved. The design process has more than 100 input parameters across many areas, including, climatic conditions, material properties for each layer of the pavement, and information about the truck traffic anticipated. Many of these parameters are known to have insignificant influence on the predicted performance During the development of this procedure, input parameter sensitivity analysis varied a single input parameter while holding other parameters constant, which does not allow for the interaction between specific variables across the entire parameter space. A portion of this research identified a methodology of global sensitivity analysis of the procedure using random sampling techniques across the entire input parameter space. This analysis was used to select the most influential input parameters which could be used in a streamlined design process. This streamlined method has been developed using Multiple Adaptive Regression Splines (MARS) to develop predictive models derived from a series of actual pavement design solutions from the design software provided by NCHRP. Two different model structures have been developed, one being a series of models which predict pavement distress (rutting, fatigue cracking, faulting and IRI), the second being a forward solution to predict a pavement thickness given a desired level of distress. These thickness prediction models could be developed for any subset of MEPDG solutions desired, such as typical designs within a given state or climatic zone. These solutions could then be modeled with the MARS process to produce am “Efficient Design Solution” of pavement thickness and performance predictions. The procedure developed has the potential to significantly improve the efficiency of pavement designers by allowing them to look at many different design scenarios prior to selecting a design for final analysis.

APSDS is a layered elastic tool for aircraft pavement thickness determination developed and distributed by Mincad Systems and based on the sister software Circly. As aircraft pavement thickness determination remains an empirical science, mechanistic-empirical design tools such as APSDS require calibration to full scale pavement performance, via the S77-1 curve. APSDS provides the unique advantage over other tools that it models all the aircraft in all their wandering positions, negating the need for designers to use pass to cover ratios and acknowledging that different aircraft have their wheels located at difference distances from the aircraft centerline. APSDS requires a range of input parameters to be entered, including subgrade modulus, aircraft types, masses and passes and a pavement structure. A pavement thickness is then returned which has 50% design reliability. Greater levels of reliability are obtained by conservative selection of input values. Whilst most input parameters have a linear influence on pavement thickness, subgrade modulus changes have a greater influence at lower values and less influence at higher values. When selecting input values, designers should concentrate their efforts on subgrade modulus and aircraft mass as these have the greatest influence on the required pavement thickness. Presumptive or standard values are generally acceptable for the less influential parameters. S77-1 pavement thicknesses are of a standard composition with only the subbase thickness varying. Non-standard pavement structures are determined using the principle of material equivalence and the FAA provides range of material equivalence factors, of which the mid-range values are most commonly used. APSDS allows direct modelling of non-standard pavement structures. By comparing different APSDS pavements of equal structural capacity, implied material equivalences can be calculated. These APSDS implied material equivalences lie at the lower end of the ranges published by FAA. In order to obtain consistence between APSDS and the FAA guidance, the following material equivalence values are recommended: * Asphalt for Crushed Rock. 1.3. * Crushed Rock for Uncrushed Gravel. 1.2. * Asphalt for Uncrushed Gravel. 1.6. Proof rolling regimes remain an important part of the design and construction of flexible aircraft pavements. Historically, designers relied on Bousinesq's equation and the assumption of point loads on semi-finite homogenous materials to determine proof rolling regimes using stress as the indicator of damage. The ability of APSDS to generate stress, strain and deflection at any depth and any location across the pavement allows these historical assumptions to be tested. As the design of a proof rolling regime is one of comparing damage indicators modelled under aircraft loads to those under heavy roller loads, the historical simplifications are generally valid for practical design scenarios. Where project specific data is required, APSDS can readily calculate stresses induced by proof rollers and aircraft at any location and depth for comparison. APSDS is a leading tool for flexible aircraft pavement thickness determination due to its flexibility, transparency and being free from bias. However, the following possible areas for improvement are considered worthy of future research and development: * Improvements to the user interface. * Ability to model aircraft masses as frequency distributions. * Ability to copy stress with depth data to Excel(tm) spreadsheets. * Ability to perform parametric runs. * Inclusion of a reliability based design module.