Comparative analysis of empirical and mechanistic -empirical design methods for airports flexible pavement

Abstract

Pavement design must ensure sufficient protection of the subgrade by keeping stress levels low enough to avoid excessive deformation. To address the varying demands of different climates and traffic loads, a range of pavement design methods have been developed. The scientific basis for pavement design began in the 18th century with British engineer John Loudon McAdam, who introduced systematic methods primarily for road construction. However, the invention of the airplane by the Wright brothers in the early 20th century created the need for dedicated airfield pavement design to accommodate the rapidly growing aviation industry. Empirical methods rely heavily on historical data and require minimal material testing, whereas the mechanistic-empirical approach evaluates the stresses and strains within each pavement layer, providing a more scientific basis for design. This research will focus on assessing both methods in terms of their performance reliability and cost-effectiveness. In this research, six airfield pavement sections from three major airports in Rwanda—New Bugesera International Airport, the existing Kigali International Airport, and Kamembe Airport—will be evaluated. The analysis will be conducted using both the mechanisticempirical design approach and the empirical CBR design method, utilizing the United States Army Corps of Engineers' software PCASE (Pavement-Transportation Computer Assisted Structural Engineering). For consistency, similar traffic loading conditions and subgrade strength will be applied to each airport. Merits and demerits of each method is highlighted. Following the initial design computations, the resulting pavement thicknesses will be further assessed using the FAARFIELD software (FAA Rigid and Flexible Iterative Elastic Layered Design). This step will involve converting traffic data into equivalent load repetitions to estimate pavement life and compare the design outcomes for each airport. At Kigali International Airport, the PCASE-CBR method produced a pavement design with a thicker base layer, resulting in a structurally robust pavement expected to last the 20-year design period without structural failure. However, it’s important to note that environmental factors cause deterioration in flexible pavements regardless of layer thickness. Even with a thicker pavement, environmental degradation will still occur. Specifically, the PCASE-CBR design for Kigali resulted in a 257 mm base and a 102 mm subbase, whereas the PCASE-Layered Elastic Design (LED) yielded a 153 mm base and a 211 mm subbase. Since the upper pavement layers protect the lower ones, this difference leads to higher costs, especially because stabilized base materials are significantly more expensive than subbase materials. Despite the robust structure, environmental factors will still require maintenance, such as milling and overlay or simple overlays, during the pavement’s life cycle. For New Bugesera International Airport, a similar trend was observed. The PCASE-CBR method produced a thicker base (267 mm) compared to the PCASE-LED (194 mm), resulting in a stronger but more expensive pavement structure. However, considering all other parameters such as testing materials, required personnel and equipment for ME designed pavement structures, the overall initial construction cost is higher than the Empirically

vi designed pavement, however reliability itself has its cost implication, see 71 the conclusion part for overall cost benefit analysis Kamembe Airport presented a different scenario. Here, the PCASE-CBR design led to thinner pavement layers compared to the PCASE-LED method. Kamembe Airport serves smaller Code C aircraft, which have lower load demands. As a result, the software recommended minimum practical pavement thicknesses, focusing more on constructability and resistance to environmental degradation rather than structural strength. This indicates that for airports with lighter traffic loads, like Kamembe, pavement deterioration is more likely due to environmental conditions than traffic loading. Additionally, the PCASE-CBR and PCASELED methods have different minimum layer thickness standards, as reflected in the design outcomes. In comparison, the CBR design method tends to underestimate the strength contribution of the aggregate subbase, often resulting in unnecessarily thick base layers. In this study, the pavement designs based on the CBR method produced significantly thicker base courses than those designed with LED method, however the other cost parameters related to this LED design method make the pavement initial cost goes higher, however the life cycle cost analysis proven that empirically designed pavement might be cheaper but not economical in the long run. Based on these findings, the following recommendations are made: • Avoid constructing airfield flexible pavements on weak subgrades. If subgrade strength is insufficient, ground improvement techniques should be applied to achieve more practical and cost-effective pavement thicknesses. • Consider using thinner surface courses with stabilized bases. The Layered Elastic Design suggests a surface course of approximately 100 mm, compared to the 127 mm typically used in the CBR method. A stabilized base layer provides better structural value at a lower cost than excessively thick surface courses. • Limit design life to a maximum of 20 years. Due to the inherent uncertainty in traffic forecasting, shorter design periods are recommended to reduce the risks associated with over- or under-design, as pavement thickness requirements may vary depending on future traffic conditions.