Use of Compaction Index Concept to design a Layered Aircraft Pavement Subgrade

Abstract: The design of flexible and rigid aircraft pavements using FAARFIELD assumes that the subgrade is semi-infinite and uniform. At a recent greenfield airport project in Visakhapatnam, India, the CBR of the insitu subgrade material was found to vary considerably throughout the site and throughout the depth of strata. The project required a significant amount of cut and fill to achieve the required grade, and further excavation was required to improve the compaction of the existing soil. This paper details how the Compaction Index (CI) concept was used to design a layered subgrade to ensure that both strength and compaction requirements were met. CI is defined as the CBR required at a given depth for a particular wheel configuration, assembly load and tire pressure. FAARFIELD calculates CI at 10 inch intervals below the pavement surface, and is based on the computed vertical stresses at each depth. Since the CI concept was derived from the CBR design method, FAARFIELD uses the Stress Ratio S3:2 to convert the computed LEAF vertical stress with stress concentration value n=3 into a stress for the same structure with n=2. In order to compare actual subgrade CBR requirements, it was therefore necessary to use the Stress Ratio S3:2 to back calculate computed CI to obtain the CBR requirements of the subgrade soil at various depths below the top of the pavement. The results of the CI analysis allowed the designer to specify better quality material for use in the upper subgrade layers where the stresses are higher, and poorer quality material to be used in the lower subgrade layers where the stresses are lower.