Influence of Subgrade Stiffness on Flexible Pavement Responses – A Case Study-Alexandria City, Egypt

Alexandria city lies at the north of Egypt on the Mediterranean Sea. The top layers of soil in Alexandria differs from bed rock (Borg El-Arab), sand (near the sea, north Abo-Qir street) to silt and clay (Elsyouf, Smouha, Khorshed, etc..). Thus, a great attention should be considered in the design of pavements on these dissimilar subgrades because the performance of a pavement depends on the quality of its subgrade and subbase layers. The non-linear finite element program MICHPAVE is used in this study to analyze the flexible pavement responses. Displacements, stresses and strains are computed only within the region modeled by finite elements. At the end of the analysis, MICHPAVE outputs an equivalent resilient modulus for each pavement layer. Results from the non-linear mechanistic analysis, together with other parameters, are used as input to two performance models, to predict the fatigue life and rut depth. These performance models are currently restricted to three-layer pavements with asphalt concrete (AC) surface, base and roadbed soil, and four-layer pavements with AC surface, base, subbase and roadbed soil. Analysis of research results has shown that using a reasonable subbase layer reduces the rut depth of pavement while the increase in subbase thickness causes increase in rutting. Total expected rut depth of pavement as well as expected rut depth in the asphalt course increase with the decrease of subgrade stiffness, while fatigue life of pavement increases with the increase of subgrade stiffness