Impact of Subgrade Strength on Pavement Performance

Abstract

Based on the standard design of flexible pavement structure conducted in Turkey, only subbase course layer is affected by the variation of subgrade strength and other pavement layers (wearing course, binder course, asphalt concrete (AC) base course, and crushed stone base course) are not influenced by the variation of subgrade strength. Therefore, the aim of this study is to investigate the impact of subgrade strength on flexible pavement distresses. To achieve the objectives, firstly, three different flexible pavement structures designed using the Turkish Flexible Pavement Design Guide 2008 were chosen and secondly, the effect of variation of subgrade strength [5 to 50 California Breading Ratio (CBR)] on pavement performance (rutting and fatigue cracking) for three different traffic levels [5000, 10000, and 15000 average annual daily truck traffic (AADTT)] for climatic condition of Izmir region using AASHTOWare Pavement ME Design v2.5.5 were analyzed. The results of the study show that, reducing subgrade CBR values from 50 to 5 CBR resulted in an increase by 3.7mm in total rutting depth and 5.6% in alligator cracking magnitude. Also, results demonstrated that the influence of subgrade strength on rutting within asphalt concrete layers (AC rutting) is negligible. By contrast, increasing subgrade CBR value from 5 to 50 resulted in a significant increase (65.1 to 314.7 m/km) in longitudinal cracking magnitude while thermal (transverse) cracking is not sensitive to the variation of subgrade CBR values. This study can assist highway agencies and individual pavement designers in the design a more practical pavement.