Subgrades in arid and semi-arid regions are often subjected to seasonal moisture variations that trigger volume change. To account for moisture changes in a vadose zone during pavement design, matric suction (ψ_m) is unavoidably required. In that context, ψ_m inclusion in CBR design becomes imperative. This study presents two CBR design approaches of flexible pavement, i.e. the conventional CBR design, and unsaturated CBR design methods. To compare these design approaches, subgrade soils were selected and a series of suction tests, CBR, and unsaturated CBR tests were performed to obtain the CBR design values of the subgrade materials. The results illustrate a linear relationship between suction and CBR values of the subgrades. The test results, further revealed that the unsaturated CBR design values are 2 to 2.5 times greater than the conventional CBR values. Based on the experimental results, the design analysis confirmed that the unsaturated CBR design approach is more conservative and rational compare to the conventional CBR design method.

Unsaturated CBR; conventional CBR; suction; design approach