The Correlation between the CBR and Shear Strength in Unsaturated Soil Conditions

In pavement design, the CBR and direct shear tests are two very common laboratory investigations for predicting the strength of a subgrade layer. The relationship between the CBR and water content has been commonly presented in analyses, with the result of the direct shear being expressed from the aspect of effective cohesion and the internal friction angle. Even though most natural soil is in an unsaturated condition, the effect of soil suction on CBR has not been taken into account in practice. The information on the CBR based on soil suction is very rare. A new CBR test technique using suction measurement was recently implemented by the authors, namely the suction-monitored CBR test. The aim of this study is to make a correlation between the unsaturated CBR measurement and the unsaturated shear strength of a subgrade layer. The data was taken from suction-monitored CBR tests and suction-monitored direct shear tests on sand and sand-kaolin clay mixtures. The results indicate that there is a positive correlation between the CBR and the unsaturated shear strength. By using this correlation, the suction-affected CBR of a particular soil can be predicted using the suction-monitored direct shear test.