Prediction of stiffness of unbound granular materials based on the CBR value

Abstract

The unbound granular materials (UGM), base and subbase layers, play an essential role in the overall structural performance of thin pavement structures. They show complex stress dependent elasto-plastic behaviour under external loading. Therefore the UGM are commonly tested using the Repeated Load Triaxial (RLT) testing method to estimate the stiffness of the material by applying haversine loading pulses. The RLT testing method
represents the actual stress situation quite adequately and gives satisfactorily estimates of the stiffness characteristics of UGM. However a RLT test is an elaborated test and rather expansive. A much simpler test that has been used for a long time in structural design of flexible pavements is the CBR (California Bearing Ratio) test. The CBR test is a simple and cheap test where the load-deformation curve is acquired while a plunger is penetrated into the material at a constant rate. In the literature one can find a number of relationships for UGM
where the CBR value is used to predict the stiffness. To investigate if a connection between the two tests exists, twenty materials have been tested with both methods and the test results compared. The materials were of varying quality but all were fairly well graded. The results indicate that a simple power law can be used to predict the stiffness if the CBR-value is known.