Correlation between CBR value and effective strength parameters for engineered fills

Abstract

It is a tradition in Tanzania to use the California bearing ratio (CBR) value for the design of embankments. To assess the stability of embankments, slope stability calculations are performed, for which the effective strength parameters are needed. CBR values can be determined from the effective strength parameters based on Meyerhof’s general bearing capacity equations. Contrary to that, from a given CBR value, the effective friction angle is not easily calculated directly, as the relationship between the non-dimensional bearing capacity factors and the friction angle is complex. Moreover, only the resulting CBR value is known, not the strength contribution of the effective cohesion. Present study presents results from disturbed samples obtained from test pits and boreholes which were tested in order to determine the particle size distribution curves and Atterberg limits. On representative samples from different soil groups (AASHTO classes), CBR tests were performed to assess the quality of the fill material. Moreover, a set of isotropically consolidated, drained (CID) triaxial tests were carefully planned on the granular materials (AASHTO classes: A-2-4, A-2-6 and A-2-7) in order to investigate the relationship between CBR value and effective strength parameters of the materials when used as fill (normally consolidated conditions) at different stress levels. Based on 18 CID triaxial tests, a correlation was established which can be used directly to calculate effective strength parameters from CBR values.