Proposed uniform scale for stiffness of unbound pavement materials for pavement design

A review of several relationships commonly used by pavement engineers for structural characterization of subgrade soils, subbase, and base is presented. A careful examination of the literature on the subject revealed that some of the cross-correlations presented in the 1986 American Association of State Highway and Transportations Officials (AASHTO) thickness design guide (Guide for Design of Pavement Structures) are either invalid orinaccurate. Problems associated with many of these correlations are put in the context of their historical development. A clarification is provided to the current inaccurate cross-correlation information reported in the 1986 AASHTO guide with regard to the Kentucky version of California bearing ratio (CBR). A common scale for correlating “soil-strength/ stiffness” parameters, including the resilent modulus, is presented for use with the AASHTO guide as well as mechanistic pavement-design procedures. A refinement of the AASHTO procedure for selection of resilient modulus of granular base and subbase materials is also presented. An analysis of issues related to measurement of resilient modulus indicates that there is a great need for standardization of equipment and test procedures. In this regard, some of the issues related to the field versus laboratory measurement of resilient modulus are discussed.