Measurement and Evaluation of Subgrade Soil Parameters, Phase 1 - Synthesis of Literature

The objective of this was to conduct a comprehensive literature review of the state of the practice pertaining to testing and evaluation of subgrade soils.

A critical component of the pavement design involves a thorough and reliable characterization of the subgrade; i.e., the foundation of the pavement riding surface. A number of laboratory and insitu methods are available to characterize the strength and stiffness of subgrade soils. The Montana Department of Transportation (MDT) uses the Resistance value (R-value) testing method, which can yield unsatisfactory or inconsistent results with some of the soils present in the state. MDT is interested in exploring alternative investigatory techniques that are more suited to the state’s soil conditions. AASHTO is in the process of adopting a new pavement design method known as the mechanistic-empirical pavement design guide (MEPDG). Upon implementation, this mechanically based design approach will be used by all state highway agencies for layer thickness calculations. A key material property to be used in design will be the resilient modulus (Mr) value, which either can be obtained from laboratory testing or can be backcalculated from measured in-situ deflection data. The determination of a representative Mr value for a given subgrade, considering seasonal variations and testing intricacies, is not an easy or straightforward task. There is no currently recognized or unified general approach for using the correlations. Most are site or region specific, and often do not account for important variations in soil type and consistency. Adding to the confusion are the various modifications, adjustments, and simplifications that have been proposed as improvements to the laboratory resilient modulus test method. There is a considerable amount of published information, both historic and recent, that needs to be synthesized before the Department may be ready to proceed with an efficient, useful, and productive testing program. This project will assist MDT in the selection of appropriate and representative subgrade design parameters for use in mechanistic-based pavement design procedures. The first phase of the study will focus on a comprehensive study and synthesis of available technical literature. The second phase of work (if deemed necessary) will focus on laboratory and field testing of selected subgrade soils from Montana sites.