Chapter 1. Introduction

Background

Material characterization plays a vital role in the design, construction, and maintenance of roadways.  Because the loads generated on these facilities are distributed through the pavement structure to the underlying soil layer (the subgrade), engineers need to determine how this soil will perform.  Over the years, several testing procedures have been developed to analyze and predict the subgrade's response to highway loading.  Some of these tests frequently used by state highway agencies include the Resistance Value (R-value), the California Bearing Ratio (CBR), and the Texas Triaxial Classification.  The latest method for evaluating the subgrade's behavior is called Resilient Modulus (M_R).  By definition, resilient modulus is a material property that measures the elastic (load-unload) response of a soil under repeated loading (Claros et al., 1990). Numerically, it is the ratio of the deviator stress to the resilient or recoverable strain (M_R = σ_d/ε_r). This subgrade property is used in the design of new pavement sections as well as in the rehabilitation of existing roadways.  Without adequate information on the roadbed soil, a pavement structure will be improperly designed and as a result, it may fail prematurely.

Problem Statement

The American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures (1993) requires selecting a value for the design subgrade resilient modulus.  This value may be based on laboratory testing, back calculation programs using deflection measurements, resilient modulus correlation studies, or the equation presented in the AASHTO overlay design procedures based on deflection measurements.  Each of these methods of determining M_R present a variety of decisions and assumptions.

First, laboratory tests may be completed on soil samples obtained from the field under undisturbed or disturbed conditions.  After completing testing, one must choose a design M_R value.  This design value can be based on an assumed deviator stress of 41.4-kPa (6-psi), suggested in the literature, or based on actual field stresses.  Second, M_R can be determined from back calculation programs.  This method uses an indirect approach and the resilient modulus values tend to be higher than the actual values.  As a result, the back calculated M_R value should be multiplied by a correction factor.  This provides a conservative M_R value that is consistent with the assumptions made in the AASHTO pavement design procedures.  Third, correlation studies require a reliable relationship between the test used for material characterization and the resilient modulus value.

Objectives

Because the above procedures for determining the subgrade resilient modulus may give variable results, one would want to know how these variations may influence the resulting overlay thicknesses for a construction project.  Therefore, the University of Wyoming and the Wyoming Department of Transportation (DOT) conducted a joint research project to address this problem.  The principal objectives of this study were to:

1. investigate the importance of several fundamental soil properties (water content, plasticity index, liquid limit, group index) on selecting a design subgrade resilient modulus value for cohesive soils,
2. define the actual relationship (correction factor) between back calculated and laboratory based M_R values for typical cohesive subgrade soils in Wyoming,
3. compare actual subgrade field deviator stresses to the deviator stress assumed in determining a design M_R value from laboratory testing, and
4. determine the effect of selecting a M_R value on the design overlay thicknesses for typical pavement sections in Wyoming.

Organization of Study

This study examined the characteristics of cohesive subgrade soils at nine sites representing typical primary highways in the State of Wyoming. The roadbed soils included in the experiment had the following AASHTO classifications:  A-4, A-6, and A-7-6.  Samples for laboratory testing, deflection data, and pavement condition surveys were collected in the summer of 1992 and the spring of 1993.  Next, an extensive laboratory testing program, several back calculation analyses, and overlay thickness designs were completed. Finally, the results were summarized in a computerized data base and a comprehensive statistical analysis was performed on the data.

Chapter 2 of this report reviews the traditional methods used to characterize subgrade soils, methods to determine resilient modulus for subgrade soils, and the AASHTO overlay design procedure.  Chapter 3 describes the data collection process and overall evaluation strategies followed in this research.  Chapter 4 discusses the laboratory testing, back calculation testing, and several important results on the factors that influence the selection of a design subgrade resilient modulus value.  Chapter 5 discusses the impacts of selecting a particular method for determining a design resilient modulus value on the resulting overlay thickness. Chapter 6 summarizes the study, presents the conclusions, and makes recommendations for needed future research.