Strength Characterization of Open-Graded Aggregates for Structural Backfills

CHAPTER 1. INTRODUCTION

1.1 PROBLEM STATEMENT

Open-graded aggregates (OGA), or poorly graded aggregates, are defined by their gradation, with most particles being uniform in size. They can be rounded or angular and classified as either sand or gravel. OGAs are used in concrete and asphalt mixes as well as for transportation structures, where requirements often specify the use of angular open-graded (OG) gravels. The primary advantages of using this type of material are ease of constructability, very low fine content, free-draining characteristics, low frost heave potential, and simple quality assurance testing, with a method specification for compaction. Angular OGAs also have the added benefit of increased strength compared with rounded aggregates. Because of these qualities, State and local transportation agencies are more routinely specifying crushed, manufactured OGAs for wall, roadway, and bridge construction. The American Association of State Highway and Transportation Officials (AASHTO) classifies manufactured OGAs according to the M43 gradation based on standard sizes for processed aggregates; the equivalent ASTM standard is D448.^(1,2)

In the design of retaining walls and foundation systems, the friction angle and cohesion are important parameters to define the strength of the backfill, commonly described by the Mohr-Coulomb (MC) failure envelope. The AASHTO OGAs are cohesionless, and the friction angle can be measured using laboratory testing devices, including plane strain (PS), direct shear (DS), and triaxial (TX) devices, although DS and TX are the most common. Oftentimes, however, designers will either have scalped samples tested or simply use a default friction angle of 34 degrees in lieu of testing. These measures are considered conservative; however, the actual strength properties of OGAs have not been systematically measured nor have the influences of various testing devices on the results been studied.

1.2 OBJECTIVES

The Federal Highway Administration (FHWA) initiated a research program to conduct laboratory testing of common AASHTO OGAs. The primary objectives of this program are to quantify the strength parameters, determine the influence of various factors on strength, and evaluate different testing devices. As a starting point, testing commenced using a large-scale direct shear (LSDS) device to measure the friction angle of uncompacted, common AASHTO OG aggregates.⁽³⁾ In addition, standard DS and large-diameter triaxial (LDTX) devices were used in this study. The study also looked at whether strength (i.e., friction angle) could be correlated to other properties of the backfill.

1.3 REPORT OUTLINE

This research report will detail the results of the testing program and is organized as follows:

• Chapter 2 provides background information on the use and design of OGAs in the transportation sector, the various testing and data interpretation methods available, and the associated implications.
  
  
• Chapter 3 details the laboratory testing program and presents the results. Beyond strength, the gradation, density, repose angle, angularity, and texture were measured using appropriate devices and standards.
  
  
• Chapter 4 describes the data analysis for different testing devices and data reduction methods. In addition, correlations between various soil properties and strength parameters were examined.
  
  
• Chapter 5 presents a summary of the testing and provides conclusions that transportation agencies can consider when designing with AASHTO OGAs. Additional research gaps and future research plans within FHWA are also discussed.