TECHBRIEF Friction Angles of Open- Graded Aggregates From Large-Scale Direct Shear Testing

Introduction State and local transportation agencies frequently use opengraded aggregates for wall, roadway, and bridge construction. The primary advantages of using this type of material in wall and abutment applications are ease of constructability, lighter in-place density than well-graded materials, very low fine content, free-draining characteristics, and simpler quality assurance testing, using a method specification for field density. The American Association of State Highway and Transportation Officials (AASHTO) classifies open-graded aggregates according to the M43 gradation based on standard sizes for processed aggregates.(1) Despite common use of these aggregates, their strength characteristics have not been systematically measured or used in design. Instead, engineers frequently use a default friction angle of 34,o leading to potential conservatism in retaining wall and foundation design. The primary purpose of this TechBrief is to present research results on the strength properties of standard open aggregates tested in a large-scale direct shear (LSDS) device to improve the state of practice for the design of structures using these materials.