The difference between strength parameters

By Timothy D. Stark

Q: What is the difference between the strength parameters c and ø, and a and δ?

A: Good question because these parameters are frequently used interchangeably. c and ø, and a and δ are shear strength parameters used to represent the strength/failure envelope for a particular material. The strength envelope is obtained from laboratory shear testing (e.g., ASTM D5321) for direct shear testing of geosynthetic interfaces. c and ø, and a and δ are typically used to represent the strength envelope for soils and geosynthetic materials/interfaces, respectively. The values of c and a represent the vertical intercept of the failure envelope and ø and δ represent the slope of the strength envelope. The strength envelope is plotted on a graph of normal stress applied to the material during the test on the horizontal axis and shear stress/strength on the vertical axis as shown below: The normal stress is calculated by dividing the weight or force applied to material being tested divided by the cross-sectional area of the specimen. If the strength envelope is stress-dependent (i.e., not a straight line), it is recommended that these strength parameters not be used and the entire stress dependent strength envelope be used in the stability analysis.

Tim Stark is a professor in the Department of Civil and Environmental Engineering at the University of Illinois at Urbana-Champaign.