Refinement of Shear Strength Properties for Geotechnical Design

The shear strength of natural occurring materials must be accounted for in the analysis and design of embankments, excavation slopes, and structural foundations. The properties of these materials are often difficult to define due to environment, sampling technique, and testing limitations. The long-term performance of embankments, slopes, and structural foundations primarily depends on the shear strength of the fill material and in-situ soils. When the induced shear stresses are greater than the shear strength of the soils, failures tend to occur. The SDDOT currently back-calculates soil parameters from observations made in the field or uses results from direct shear tests to determine shear strength properties. While direct shear tests provide essential information, there are limitations with respect to strain boundary conditions, failure plane orientations, and principal stress orientations in the test setup. Direct shear tests force soils to fail in the horizontal plane which may not be the weakest. These limitations can result in variances in the peak and residual strength parameters, which influences the factor of safety in stability analysis. The direct shear test is a relatively simple method with inherent limitations. Triaxial testing may be a more effective method to characterize and define the strength properties of natural materials commonly encountered in SDDOT infrastructure projects. Objectives are as follows: (1) Complete triaxial shear testing of soils widely used in SDDOT infrastructure projects and compare test results with those obtained through direct shear testing. (2) Develop a comprehensive guidance document that will assist in choosing the appropriate direct shear testing parameters and enable SDDOT to validate and refine shear strength properties for geotechnical design and analysis.