Numerical Analysis of Drained Direct and Simple Shear Tests

Abstract

The authors present an interesting study on direct shear testing. The discusser estimates that practicing engineers perform laboratory direct shear tests 80-90% of the time. The remainder is mostly laboratory compression tests, such as unconfined compression or triaxial compression. The reason direct shear tests are used so often in practice is because they are quick to set up, simple to run, and the results are easy to interpret. Because the direct shear is used so often, it is frequently considered to be the standard of practice. The direct shear test is typically used to determine the effective stress parameters (ASTM 0 3080-72: Direct 1986), by first consolidating the soil sample and then shearing it slow enough to enable the dissipation of shear-induced excess pore pressure. Inaccurate results are common for clayey soil because they are not consolidated prior to shearing or sheared too quickly. The authors performed a theoretical analysis and have concluded that in most cases, direct shear tests overestimate the peak shear strength by as much as 7.5%. This translates into an increase in effective friction angle up to 2°. The unconservative nature of the direct shear test is one of it's main disadvantages. The discusser was wondering what conditions cause the direct shear test to have the highest overestimate (7.5 %) of peak shear strength? What soil type would this correspond to?