Relationship Between Direct Simple Shear Tests and Triaxial Tests
In practice, direct simple shear apparatus are not as readily available compared with triaxial apparatus. However, the stress states imposed by the majority of geosystems are similar to direct simple shear (plane strain). Therefore, if we could fi nd a relationship between triaxial test results (axisymmetric condition)
and direct simple shear results, it would allow us to use results from triaxial tests to analyze
Variation of the theoretical
ratio of normalized undrained shear
strength at critical state for direct simple
shear tests to that of triaxial isotropically
consolidated tests.
geosystems for which the imposed stress state is best simulated by direct simple shear. Recall that in the triaxial test we can only rotate the principal axis of stress by 908 instantaneously, whereby in the direct simple shear test the principal axes of stress rotate gradually. In the fi eld, many structural loads impose
gradual rotation of the principal axis of stress.
The ratio of the normalized undrained shear strength of a fi ne-grained soil under direct simple shear and the normalized undrained shear strength of the same soil isotropically consolidated and then sheared under triaxial undrained condition, aDSS2ic, is
plot of Equation The normalized undrained shear strength in a direct simple shear test is always less than that in the triaxial test on isotropically consolidated samples of the same soil. Suppose the critical state friction angle of a fi ne-grained soil is 308; then the normalized undrained shear strength from a triaxial isotropically consolidated sample must be multiplied by about 0.72 to estimate the undrained shear strength of the same soil sample subjected to direct simple shear test. Recall that frcs is a fundamental soil property and, as such, it is independent of the type of test.
The practical implication is that triaxial test results from isotropically consolidated samples must be corrected to apply to the analysis of geosystems in which the stress states are best simulated by direct simple shear. Examples of these geosystems are retaining walls and slopes. In the case of slopes, the design safety margin is generally small (1 , FS # 1.5; FS is factor of safety). If you do not account for the reduction in undrained shear strength from triaxial test results, the risk of failure increases.