Triaxial advantage over direct shear

The triaxial test has these advantages over the direct shear test:

1. Drainage can be controlled. UU, CU and CD tests are possible, and pore pressure can be measured. In direct shear, drainage cannot be controlled, so effective stress parameters are hard to obtain.
2. Failure plane is not forced. The sample fails along its weakest plane, while in direct shear the plane is fixed horizontally.
3. Uniform stress and strain in the specimen, so results are more reliable. Direct shear has stress concentration at the edges and progressive failure.
4. Principal stresses are known exactly. A complete Mohr circle can be drawn from $\sigma_1$ and $\sigma_3$.
5. Stress path can be followed; different stress paths such as compression or extension can be applied.
6. Volume change can be measured during shear, as well as strains.
7. Large specimen sizes can be used for coarse soil, and the test suits all soil types.
8. Stresses can be varied independently, which allows tests under in-situ stress conditions.