Control on the drainage level:
One major difference between the direct shear test and triaxial shear test is the control on the drainage level during the test. The direct shear test is conducted under drained conditions, while the triaxial shear test can be conducted under both drained and undrained conditions.

Direct Shear Test:
- In the direct shear test, a sample of soil is placed in a shear box and sheared along a pre-determined plane.
- The test is conducted under drained conditions, which means that the water is allowed to freely drain out of the sample during the test.
- As a result, the pore water pressure remains relatively low and the drainage conditions are fully controlled.
- The test is typically used to determine the shear strength parameters of the soil, such as the cohesion and angle of internal friction.

Triaxial Shear Test:
- In the triaxial shear test, a cylindrical sample of soil is placed in a rubber membrane and subjected to a confining pressure.
- The test can be conducted under both drained and undrained conditions, depending on the purpose of the test.
- In the drained test, the water is allowed to freely drain out of the sample, similar to the direct shear test.
- In the undrained test, the water is not allowed to drain out of the sample, and the pore water pressure builds up during the test.
- The undrained test is typically used to determine the undrained shear strength of the soil, which is relevant for quick clay or sensitive soil conditions.

Conclusion:
In summary, the major difference between the direct shear test and triaxial shear test is the control on the drainage level. The direct shear test is conducted under fully drained conditions, allowing the water to freely drain out of the sample. On the other hand, the triaxial shear test can be conducted under both drained and undrained conditions, depending on the test requirements.