A triaxial shear test is prefered to direct shear test, because

ANS:A - it can be performed under all three driange conditions with complete control

"it can be performed under all three drainage conditions with complete control." In a triaxial shear test, the drainage conditions can be controlled, allowing the test to be conducted under undrained, partially drained, or fully drained conditions. This flexibility provides valuable insights into the behavior of soils under various drainage conditions, which is essential for understanding their engineering properties and performance in different scenarios. The other options provided are not accurate reasons for why a triaxial shear test is preferred:

• Precise measurement of pore pressure and changes in volume during the test is indeed possible in a triaxial shear test, contrary to the statement provided.
• While stress distribution on the failure plane may not be perfectly uniform in some cases, it's not a primary reason for preferring a triaxial shear test over a direct shear test.