Soil Mechanics and Foundation
A tri-axial shear test is preferred to direct shear test, because

Stress distribution on the failure plane, is non uniform
It can be performed under all three drainage conditions with complete control
Precise measurement of pore pressure and change in volume during test, is not possible
None of these

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Soil Mechanics and Foundation
In a triaxial compression test when drainage is allowed during the first stage (i. e. application of cell pressure) only and not during the second stage (i.e. application of deviator stress at constant cell pressure), the test is known as

unconsolidated undrained test
consolidated undrained test
consolidated drained test
unconsolidated drained test

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