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

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

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Soil Mechanics and Foundation
The shear strength of a soil

Decreases with decrease in normal stress
Decreases with increase in normal stress
Is directly proportional to the angle of internal friction of the soil
Is inversely proportional to the angle of internal friction of the soil

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