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
None of these
It can be performed under all three drainage conditions with complete control

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Soil Mechanics and Foundation
The major principal stress in an element of cohesion-less soil within the backfill of a retaining wall is

None of these
Vertical if the soil is in a passive state of plastic equilibrium
Inclined at 45° to the vertical plane
Vertical if the soil is in an active state of plastic equilibrium

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