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
None of these
Precise measurement of pore pressure and change in volume during test, is not possible

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Soil Mechanics and Foundation
Coefficient of compressibility is

different for different types of soils and also different for a soil under different states of consolidation
different for different types of soils but same for a soil under different states of consolidation
constant for any type of soil
independent of type of soil but depends on the stress history of soil

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