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
It can be performed under all three drainage conditions with complete control
Stress distribution on the failure plane, is non uniform
None of these

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Soil Mechanics and Foundation
The admixture of coarser particles like sand or silt to clay causes

increase in both liquid limit and plasticity index
decrease in liquid limit and increase in plasticity index
decrease in both liquid limit and plasticity index
decrease in liquid limit and no change in plasticity index

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