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

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

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

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

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