Question

How to obtain peak deviator stress from the three triaxial tests plotted on a graph? HINT.

Answer 1

As first we have to know, what is deviator stress, If we substract the value of major principal stress( sigma 1) and minor principal stress (sigma 3) i.e confining pressure, we get the value of Deviator stress. In any undisturbed soil sample, when three types of triaxial test is performed mainly devaitor stesss and confining pressure are applied. If we add the both of these deviator stress and confining pressure, we get the value of major prinicipal stress( sigma 1).

In UU( unconfined undrained) triaxial test generally, peak deviator stress i.e failure deviator stress is same for all the test elements, as in UU test friction angle is zero.

In CU(consolidated undrained) test, water is not allowed to drain, so we get another pressure acting on the elements ,that is pore water pressure, so in CU test we get two types of peak or failure deviator stress, one is for effective and other is for total.For both types, high diameter({(sigma 1-sigma 3)or (sigma 1'-sigma 3')}gives the peak or failure deviator stress.

In CD( consolidated drained) test, water is allowed to pass or drain.so there is no pore water pressure value.Only total deviator stress we get. The higher difference in the two principal stresses, get the peak or failure deviator stress.