Soil Mechanics/Mohr's circle
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Mohr's circle is a graphical depiction of two dimensional stress states. Amongst others, it can be applied to stresses and strains. In soil mechanics, Mohr's circles are used to visualize relationships between normal and shear stresses, and to estimate the maximum stresses, based on three or more soil samples extracted from a site. 
In practice, Mohr's circles are constructed based on the results of triaxial compressions tests for undisturbed soils. This test provides for the measurement of the total stresses applied to a specimen, uncorrected for pore pressure, and it provides data for determining strength properties and stress-strain relations for soils. It "determines the unconsolidated, undrained, compressive strength of cylindrical specimens of cohesive soils in an undisturbed condition, using a strain-controlled application of the axial compression-test load where the specimen is subjected to a confining fluid pressure in a triaxial chamber".
The shear strength of an extracted soil sample when tested in a triaxial compression test may differ from that of the same sample in situ, owing to the mechanical processes involved in soil extraction such as drilling, transportation and storage. Interpretation of the graphical results need to take this into account for greater accuracy.