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Despite the fact that there are shear stresses also involved while considering deformations in soil why are the horizontal and vertical stresses on an element as shown in below figure considered as principle stresses while analyzing the active and passive earth pressures on retaining walls? In other words how does one conclude here in the first place that shear stresses on horizontal and vertical planes are zero when the soil mass is in active or passive stage?

enter image description here

inquisitive learner
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These are not principal stresses in the element. In my opinion this is simply a way of conveniently showing the stresses applied on the element, but not necessarily the principal ones (remember a triaxial test. Of course then the weakest plane on the element will be sloped). Showing stresses vertical and horizontal is convenient and can take us to solutions when solving the retaining wall.

upstream
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