Understanding Principal Stresses In Soil Mechanics

Hello all

I was wondering if someone could help with the following:-

I am trying to understand the concept of principal stresses in terms of soil mechanics.

I have a soil element that is being loaded by 25kN. I am trying to draw all of the stresses the soil element experiences.

I have attached a drawing of my soil element, the bit I cannot get my head round is the last cube which I got from an online source which shows the Principal stresses 1 3, would it be correct to say that:-

Sigma1 = 25kN worth of stress acting downwards & upwards
Sigma3 = ?????? No idea where this comes from?

Any ideas?

Thank you.

You need to study up on Mohr's Circle for plane stresses.

Start here..
[url]https://www.engineersedge.com/material_science/mohrs-circle-stress-plane.htm[/url]