Shear strength characterization is described in detail starting on p. 98. The use of a modified Mohr-Coulomb diagram is explained from which I will quote:
"Several different forms of modified Mohr-Coulomb diagrams can be used. All modified Mohr-Coulomb diagrams are based on the fundamental relationship between the principal stresses and the Mohr-Coulomb shear strength parameters, c' and φ' or c and φ.
Referring to Figure D-5 and the triangle formed by points, def, the following expression can be written:
Equation D-3 can be rearranged to obtain a number of different relationships between the principal stresses and the shear strength parameters, c and φ. Two of the most useful forms of Equation D-3 and the resulting modified Mohr-Coulomb diagrams are described in the following text......." (p.106, USCE).
That is about the meat and potatoes of the publication, I just wanted to share my excitement in finding such wonderful publications. There are a few others I found that are really interesting, and once I have a chance to read through them I will write about it. I have always just loved the Mohr circle, it was the one thing on exams in structural geology I could whip through quickly and get 100% correct. There is something about using a compass and drawing tangent lines that brings out the best in me I guess. Now if I could just get that one thing in structures that I am horrible at mastered, I will be a happy camper. Hope you enjoyed my Monday "show and tell".