This book focuses on the unified solutions and analysis for the problems in soil mechanics based on the unified strength theory, which is a new theory on the yield and failure of materials under multi-axial stresses. Then, it provides a system of yield and failure criteria adopted for most materials, from metallic materials to rocks, concretes, soils, polymers, etc. It includes the Tresca criterion, Mohr–Coulomb theory, and Mises criterion as well, which are special cases or linear approximation of the UST.