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.