This paper by D. C. Drucker and W. Prager explores the implications of treating soil mechanics problems as plasticity issues. The authors assume the soil to be a perfectly plastic body, focusing on the yield function and stress-strain relationship. They derive a yield function that generalizes the Mohr-Coulomb hypothesis and introduce the concept of plastic potential to describe the stress-strain relation. The paper discusses the collapse or limit design theorems, which state that collapse occurs when the rate of external work equals or exceeds the rate of internal dissipation. The authors also analyze plane strain conditions, where the yield function reduces to the Mohr-Coulomb rule, and provide a detailed derivation of the rate of energy dissipation and dilatation. They illustrate these concepts with examples, such as the critical height of a vertical bank, and propose a modified stress criterion to account for tension in the soil. The paper concludes by highlighting the far-reaching consequences of plasticity theory and limit design in soil mechanics, including the replacement of circular sliding surfaces by logarithmic spirals and the necessity of volume expansion during shearing deformation.This paper by D. C. Drucker and W. Prager explores the implications of treating soil mechanics problems as plasticity issues. The authors assume the soil to be a perfectly plastic body, focusing on the yield function and stress-strain relationship. They derive a yield function that generalizes the Mohr-Coulomb hypothesis and introduce the concept of plastic potential to describe the stress-strain relation. The paper discusses the collapse or limit design theorems, which state that collapse occurs when the rate of external work equals or exceeds the rate of internal dissipation. The authors also analyze plane strain conditions, where the yield function reduces to the Mohr-Coulomb rule, and provide a detailed derivation of the rate of energy dissipation and dilatation. They illustrate these concepts with examples, such as the critical height of a vertical bank, and propose a modified stress criterion to account for tension in the soil. The paper concludes by highlighting the far-reaching consequences of plasticity theory and limit design in soil mechanics, including the replacement of circular sliding surfaces by logarithmic spirals and the necessity of volume expansion during shearing deformation.