Abstract: Dilatancy is a common concept in the constitutive relationships in the study of rocks, soils, and concretes, etc. In its measurements, dilatancy angle is a primary parameter that, an objective quantity though, is closely related to constitutive models, particularly the Mohr-Coulomb model. This paper presents a comprehensive review for clarification of the origin and nature of dilatancy angle, discusses typical misconceptions about it, and demonstrates the internal connections and implicit assumptions between its different forms of expression under plane strain and triaxial testing conditions. To distinguish from the actual failure surface, we classify the theoretical failure surfaces into two categories－conventional strength-based failure surfaces defined by the Mohr-Coulomb strength criterion, and deformation-based failure surfaces defined by the zero extension line. The relationships of failure surfaces versus dilatancy angle, internal friction angle, and zero extension line are explored. We also discuss the reason that the angle between the displacement increment direction and the theoretical failure surface equals the internal friction angle, by using the Mohr-Coulomb associated flow model under plane strain conditions. And, the objectivity of the indexes in geomechanics is examined and classified. Further, combined with the definition of shear strength, we reveal that the measurement bases of shear strength indexes adopted by direct shear tests and triaxial tests are not exactly the same. For practical measurement of strength indexes, test methods should be selected according to different scenarios. The findings would deepen our understanding of fundamental concepts－such as dilatancy angle, zero extension line, failure surface, and shear strength－and their applicable conditions, thereby avoiding confusion and ambiguity.

Key words: dilatancy angle, zero extension line, friction angle, failure surface, shear band (shear zone), Mohr-Coulomb model