Abstract: This paper presents a study of shear deformation whichincludes a theoretical approach to progressive - retrogressive failure,involving discontinuous, elastic-elatoplastic finite element method.It has been concluded that choice of boundary conditions exertd an important control on failure mechanism. Under certain boundary conditions,the ultimate failure of the test block is a consequence of multiple fracturemode. "In-situ" shear tests on geological materials should be interpretedin more sophisticated terms; i.e., as a consequence of variable combinedstress state, involving inhomogeneous stress field, one / several of principal stresses being tensile; extensive stress reorientation; multiplecrack propagation. The theoretical approach to the mechanism of sheardeformation. The theoretical approach to the mechanism of shear deformation failure characteristics of "ın-situ" shear tests, utilizing finiteelement method, appears to be a valid approach for prediction of certainexperimental results.