A laboratory study of post-rupture strength

Stiff intact clays exhibit rapid loss of strength after failure, with the f ormation of slip surfaces and the development of a well-defined 'post-ruptu re' strength envelope. This paper describes an experimental study of the in fluence of a number of factors on the post-rupture strength of initially in tact samples of Vallericca Clay. These include changes in confining pressur e after the formation of a slip surface, the magnitude of the relative disp lacement across the surface, comparison between the post-rupture strength e nvelopes in triaxial compression and extension, and the influence of existi ng slip surfaces on the formation and strength of subsequent rupture planes formed by rotation of the principal stresses. The results show that the po st-rupture strength envelope is well defined, repeatable, and not very sens itive to stress history or the rotation of the principal stresses. Little r eduction in shearing resistance takes place for relative displacements up t o about 10 mm. The results are believed to be of significance in understand ing the formation of fissures in stiff clays, and in evaluating their opera tional strength.