Experimental Proof of the Existence of a Bifurcation Process During the   undrained test in Clay
  Recent papers, based on a new simple incremental modelling which assumes an "isotropic" response, predicts that the trajectory followed during an undrained compression test exhibits a bifurcation process when the stress field arrives at q=M'p', where q stands for the deviatoric stress, p' for the mean stress and M' is a coefficient describing friction. This indicates a discontinuous change of solution when arriving at q=M'p'. This paper looks at experimental data on dense sample obtained at p'=constant, and it shows that the trajectory (p'=constant, v=constant) does continue to exist at and beyond the q=M'p' plane. So, this demonstrates the validity of the analysis which uses the bifurcation theory and this strengthens the proposed modelling. Indeed, this demonstrates the reality of the bifurcation process during undrained compression. Pacs # : 5.40 ; 45.70 ; 62.20 ; 83.70.Fn 
P. Evesque and M. Hattab