  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
