I've also found it very surprising. The good answer to your question gives 'The Mechanics of Soils' by J.H. Atkinson & P.L. Bransby - chapter 13. Crosssection of undrained plane and elastic wall is a vertical line. Until you reach state boundary surface (plasticity) the soil moves on the elastic wall. In v/p' space its projection is a line with slope kappa. In undrained conditions v remaines constant, so the only movement on the wall you can make without changing v is vertical( delta p' equals zero).