Abstract: This paper presents the principles of the experimental study by the torsional plastometer with the multi-force systems (i .e .the case of axial compression under torsion and hydrostatic pressure ).Stress analyses of the pure shear tests and the three –dimensional stress –state tests are made .A simple and accurate testing method is suggested for studying the effect of the intermediate principal stress level on the strength and the stress –strain relationship of the soil .At the end ,the extensive uses of this apparatus are given .