Experimental study with complete stress state interpretation of undrained monotonic and cyclic simple shear tests with flexible boundaries

In many geotechnical problems, the stress state may be approximated by a simple shear stress state. Owing to that, simple shear tests, in which the principal axes of stresses and strains are free to rotate, should be preferred to investigate the soil mechanical behaviour. However, the most common simple shear devices (rigid boundaries) do not allow to completely know the stress state of the specimen. Therefore, the interpretation of this test type has always been developed only theoretically. In order to improve the basic understanding of simple shear stress paths, a more innovative simple shear device with flexible boundaries was used in this research. The specimen is enclosed with an unreinforced membrane and confined by cell pressure. The diameter is kept constant through a sophisticated control system, which well approximates a K-0 condition. The undrained monotonic and cyclic simple shear tests on an Italian sand are presented and discussed. Based on some hypotheses, the stress state is reasonably determined and represented by Mohr's circles. The theoretical interpretations show that the soil failure-under monotonic and cyclic loading-is reached when the effective intermediate principal stress is midway between the major and minor principal effective stresses (b = 0.5; theta = 0), while the principal stress directions tend to reach a slope of 45 degrees. Finally, the mechanical response of the tested sand is compared with the results of triaxial tests from a static and cyclic point of view. In agreement with several results reported in the literature, the friction angle in critical state conditions is higher in simple shear tests compared to that achieved in triaxial tests due to the rotation of principal stress directions. Moreover, the results of cyclic simple shear tests, in terms of liquefaction resistance (CRR15 approximate to 0.13), are consistent with those performed in cyclic triaxial conditions.