Influence of grain characteristics on stress-dilatancy relationship and failure surface geometry

Stress-dilatancy relationships of granular soils are influenced by both mechanical and physical soil properties. However, existing empirical correlations which define the influence of dilatant behavior on soil strength depend only on the interpretation of shear test data conducted at different relative density and confining pressure ranges. Therefore in this study a wide range of consolidated, drained triaxial testing program was carried out on granular materials consisting of varying gradation and grain shapes in order to reveal the effect of physical grain characteristics on existing stress-dilatancy and strength correlations available in literature. In addition, soil specimens were shared with different stress paths in order to observe the influence of loading condition on dilatant behavior. In the end, unique empirical correlations were proposed which link initial physical characteristics of granular materials with line-fitting stress-dilatancy correlation parameters and stress-dilatancy relationship constants. Within the scope of this research, influence of dilatant behavior on the evaluation of shear band geometry at active failure state of granular backfills were also investigated. For this purpose, small-scale physical retaining wall model experiments were conducted in 1g conditions. Tests were applied on variant graded granular materials of distinct grain characteristics. Arching effects on shear band generation were eliminated by proper selection of model width. Failure surfaces were revealed by particle image velocimetry (PIV) method. Mechanical properties of tested soils were evaluated by positioning pressure sensors at vertical and horizontal walls of the physical model. Consequently, variance on the slip surface geometry and peak friction angles of granular materials which were composed of different grain size and shapes were investigated. Actual results were compared with existing empirical equations which proposed the coordinates of any point located on the failure surface of the backfill. It was revealed that physical grain characteristics have a considerable effect on the dilatancy of granular soils especially under high surcharge pressures. Hence, existing correlations were modified to take grain size and grain shape effects into account.