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UBC Theses and Dissertations

Discrete element modeling of direct simple shear response of granular soils and model validation using laboratory tests Dabeet, Antone


The direct simple shear (DSS) device is one of the most commonly used laboratory testing tools to characterize the shear behavior of soils. In the Norwegian Geotechnical Institute (NGI) version of the DSS test, where a cylindrical soil specimen is confined by a wire-reinforced membrane, only normal and shear stresses on the horizontal planes are measured. The knowledge of these stresses alone does not provide adequate information to calculate friction angles for use in geotechnical design. Further, the absence of complementary shear stresses at the soil-membrane interface causes stress non-uniformities within DSS specimens, which makes the task of interpreting DSS testing results even more difficult. With the recent advances in computers, it is now possible to model soil in a realistic manner as a collection of particles using the discrete element method (DEM). With this background, a DEM model of a cylindrical DSS specimen was developed to provide insight on the state of stress and strain in DSS specimens. A laboratory DSS testing program was undertaken on glass beads as part of this study. The results of the glass beads tests were used for comparison with the DEM model results. Further, free-form sensors (paper-thin flexible pressure sensors mounted on the reinforced part of the DSS membrane) were used to measure lateral stresses acting on reconstituted Fraser River silt specimens. It was shown that: i) the adopted DEM modeling approach is effective in capturing the salient characteristics of the DSS behavior of the tested glass beads; ii) during the shearing phase, the distribution of shear strains across the specimen is more uniform at lower shear strain levels; iii) significant stress non-uniformities during shearing are limited to a narrow zone of about two particles diameter near the lateral boundaries, while stresses at central specimen locations are relatively more uniform (i.e. most representative of “ideal” simple shear conditions); and iv) at large shear strains, the horizontal plane becomes the plane of maximum obliquity, and the friction angle calculated using the stress state on the horizontal plane is a good approximation to the mobilized friction angle at such strain levels.

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