A combined experimental and DEM investigation of grain interlocking in sheared granular assemblies

Compacted granular material, integral to geotechnical engineering, undergoes translation, rotation, and interlocking when subject to shear displacements or external loads. The present study focuses on the interlocking of heterogeneous granular materials, a complex behavior influenced by gradation, compaction, and varying particle geometry, and has consequently received limited attention in existing research. To address this research gap, we conducted an analysis on the effect of grain interlocking on the shear resistance of granular assemblies, using a combination of laboratory testing and discrete element method (DEM). Initially, large-scale direct shear tests were conducted on gravel−sand mixes with varying degrees of compaction and normal pressure. One of the mixes also underwent subsequent shear reversal to explore the differences in grain interlocking between the two shearing processes on the shear plane. After analyzing the laboratory results, a mesoscopic scale investigation was performed by replicating the test using discrete element simulations. To facilitate this, granular particle geometries were measured using 3D laser scanning based on the physical lab tests. Subsequently, based on these scans, discrete element R-block and ball models were utilized to construct both the coarse and fine particles within the mix. Surface vibro-compaction was employed to regulate the degree of compaction. The results indicate that an increase in vertical pressure, coupled with a zero dilatancy angle, results in a rising stress ratio, indicative of grain interlocking. This interlocking exhibits a positive correlation with both the coarse content and the degree of compaction, and varies depending on the shear displacement. As interlocking progresses, the shear band, induced by particle movement, expands and is associated with reduced particle rotation near the shear band. The study further reveals a consistent positive correlation between interlocking and the principal orientation angle of strong normal contact forces, as well as a correlation between interlocking and mobilized contacts.