Mesoscopic Analysis Of The Interface Properties Of Geogrid-reinforced Coarse Aggregate With The Optimization Of Reinforcement Performance

Being a natural geo-material, soil is weak in bearing tensile forces due to its loose nature, as a result, the direct application of soil seems unreliable for geotechnical engineering practice, especially for cases where horizontal disturbances are involved. Recently, geosynthetics have been extensively used in road embankments, retaining walls and slopes to improve the overall performance. Comparatively, geogrid has received increasing popularity due to its high technical and economic efficiency. However, limited by the measurement accuracy, the geogrid-soil interaction has not been clarified, which inevitably inhibit the development of corresponding design method. Thus, deep insight into the interaction between backfill soil and geogrid is of great importance for proper design and construction of geogrid reinforced earth structures. Based on the calibrated constitutive models and corresponding parameters, this dissertation presents a simulation procedure using PFC3D to reproduce the realistic pull-out behaviors of a triaxial geogrid embedded in sand and ballast, respectively. Under special consideration of the actual aperture structure, the angular characteristics of granular soils and the aggregate orientation, the load transfer behaviors across the interacting components (i.e., geogrid and backfill soil, respectively) are visualized, and factors affecting the interface strength of geogrid-reinforced granular soils are analyzed synchronously. The main contents and the results obtained from numerical modeling are summarized below:(1) A DEM model is established to reveal the structural evolution of particles within the interface at a microscopic view, simultaneously, influencing factors regarding the frictional property of geogrid and the particle size are discussed. Numerical results show that the enhancement of geogrid friction contributes to accelerate the rearrangement of backfill particles; while for particles within lower diameter, the improvement of critical edge load can be attributed to the improvement of specific surface area, which accelerates the consumption of energy inputted into the particle assembly.(2) Pull-out behaviors such as the geogrid deformation and force distribution, the particle displacement as well as the contact force are systematically examined, and the cooperation of above two interacting components are addressed. Meanwhile, a new variable named fabric anisotropy coefficient is introduced to evaluate the inherent relationship between macroscopic strength and microscopic fabric anisotropy. A correlation analysis is adopted to compare the correlation between the newly-proposed coefficient and the most commonly used one. Furthermore, additional pullout tests on geogrid with four different joint protrusion heights have been conducted to investigate what extent and how vertical reinforcement elements may result in reinforcement effects from perspectives of bearing resistance contribution, energy dissipation, as well as volumetric response. Numerical results show that both the magnitude and the directional variation of normal contact forces govern the development of macroscopic strength; and the reinforcing effects of joint protrusion height can be attributed to the accelerated energy dissipation across the particle assembly and the intensive mobilization of the geogrid.(3) A new PFC3D model is established in order to study the influence of ballast angularity on the reinforcement effect, at the same time, the inherent microscopic changes accounting for the regression of macroscopic strength are addressed. Moreover, the differences in macro performance and the particle responses produced by different shapes of particles are also systematically discussed to justify the significant role of particle geometry. Numerical results show that geogrid embedded in more angular particles exhibits greater elongation, suggesting that more intensive ballast-geogrid interaction arises due to the greater interlocking effect of particles. Meanwhile, the regression of macroscopic strength indicates the failure of self-organizing structure in the particle assembly.(4) Through graphical method and function fitting, six aggregate sizes in total are attained to investigate the influence of the ratio of aperture size to particle diameter on the reinforcement effect. A representative particle size that equals to the diameter of inscribed circle of the triangular aperture is selected to quantity the region where intensive interaction occurs. Numerical results show that the intensive interaction region is related to the aperture size, and unified results can be attained for different aperture geometries by substituting the length of aperture (in one direction) with the size of aperture inscribe circle.