Optimizing the Microanchor Attachment Angle for Maximum Interaction Enhancement at Granular Soils–Geogrid Interface under Direct Shear Mode

This paper presents the results of a laboratory study aimed at optimizing the attachment angle of a microanchor to a sand–geogrid interface under direct shear mode. Well- and poorly graded sands, together with a single configuration comprising 24 microanchors attached at angles of 0°, 30°, 45°, 60°, and 90° to the interface are considered. All samples were prepared under dry conditions at 80% relative density and tested under normal pressures of 12.5, 25, and 50 kPa using a 300 × 300 × 170-mm direct shear box. The results show that the enhancements achieved for fine uniform sand samples were greater than for equivalent well-graded nonuniform sand samples. The optimum microanchor attachment angle for maximum interaction enhancement at the interface with well-graded sand varied between 30° and 60°, whereas this angle was 45° with the fine uniform sand. Microanchors attached at 0° and 90° angles adversely affected interaction. The greatest enhancements—of 13% and 21%—were, respectively, achieved for the well- and poorly graded sand–microanchored-geogrid samples compared with normal sand–geogrid samples. These improvements are attributed to the mobilization of passive sand resistance in front of the microanchors during shearing, with sand particle size and grading, together with normal pressure, also proving to be influential factors on the interactions. Improving sand-reinforcement interactions can reduce the amount of backfill and reinforcement required, thus reducing costs.

Soils are abundantly used in the construction of earth structures. To improve the bearing capacity, reduce deformation, increase stability, and the tensile characteristics of soils, various methods, including soil reinforcement using geosynthetics, are used. Geosynthetics, such as geogrids, are widely used for reinforcing embankments and slopes, for example. The behavior of reinforced soil structures is largely governed by complex interaction mechanisms that develop between the reinforcement and the fill material. Soil–geosynthetic interactions comprise: (1) soil–geogrid skin friction; (2) soil–soil shear strength in apertures; and (3) bearing resistance against transverse ribs. To develop an understanding of the interaction mechanisms, direct shear or pullout tests are usually performed. In direct shear tests, Factors (1) and (2) are dominant, with Factor (3) being dominant in pullout tests. Passive soil resistance against transverse ribs under pullout accounts for 75% to 90% of the pullout resistance, whereas in direct shear, it contributes 10% to 12% to the overall shear resistance. To increase the contribution of passive resistance in direct shear mode, researchers have investigated different methods, including the attachment of elements as anchors to the geogrid surface. Improved interaction leads to increased stability in earth structures, reduces backfill and the amount of reinforcement required, and saves costs.