Large-scale direct shear testing of geogrid-reinforced aggregate base over weak subgrade

Geogrids have been widely used to construct stable subgrade foundations and to provide a working platform for construction over weak and soft soils. Use of geogrid reinforcement in a pavement system ensures a long‐lasting pavement structure by reducing excessive deformation and cracking. In this study, a series of large-scale direct shear tests were performed to evaluate the mechanical interaction between a subgrade soil and an aggregate base layer with and without a geogrid at the interface. The subgrade and base materials consisted of glacial till and well-graded gravel, respectively. Seven types of biaxial geogrids were tested. Normal stresses of 50, 100 and 200 kPa were applied to the top of the samples compacted at optimum moisture content (OMCsoil = 16.4% and OMCaggregate = 8.2%) to relative compaction values of 93–98% (RCsoil = 94–98% and RCaggregate = 93–96%). The test results show that aperture area and junction strength of geogrids are important factors that affect the soil–aggregate interface shear strength response. The average values of the peak interface shear strength coefficient, defined as the ratio of the peak interface shear strength of the soil–aggregate systems with and without geogrid at the interface, range from 0.96 to 1.48. Based on the results of this study, the geogrid aperture area, the normalised aperture area, the normalised aperture length, and the junction strength to optimise the peak interface shear strength of soil–aggregate–geogrid systems are 825 mm2, 4.7, 5.4, and 11.5 kN/m, respectively. These values are restricted to the materials and test conditions used in this study.