Insights into geogrid–soil interaction using a transparent granular soil

Geogrid pullout tests were carried out in a large pullout box filled with a transparent granular soil. Geogrid displacements and displacements of a dispersed layer of opaque particles located immediately above and below the plane of the geogrid were measured from image analysis of pictures taken through a window at the bottom of the box. Longitudinal displacements of an extensible biaxial polypropylene geogrid were converted to strains and strain rates and these values used in a rate-dependent load–strain model to calculate load along the entire length of the specimen and load transfer to the surrounding soil. Plots of load transfer, expressed as equivalent shear stress acting over the area between transverse members, against relative horizontal deformation between the geogrid inclusion and the surrounding soil were generated. These data show that for the combination of geogrid and granular soil used in this investigation, load transfer is largely due to shear between the soil particles trapped in the plane of the geogrid between transverse members, and the surrounding soil. This observation suggests that the results of direct shear box tests can be used to characterise the stiffness of the shear interface between the horizontal inclusion representing the geogrid, and the surrounding soil.