Geogrid pullout load-strain behaviour and modelling using a transparent granular soil

The paper describes the results of a series of geogrid pullout tests that were carried out in a novel large pullout box with a transparent granular soil. The test apparatus and methodology allows the entire geogrid specimen to be visible through the bottom of the box. Specimen displacements are computed from image analysis of pictures taken through the transparent bottom of the pullout box during each test. The geogrid material was an integral punched and drawn biaxial polypropylene geogrid. A series of reference tests were carried out on geogrid specimens 2000 mm long and loaded at a front clamp displacement of 1 mm/min. The results of these tests have been reported in previous publication by the writers. In the current study, this earlier database of test results and interpretation of results is extended to include tests with specimens of different (shorter) length and loaded at other displacement rates. The tensile loads in the specimens are estimated using two different nonlinear rate-dependent load-strain models with parameters determined independently from in-air tests. The predicted loads are compared to measured loads recorded at the front and end of specimens trimmed to short lengths. In addition, the paper compares the load-displacement response of an unmodified geogrid specimen to that of a nominal identical specimen with the transverse members removed. The load-strain models are used to deduce the component of total tensile load in the geogrid carried by the transverse geogrid members. Finally, test results for nominal identical specimens tested in air and in soil show that there is no detectable influence of soil confinement on longitudinal tensile stiffness of the geogrid product used in this study.