A Transparent Pullout Testing Device for 3D Evaluation of Soil-Geogrid Interaction

This paper presented a newly developed transparent pullout testing device conceived with the objective of studying the soil-geogrid interaction under small displacements and strains. The bottom plate and the side walls of the pullout box were transparent. The system involved a transparent soil, which was tested as a surrogate for sands in the testing program. The setup led to 3D visualization of the soil-geogrid interaction as it allowed direct visualization of the plan view of the geogrid as well as of the side view of the soil-geogrid interface. Markers embedded in the transparent soil mass allowed tracking of soil particle displacements during the test. The tests were conducted using a polypropylene biaxial geogrid and both transparent soil and a conventional sand. Displacements along the geogrid were obtained continuously using digital image correlation (DIC) techniques in tests with transparent soil, and using tell-tails at five junctions in tests with sand. Comparison of test results with both soils showed that the transparent soil constitutes a good surrogate for sands in pullout studies. The pullout test with transparent soil also indicated that displacements along the geogrid could be properly described using an exponential function. Moreover, exponential fitting to the displacement data led to an exponential distribution of strains along the geogrid during pullout testing. Deflections of transverse ribs were first observed at early stages of the test when only 25 % of the maximum pullout force developed. The observed displacement patterns of the soil markers were useful in defining the zone of influence of the geogrid, which could be successfully quantified using the newly developed testing device. Overall, the new equipment was found to represent an effective tool to better understand the mechanisms involved in soil-geogrid interaction, particularly those that are relevant to quantify the interface stiffness.