Evaluation of soil-geogrid interaction at different load levels using pullout tests and transparent soil

Geogrids have been used for decades as reinforcement for mechanically stabilized earth retaining walls and base layers of pavements. However, literature on these applications is contradictory regarding the displacement and strain levels at which the bearing mechanism of interaction between soil and geogrid is developed along the transverse ribs of geogrids. No data are available on the deflections and displacement profiles of transverse ribs during loading of geogrids. Field and laboratory data on strain distributions along geogrids are limited. Accordingly, the objective of this study is to better understand the mechanisms of soil-geogrid interaction that develop at different stages of pullout tests, especially at small displacements and strains. Moreover, the behavior of transverse ribs throughout pullout testing is evaluated. Pullout loads were obtained from a load cell synchronized with two 5 MP cameras. Images of the geogrid were analyzed using the Particle Image Velocimetry (PIV) technique to obtain displacement profiles along the entire geogrid specimen throughout the duration of the test. Five transparent pullout tests were conducted using a confining pressure of 35 kPa (5 psi) on polypropylene geogrids with different configurations. Displacements along the polypropylene geogrid used in this study are well represented by an exponential equation. The bearing mechanism along transverse ribs was observed to develop at small viii displacements. The contribution of the bearing mechanism was first observed at 25 % of the maximum pullout force. Interference between transverse ribs was first observed at approximately 60 % of the maximum pullout force. High interference between transverse ribs was observed when the ratio of spacing between transverse ribs (S) over the thickness of the transverse ribs (B) was equal to 24. Negligible interference was observed when S/B was equal to 57. Displacements of soil particles were observed when the ratio distance from the soil-geogrid interface (d) over the D₅₀ of the soil was equal to 3, but they were orders of magnitude smaller than the displacements of the geogrid specimens. The observed boundary of the zone of influence of geogrids was for values of 3 < d/D₅₀ < 7 for the transparent soil used in this study.