Evaluation of the Zone of Influence and Stiffness Improvement from Geogrid Reinforcement in Granular Materials

Abstract

The longevity of a pavement system is closely related to the amount of deformation of both the asphalt surface and the underlying layers. Placing a geogrid in the granular base decreases the amount of rutting at the surface by providing some strength to the base and increasing stiffness and thus limiting elastic deformations and loads transmitted to the sub-grade. However, the zone of influence of the geogrid layer on surrounding soil particles and the increase in modulus in this zone are not well known. A testing scheme aimed at quantifying both the zone of influence and the increase in the modulus caused by the presence of a geogrid in granular materials was developed. Both P-wave velocity and the shear strain induced by loading a 150-mm diameter plate were examined. P-wave velocity results indicated a change in modulus across the geogrid from a minimum of 1.35 (at 75-mm geogrid depth) to a maximum of 2.66 (at 100-mm geogrid depth) over modulus expected in unreinforced soils. Expected internal soil rotations were modeled with PLAXIS and compared with laboratory tests. These analyses showed that the shearing of soil was confined to a zone above the geogrid. Rotation tests showed a zone of influence of 30 to 40 mm on both sides of the geogrid reinforcement; however, the zone of influence depends on the position of the geogrid, with a geogrid 100 mm in depth seeming most able to constrain subsurface soils and distribute the shear stresses caused by the 150-mm-diameter loading plate.