Three dimensional bearing capacity analysis of granular soils, reinforced with innovative grid-anchor system



Reinforcing soils with biaxial geogrids have been shown to be an effective method for improving the ultimate bearing capacity of granular soils. The pull-out resistance of reinforcing elements is one of the most significant factors in increasing bearing capacity. In this research a new reinforcing element that includes attaching elements (anchors) to ordinary geogrid for increasing the pull-out resistance of reinforcements is introduced. Reinforcement therefore consists of a geogrid and anchors with cubic elements attached to it, named (by the authors) Grid-Anchor. Three-dimensional numerical study was performed to investigate the bearing capacity of square footing on sand reinforced with this system. The effect of depth of the first reinforcement layer, the vertical spacing, the number and width of the reinforcement layers, the angle of anchors, the stiffness of reinforcement and anchors and the distance that anchors are effective were investigated. Three-dimensional finite element analysis by "PLAXIS 3D Tunnel" software, indicated that when a single layer of reinforcement is used there is an optimum reinforcement embedment depth for which the bearing capacity is greatest (u=0.5B). There also appeared to be an optimum vertical spacing of reinforcing layers for multi-layer reinforced sand (h=0.25B). The bearing capacity was also found to increase with increasing the number of reinforcement layers, if the reinforcement was placed within a range of effective depth (d=1.25B). In addition, analysis indicated that increasing reinforcement and anchor stiffness beyond a threshold value does not result in further increase in the bearing capacity (1kN for anchors and 100 kN/m for geogrid). Results show that the Grid-Anchor system of reinforcing can increase the bearing capacity 2.74 times greater than that for ordinary geogrid and 4.43 times greater than for non-reinforced sand.