Geotechnical centrifuge modelling of the behaviour of a compressible clay horizon underlying a reinforced sand foundation

Abstract

Basal reinforcement, where high tensile geogrids are employed beneath structures, is becoming an increasingly accepted construction technique along the eastern coast of southern Africa. The presence of compressible, soft, thin and shallow clay horizons usually associated with complex estuarine or lagoonal deposits are a major consideration when using basal reinforcement as a founding technique. Basal reinforcement involves the use of high tensile strength geogrids beneath a structure to form a reinforced sand foundation. Deformation behaviour under loading is an important component of stability analysis of earth structures. If reinforcement is used, the mechanisms become altered. Geotechnical centrifuge modelling is a unique physical modelling technique, as it allows replication of in situ stresses, which is most important because soil behaviour is a function of stress. This is achieved by placing the model at the end of the centrifuge arm, and subjecting it to an increased gravitational field, which creates the correct stress distribution in the model. Centrifuge modelling provides an appropriate technique to observe the behaviour of compressible, soft, thin and shallow clay horizons when basal reinforcement is utilized. An appropriate centrifuge model was constructed and compared the behaviour of the clay horizon under unreinforced and reinforced conditions. Reinforcement configurations were adjusted to observe the influence of additional geogrid layers, and extension of the width of the reinforcement. It was found that deformation behaviour is distinctly different between unreinforced and reinforced tests. Vertical deformation in the unreinforced test localised to the region directly beneath the platform, with little lateral disturbance to the clay horizon beyond the platform edge. As such, the sand directly beneath the platform acts as a deeper rigid platform. The deformation behaviour of the clay horizon changes with the inclusion of reinforcement. When reinforcement is included a wider portion of clay is deformed. The lateral width of this deformation zone is controlled by the width of the reinforcement, as the applied load is spread. A ‘wide-slab’ effect is evident with an increase in the geogrid width, as the tensioned membrane-effect is mobilised to increase the capacity of the reinforced foundation sand. This results in a wider portion of the clay deforming. Addition of geogrid reinforcement to the sand foundation under a wide platform load enhances deformation of the clay, but has the advantage of an increased load-bearing capacity of the system. Furthermore, the addition of multiple layers of reinforcement contributes to this increase in load-bearing capacity. Additionally, increasing the installation width of the reinforcement contributes to an increased vertical load-bearing capacity. However, this resultant increase is only mobilised after a certain amount of vertical displacement. This is likely due to the reinforcement requiring a certain amount of vertical displacement to mobilise tension in order to support the applied load. The behaviour of a thin compressible clay horizon changes with the inclusion of reinforcement under a wide platform load. The deformation behaviour of the clay is increased by additional layers of reinforcement as well as an increase in the width of the reinforcement. However, the increase in deformation comes at the benefit of an increased vertical load-bearing capacity of the reinforced foundation sand.