Behaviour of Prestressed Reinforced Granular Bed Overlying Soft Soil

Abstract

The use of geosynthetics to improve the bearing capacity and settlement performance of shallow foundations has proven to be a cost-effective foundation system. In marginal ground conditions, geosynthetics enhance the ability to use shallow foundations. This is done by either reinforcing cohesive soil directly or replacing the poor soils with stronger granular fill in combination with geosynthetic reinforcement. In low-lying areas with poor foundation soils, the geosynthetic reinforced granular bed can be placed over the weak soil. The resulting composite ground (reinforced granular bed) will improve the load carrying capacity of the footing and provide better pressure distribution on top of the underlying weak soils, hence reducing the associated settlements. It is now well established that geosynthetics demonstrate their beneficial effects only after considerable settlements, since the strains occurring during initial settlements are insufficient to mobilize significant tensile load in the geosynthetic. This is not a desirable feature for foundations of certain structures, since their permissible values of settlement are small. Thus there is a need for a technique which will allow the geosynthetic to increase the load bearing capacity of soil without the occurrence of large settlements. One technique yet to be comprehensively studied is the effect of prestressing the geosynthetic layer before implementing them as reinforcement in field applications. In this thesis, extensive investigations are made to study the effects of prestressing the geosynthetic reinforcement on the behaviour of reinforced granular bed overlying weak soil. The study involved laboratory scale plate load tests to observe the physical behaviour of prestressed geosynthetic-reinforced soil system. Non-linear FE analyses were carried out using the FE program PLAXIS, version 8, and the results were compared with those obtained from model tests. The parameters studied are effects of magnitude of prestress, direction of prestress, number of layers of prestressed reinforcement, type of geosynthetic reinforcement, size of reinforcement, thickness of granular bed, strength of underlying weak soil, presence of voids in granular bed and weak soil etc. In this thesis an analytical model is proposed to predict the improvement in bearing capacity due to prestressing the reinforcement in the granular bed. The results of the analytical model are validated by comparing it with those obtained from experimental and finite element studies.