LOAD-SETTLEMENT BEHAVIOUR OF GEOGRID-REINFORCED SAND CUSHION

Soft clays are those which have their natural water content close to their liquid limit. As a result, they have a low bearing capacity and undergo large settlements even at low values of imposed stress. Hence, structures formed in this soil are subjected to distress and instability. Various ground improvement techniques have been devised for improving the bearing capacity and reducing settlements of these soils. Chemical stabilization, lime-soil columns, grouting, vibro-compaction, stone columns, preloading with sand drains, heavy tamping, and electro-osmosis have been some of the successful ground improvement techniques used widely all over the world. Reinforcing earth is also an effective technique to improve the bearing capacity. Geo-synthetic reinforcement of ground has been a recent development in reinforced earth. Geo-textiles, geo-membranes, geo-fabrics and geogrids have been the reinforcement component used for improving bearing capacity of weak soils. In this thesis, the efficacy of geogrid-reinforced sand cushion in improving the load-settlement behaviour of a soft clay bed was explored. A high density polyethylene (HDPE) geogrid was used as horizontal reinforcement in the sand cushion overlying a soft clay bed. A series of laboratory model plate load tests was conducted to study the load-settlement behaviour of a soft clay bed underlying sand cushion prepared with fine sand, medium sand and coarse sand. The soft clay bed was prepared at a consistency index (CI) of 50%. All the sand cushions were compacted to a constant relative density of 60%. Circular geogrids of diameter 120 mm were used as reinforcement layers in the sand cushion. The number of geogrids and the spacing between them were varied. It was found that sand cushion layers improved the load-settlement behaviour of the clay bed even in the un-reinforced condition. When geogrid reinforcement was provided in the sand cushion the behaviour further improved with increasing number of geogrid layers and with decreasing spacing between them. Different types of sand cushion were provided to study the influence of particle size on the settlement response of geogrid reinforced sand