Highway Embankment Constructed on Soft Soil Improved by Stone Columns with Geosynthetic Materials

Great areas all over the world, particularly along the rivers and the seas, are covered with soft clay. Construction on soft natural soil is considered a risk due to its low shear strength and high compressibility. Stone columns are an effective improvement method for soft soils under light structures such as rail or road embankments. The stone columns derive their load carrying capacity from the passive earth pressure resistance developed against the bulging of the column which thereby depending on the shear strength of the surrounding soil. To avoid dispersion of the stones into the clay and to improve the stone columns as reinforcing elements, geosynthetics are used as an encasement of the stone columns. Increasing lateral and vertical deformations over the consolidation time controls the serviceability state and affects the economy of the embankments. Therefore, additional efforts to predict the long-term behavior of the reinforced soft soil with ordinary and encased stone columns foundation are required. In this research full scale stone columns in Bremerhaven clay, are analyzed using the finite element program Plaxis. Firstly, the stone columns are only loaded to investigate the effect of varying parameters like spacing distance between columns, column diameter, geogrid encasement and stiffness of the geogrid, and encasement depth on the behavior of the stone column in short and long term conditions. The results showed that the ordinary stone columns with narrower spacing distances and smaller diameters have a greater bearing capacity and show smaller settlement as well as lateral bulging than wider spacings and greater diameters of stone columns. When using geogrids as encasement for stone columns, a huge increase in the bearing capacity of the stone column as well as a huge reduction in the bulging occurs. More improvement occurs in the behavior of the encased stone columns with increasing encasement stiffness in both short and long term conditions. The bearing capacity of the partially encased stone columns increases with increasing encasement depth. The increase in the bearing capacity in long term is more significant than that in short term conditions under working loads. Secondly, the non-reinforced and the reinforced soft clay with ordinary and encased stone columns have been loaded by a highway embankment fill. Two types of soft clay have been used which are the Bremerhaven clay and the Hamburg clay. The analysis is performed to study the effect of spacing distance between columns, column diameter, geogrid stiffness and encasement depth on the behavior of the reinforced soft soils during and after the consolidation. A case history of an embankment constructed on the reinforced soft soil with stone columns is also simulated and gave a good agreement. Using stone columns in soft clay reduces the settlement and the production of the initial pore water pressure and accelerates the consolidation time to minimum values. The smaller the spacing distance between the columns is, the faster the consolidation is and the smaller the settlement, the bulging of the column and the generated excess pore water pressure are. The construction time of the reinforced clay decreases also with decreasing diameter of the column. But the settlement has no significant decrease with decreasing diameter of the column. Once the stone columns are encased with geogrid under embankment loads, the consolidation time, the settlement, the column bulging and the excess pore water pressure are reduced with a high degree. Further reduction occurs in the deformation and the excess pore water pressure with increasing stiffness of the encasement. Keywords: stone columns, soft clay, encasement, geogrid, consolidation