Abstract: A new type of artificial soft clay composed of sodium bentonite and glycerol was made to study the mechanical feature of composite foundation reinforced with geosynthetic-encased stone columns (GECs) in soft clay foundation under undrained or poorly drained conditions. Four groups of model tests of GECs composite foundation with varied reinforcement stiffness and length under embankment load were carried out for this purpose. The test results were compared with those in well drained condition. Results showed that: the settlement of the GECs composite foundation obviously decreases with an increase of reinforcement stiffness. The stress concentration ratio is much larger than that in well drained condition. The columns with high reinforcement stiffness are likely to tilt outward the embankment. The columns with low reinforcement stiffness, however, bend inward the embankment. For the partially encased stone columns, the settlement is significantly greater than the fully encased stone columns. The stress concentration ratio of the partially encased stone columns is smaller than that of the fully encased stone columns, but larger than that of stone columns. The GECs bend inward the embankment in the encased parts and bulge in the non-encased parts. Drainage condition significantly affects the displacement and stability of GECs. Generally, the bending deformation is smaller in the columns with well drained conditions and vice versa. It is found from the study that fully encased stone columns with high reinforcement stiffness and well drained conditions are of great importance to improve the stability of road embankment.

Key words: geosynthetic-encased stone column, artificial soft clay, model test, composite foundation, stress concentration ratio, failure mode