Compacted granular columns formed by either sand or crushed stone aggregates are one the most commonly used methods to reduce the total deformations and to improve the load carrying capacity of soft soil underneath the embankment. In very soft and highly compressible soil layers with undrained strength below S_u< 15 kPa, the conventional granular column may not help sufficiently as columns are undergone excessive bulging along their length in the soft soil layer. The most applicable and suitable approach to restrict column excessive bulging is to encase the granular column with a high stiffness geosynthetic material along its entire or partial length. The hoop tensile force mobilized in the geosynthetic encasement will consequently provide an additional radial stress acting along the column-reinforced length thus the upward thrust load developed in column increases while the surrounding very soft soil is already yielded. 

In this study a series of numerical analysis was performed using PLAXIS 2D finite element code, aiming to investigate the influence of the encasement stiffness, area replacement ratio, thickness of the compressible layer, oedometric modulus of soft soil, friction angle and elastic modulus of both the column and the embankment materials on the behavior of the improved soft foundation. The data available from a full-scale load test was used to validate the finite element model in which 36 geotextile-encased granular columns stabilized 10 m-thick very soft soil (Almeida et al. 2015). The parametric analysis was then performed by a hypothetical model consisting an encased granular column and its influence area loaded by an embankment on top.

Results of the numerical analyses showed that among the parameters analyzed, geosynthetic stiffness, area replacement ratio and thickness of the compressible soft soil were those which most significantly affected the settlements and the total vertical stresses below the embankment. The differential settlements on top and bottom of the embankment were also studied and the results were compared with the differential settlements proposed for the piled embankment. The results demonstrated that the critical height for encased granular columns supporting embankment was smaller than that obtained for the piled embankment.