The reinforced column in the field is subjected to increasing axial stress while the confining pressure is coincidently increased. Moreover, the confining pressure arising from in-situ lateral earth pressure may vary along the column length. These complications in analyzing column behaviour can be overcome in theoretical and numerical analyses by modeling the mechanical properties of the material as a function of the monotonously increased confining pressure.

The numerical method has advantages over the theoretical method in analyzing structures with complicated geometric and load conditions such as columns embedded with limited radial spacing and with non-uniform pressure distributed along the column length. Using a representative single column and its influence boundary, a "unit cell" concept is introduced to the numerical method for a group column analysis.

The confining pressure of an encased column is highly dependent on the volumetric behaviour of the column because volumetric strain induces column expansion and radial strain, which in turn increases the circumferential stress in the encapsulating reinforcement and cavity pressure offered by the surrounding soft soil. As a consequence, a numerical model capable of accurately evaluating volumetric strain is essential in analyzing reinforced granular column behavior, single or grouped while embedded in soft soil. However, numerical modelling for volumetric strain requires elaborate effort.

This study analyzes the behavior of geosynthetic-encased sand columns assuming that the encased column deforms with constant volume. A simple relationship between the confining pressure and encasement stiffness to column diameter ratio can be achieved by the constant volume assumption. Simple steps were also provided to evaluate axial stress for an encased column.

The assumption provides conservative, yet practically accurate results without elaborate analytical procedures. The only material properties and column geometry needed are the encasement stiffness, strength and column diameter. The results from the present approach are compared with those obtained from experimental and numerical analysis using a more complicated model.

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