The purpose of this paper is to compare the behaviour of deep soil mixing columns predicted by Plaxis 2D and 3D programmes in terms of stresses acting in columns and calculated factor of safety (FoS). Deep soil mixing column has been used as one of the remedial solutions on slip repair works of state highways in the Northland region of New Zealand. Slope failure is common in Northland region due to the unique characteristic of the geology in this area and also the heavy rainfall experienced particularly in winter months. Prior to installing the deep soil mixing columns, the road had to be repaired regularly by smoothing of the pavement. A heavy rain event eventually caused sufficient significant damage to the road such that slope stabilisation work was considered necessary.

One such study was selected to both illustrate the repair method, integration with Plaxis, and also to illustrate the issues with the use of FE technology in routine design. Experience to date suggests that conventional limit equilibrium design was not capturing the field behaviour of the columns, and greater understanding of the real behaviour was needed. In this particular study, 2-D and 3-D Plaxis was used, as concerns were raised about the suitability of only two rows of columns in the design by the project reviewers. Field experience also indicates that the effectiveness of the columns is bounded. If columns are too close there is no further gain in overall strength, and too far apart, and the columns begin behaving individually with no net benefits. This paper illustrates some of these issues.