Challenges in modelling DCM column-supported embankments with post yield strain-softening behaviour

The most realistic approach to numerically simulate a Deep Cement Mixed (DCM) column-supported field embankment is the use of a three-dimensional (3D) finite element model. However, two-dimensional (2D) plane-strain numerical models are popular compared to 3D models due to the efficiency in computational time and computer memory. When a column-supported embankment is modelled using a 2D plane-strain model, the individual columns are converted into equivalent column walls and it is assumed that the strains do not change in cross sections along the longitudinal direction of the embankment. During the conversion, material properties or the geometry of the columns are modified based on the conversion method applied. According to the literature, 2D plane-strain idealisation based on equivalent area (EA) method yields the results closest to the 3D model predictions and field measured data. However, all these studies are carried out considering the elastic or elasto-plastic behaviour for columns. As far as the mechanical behaviour of DCM columns is concerned, DCM columns experience strain-softening beyond yielding. The column strength reduces during the strain-softening resulting large deformations in the improved ground. If a constitutive model, which has the ability to simulate strength reduction based on the level of plastic strains, is adopted to simulate the softening behaviour, the accuracy of predicted embankment behaviour is largely dependent on the plastic strains developed in DCM columns. Hence, results of a 2D model and a 3D model differ to a large extent. The aim of this study is to investigate this problem in detail and to propose an approach to obtain an equivalent set of parameters for the softening incorporated constitutive model to obtain realistic predictions for a field problem using both 3D and 2D numerical models.