How to define the soil and deep mixing column interaction in Plaxis-2D?

Hi, I am a civil engineering student and am doing a project that requires the simulation of deep mixing columns in a reclamation plan. I have built the model, but my supervisors told me that I have to define the boundaries between the soil and the DCM columns for a more realistic analysis. I tried to follow his advice. However, I have no idea what should be defined and how to define the interaction between the soil and DCM columns and there are no relevant existing models online for me to learn. Could someone that has done similar models give me some advice? Thanks!