There is a stress jump on the interface between the elastic and the plastic zones. Using the stress jump values as mesh refinement criteria
one can not only greatly improve the accuracy of the stresses calculated but also solve the form of plastic zone. For the specific problems of tracing the interface between the two zones
a modified mesh refinement criterion is presented. By changing several main factors that effect the shape and size of plastic zone
a lot of numerical computations are carried out. The results have shown that the criterion used in this paper is effective for determining the shape and size of the plastic zones of caverns.