Nowadays, convergence-confinement method is one of the most important methods in determining  support systems for both underground circular and non-circular spaces. Selecting an appropriate constitutive model including elastic-perfectly plastic, elastic-brittle-plastic and elastic-strain softening models has a significant role in the application of this method. In some cases, the difference of displacements resulting from choosing different constitutive models is more than 100%.
In this research, we attempted to investigate the effect of different constitutive models on the ground reaction curve and plastic zone made around a circular opening. It has been supposed that a rock mass is initially carries an elastic behavior and undergoes an isotropic pressure,. The failure criterion used here is non-linear Hoek-Brown yield criterion. The problem is solved in a stepwise procedure proposed by Brown et al (1983), in which rock mass strength parameters change in a stepwise method from the peak to the residual values. Finally, a sensitive analysis is performed on the Hoek-Brown strength parameters.