The nonlinear volumetric increase of rock masses can influence the safety of tunnel excavation; thus it is important to accurately evaluate the dilation mechanism of surrounding rock masses in the plastic zone. However

most of the existing studies solve the stress and strain states in strain-softening rock masses of deep circular tunnels by using the constant or linear dilation model. In order to overcome this deficiency

based on the finite difference method

a new analysis approach is proposed. It can consider the nonlinear dilatancy effect as well as the strain-softening behavior of surrounding rock masses. Moreover

the calculated results are compared with those from other existing methods to verify the calculation accuracy of the proposed method. And then a further study is made to investigate the influence factors of the dilatancy coefficient in the plastic zone with different qualities and support pressures of limestone rock masses

and to compare deformations of surrounding rock masses by the constant and nonlinear dilation models. The results indicate that: for rock masses with lower value of geotechnical strength index (GSI) and poor quality

the confining stress primarily controls the degree of the dilation in the plastic zone; and the deformations at the opening surface with constant and nonlinear dilation models distinguish greatly.