The present work is the further development of the previous works of the writers on internal variable gradient model for zonal disintegration phenomenon near deep tunnels. Strain gradient model is used to study zonal disintegration phenomenon. As an additional variable

strain gradient

is introduced. The equilibrium equations

boundary conditions and flow rule are obtained by using virtual work principle. Constitutive equations are obtained by using Clausius-Duhem inequality. For deep circular tunnels the governing equations are obtained from the general equations of the proposed model for elastic deformation regime

elastoplastic deformation regime and plastic deformation regime without consideration of elastic deformations. The solutions of the governing equations are obtained; and the behaviour of the solutions is analyzed. The proposed model not only expands the classical elastoplastic model for surrounding rock near tunnels

but also serves as theoretical basis for numerical study of zonal disintegration phenomenon near deep tunnels.