Abstract: Based on according completely with practical constitutive relation of rock, i.e. relation between stress and strain that is linked glossily by three segments composed of elastic, strain nonlinear hardening and nonlinear softening segment having inflexion, the stress distribution laws of surrounding rock in plastic hardening and softening zone of circular tunnel under the condition of hydrostatic pressure is deduced. It is proved that the obtained radial and tangential stress curve link glossily at hardening zone radius and softening zone radius , and curve of tangential stress - has not up-pinnacled stress concentration especially, the result goes all the way with the experiment result of Japanese scholar Jiuwubosheng. The paper points out that there is obvious limitation in Kastener formula based on ideal elastoplastic constitutive model, and up-pinnacled stress concentration of - doesn’t correspond with practice.

Key words: nonlinear hardening, nonlinear softening, tunnel, stress distribution law