Abstract: The softening behaviour of rocks is introduced into the visco-elastoplastic solutions for the circular tunnel considering the stress release. During the softening process and residential strength stage, the plastic voluminal expansion is taken into account. When the softening coefficient k = 0 and swelling coefficient h = s, the solutions could be transformed into viscoealsto-brittle plastic solutions. Otherwise, k = 0 and h = s, the solutions could be transformed into viscoealsto-perfectly plastic solutions; furthermore, when h = s = 1, the solutions would be the viscoelasto-perfectly plastic solution without considering the plastic swelling. By the case study, the effects of distance x between the face and the setting section, softening coefficient k, plastic swelling coefficient h and s, supporting system on the radii of the plastic zone and broken zone, and displacements of surrounding rock have been analyzed in detail. When x is smaller than 4D, where D is the diameter of tunnel, with time increase, the plastic zone and broken zone enlarge rapidly. When x >4D, the increments of plastic zone and broken zone will diminish. The support system has no effects on the ratio between radii of plastic zone and broken zone; but it could limit the development of the plastic and broken zones. The calculating results of the paper are larger than the results without considering the softening behaviour and swelling behaviour of the rock mass. With softening coefficient k increases, displacements, plastic zone and broken zone will enlarge; but the ratio between the radii of plastic and broken zones will diminish. The results could be taken as reference and guide for the tunnel design and construction.

Key words: circular tunnel, visco-elastoplastic solution, softening behaviour, voluminal expansion, stress release