Abstract:  To calculate the stress and displacement of a circular tunnel in soft surrounding rock during large deformation, the elastoplastic solution of the ideal elastoplastic model for soft surrounding rock is derived using the unified strength theory of triple shear stress and logarithmic strain in Lagrangian coordinates considering the influence of principal stress sequence and seepage under three working conditions of construction, operation, and maintenance periods. Then, the effects of elastic modulus, Poisson′s ratio, pore water pressure, and strength criterion on plastic zone thickness and radial displacement of the tunnel wall are analyzed. The results reveal the following: (1) With an increase in elastic modulus, the plastic zone thickness of the surrounding rock during the construction period increases gradually and stabilizes at a certain value (small strain solution), and during the operation period, it decreases gradually and stabilizes at a certain value (small strain solution); whereas the radial displacement of the tunnel wall during construction and operation periods decreases with an increase in elastic modulus. (2) The Poisson′s ratio has little impact on the plastic zone thickness of the surrounding rock during construction and operation periods, whereas the radial displacement of the tunnel wall increases linearly with an increase in Poisson′s ratio. (3) With an increase in pore water pressure, the plastic zone thickness of the surrounding rock and the radial displacement of the tunnel wall during the construction period increase, whereas during the operation period, they decrease. (4) The plastic zone thickness and the radial displacement of the tunnel wall vary significantly under various strength criteria. (5) The variations in the plastic zone thickness and the radial displacement of the tunnel wall during the maintenance period are similar to those observed during the construction period. However, for the circular tunnel with large deformation in soft surrounding rock, the derived elastoplastic solution clearly differs from the small strain solution.

Key words: soft surrounding rock, threeshear stressunified strength theory, large deformation, logarithmic strain, seepage