Viscoplastic solutions of time-dependent deformation for tunnels in swelling rock mass considering stress release

Excavation and control of tunneling responses in swelling soft-rock tunnels of Sichuan-Tibet railway under seepage conditions were studied. For this, a fractional viscoplastic (FVP) model for swelling soft rocks was established by introducing Abel dashpot and unsteady viscosity coefficient, considering additional swelling deformation and damage of rock caused by humidity effect. In view of the FVP model, the viscoplastic deformation solutions for rock mass surrounding tunnel under seepage conditions were derived and long-term mechanical responses of swelling rocks upon tunnel excavation were analyzed. Next, a stress release coefficient considering seepage and creep was proposed, based on which control responses considering stress release and failure mechanism of stress release measures were analyzed. The results showed that: (i) The one-dimensional (1D) FVP model has a good application for swelling rock and the three-dimensional (3D) FVP model could well describe the whole creep process of rock mass despite a much higher creep attenuation rate in the first stage of creep; and (ii) An appropriate stress release and deformation of surrounding rocks could effectively reduce the supporting resistance. However, upon a large stress release, the radius of plastic region could increase significantly, and the strength of the surrounding rock mass decreases greatly. The proposed solution could provide a theoretical framework for capturing the excavation and support responses for tunneling in swelling rock mass in consideration of time effect. (C) 2023 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).