Synergistic effect of strain softening and dilatancy in deep tunnel analysis

• Published in: Tunnelling and underground space technology 2020-03, Vol.97, p.103280, Article 103280
• Main Authors: Yi, Kang, Kang, Hongpu, Ju, Wenjun, Liu, Yuedong, Lu, Zhiguo
• Format: Article
• Language: English
• Subjects: Acceleration; Bolting; Computer simulation; Deep tunnel; Dilatancy; Dilation; Displacement; Mathematical models; Mechanical properties; Nonlinear systems; Plastic deformation; Plastic flow; Plastic zones; Residual strength; Rock bolting; Rock bolting system; Rock masses; Shear strain; Softening; Strain softening; Synergistic effect; Tunnels; Underground construction
• ISSN: 0886-7798; 1878-4364
• DOI: 10.1016/j.tust.2020.103280

•Ignoring strain softening and dilatancy will induce significant errors.•Considering dilatancy without strain softening has little influence on calculation.•Considering dilatancy can accelerate strain softening process.•Strain softening and dilatancy create a significant synergistic effect.•A rock bolting system can inhibit strain softening and dilatancy. The extent of the plastic zone of the surrounding rock of a deep tunnel is significantly greater than that of a shallow tunnel, giving importance to correctly determine the post-peak mechanical behavior during a deep tunnel analysis. In this study, the nonlinearity of strain softening and dependence on the confining stress and plastic shear strain of the dilation angle were considered. Based on the incremental equations of the theory of plastic flow in FLAC3D, the influences of the strain softening and dilatancy on a deep tunnel analysis were discussed, and the modified M−C model was used in the numerical simulations for verification and further analysis. The results indicate that the extent of the plastic zone and the surface displacement of the surrounding rock are unrealistically small when the strain softening and dilatancy are ignored. When considering only strain softening, the plastic zone and surface displacement increase significantly. When considering only the dilatancy, the extent of the plastic zone exhibits no change, and the surface displacement increases only slightly. The increase in dilation angle will lead to an acceleration of the strain softening process, that is, a decrease in the strength of the rock mass occurs. There is a significant synergistic effect of the strain softening and dilatancy, that is, the influence of considering the two factors together is greater than the sum of considering the two factors individually. A rock bolting system can improve the residual strength of the surrounding rock of a deep tunnel, strengthen the confinement of the shallow to deep parts of the surrounding rock, reduce the level of strain softening and the dilation angle, and thus, significantly reduce the extent of the plastic zone and surface displacement of the surrounding rock. Therefore, a tunnel should be supported in time after an excavation to prevent a degradation of the surrounding rock and make full use of the self-support capacity of the surrounding rock. When a numerical simulation is applied to the deformation prediction and the support design of a deep tunnel, the accurate strain sof