Effect of rock nonlinear mechanical behavior on plastic zone around deeply buried tunnel: a numerical investigation

Rock shows a significant nonlinear mechanical behavior in the deeply buried tunnel, and it could affect the stress redistribution and the magnitude of the plastic zone (PZ) around the tunnel. Therefore, a series of numerical simulations were performed to highlight the effect of rock nonlinear behavior on PZ with the implementation of a novel proposed unified strain-hardening and strain-softening (UHS) constitutive model. UHS model can well describe the nonlinearity of stress-strain curves, peak strength and residual strength. The numerical model was first validated via the experimental results of one model test, followed by the parameter sensitivity of UHS model and initial stress field. The effect of strain-hardening, strain-softening, the nonlinearity of peak and residual strengths of rock, buried depth of tunnel and the lateral pressure coefficient were discussed. A detailed description method of the PZ was finally proposed considering the nonlinearity of rock. The results reveal that (i) PZ can be divided into the residual zone, strain-softening zone and strain-hardening zone; (ii) the maximum tangential stress stays in the strain-hardening zone; (iii) the initial stress state can change the shape of PZ. Moreover, for the deeply buried tunnel, the pre-stressing measures show a good potential to control PZ, and the support should be applied as early as possible to reduce the relief of initial stress.