Modified ground response curve (GRC) in strain-softening rock mass based on the generalized Zhang-Zhu strength criterion considering over-excavation

Abstract

The ground response curve (GRC) depicts the relationship between support reaction force and ground displacement, which improves the understanding of ground-support interaction and provides important references to the tunnel design. However, it is difficult to anticipate the tunneling-induced large deformation with sufficient reliability in soft rock with high geostress since the small strain theory is not applicable. When large deformation occurs, the tunnel needs to be over-excavated. Thus, the GRC should be modified considering the enlarged excavation radius since the actual excavation radius is usually greater than the designed one. To overcome the shortcomings of small strain theory in recognizing ground-support interaction under large deformation circumstances, a new large strain numerical approach for modifying the GRC was proposed considering over-excavation in strain-softening rock masses based on the generalized Zhang-Zhu strength criterion. A case study was conducted based on the Lianchengshan tunnel in China. The modified GRC was employed to investigate the ground-support behavior for different support schemes and to explore the applicability of the stress release measures. Combined with field tests, the proposed approach was validated. By comparing with GRCs proposed by previous work, the present modified GRC was proved to be superior to others. Parametric studies were conducted and it is found that over-excavation, for example, reserving a very large clearance between the surrounding rock and the support, is necessary to reduce ground pressure to a large extent. The yielding supports which can provide high support pressure during the process of deformation are highly recommended when tunneling in high geostress environment. However, if the initial geostress is not very high, it is not necessary to pursue unwarranted over-excavation since the ground pressure applied on the support is mainly the loosening stress when the deformation is large. Ample support stiffness should be provided in the process of deformation to prevent uncontrolled large deformation of surrounding rock.