Numerical evaluation of a yielding tunnel lining support system used in limiting large deformation in squeezing rock

Large deformation in squeezing soft rock is a significant challenge that complicates the safety of underground construction engineering. A yielding tunnel support system allows a certain amount of over-excavation, thereby accommodating large deformation in severely squeezing rock. In this study, a special yielding support system has been developed with a type of newly developed foamed concrete material which has a cushion effect. The special yielding support uses pre-cast foamed concrete blocks which are mounted in the primary lining, and an in situ cast foamed concrete layer which is placed between the primary lining and the secondary lining. The effect of the special yielding support on squeezing rock tunnels has been validated by comparing the numerical results with those of a lining-strengthened stiff support system. The incorporation of the foamed concrete blocks can both reduce the maximum and minimum principal stress in the primary lining. Relative to the stiff support, the maximum and minimum principal stress in the primary lining are about 50 and 60% of those of the stiff support, respectively, thereby improving the stress state of the primary lining. Further, compared with that in the stiff support, the plastic zone in the secondary lining in this yielding support is significantly improved, and the deformations at the roof and the sides of the secondary lining are 40 and 46% less than that of the stiff support, respectively, resulting in a better stress state and less deformation in the secondary lining.