Abstract:During the construction of small clear space tunnel the collapse of tunnel face and the stability of middle pillar exist commonly. Take Leigongpu small spacing tunnel for example

analysis of the surrounding rock was carried out

and some monitoring tasks have been performed at the exit of collapsed accumulative rock segment

including the ground settlement

crown settlement

convergence

the displacement of middle pillar

the axial force of bolt

pressure of surrounding rock and the stress of steel arch

etc. Then simulation and calculation were done aiming at K72+520 cross-section by finite element method. Monitoring and numerical simulation results show that the construction of three-step method in collapsed accumulative rock in outlet section has stability problem

and the reserved soil core method can effectively improve the stability of excavation face; Excavation of later hole has great influence on stress distribution and stability of first hole. The rock of the middle pile are wholly in the disturbed state

and bolt’s anchoring force has not been well play

the vault and arch waist position was prone to stress concentration. The experience and conclusions presented here can provide reference to design

construction and research of similar tunnels.