Abstract: In order to analyze the failure evolution characteristics of tunnel lining structure during service period and provide reference for subsequent monitoring and response measures, calculation models for the two common cases of weak surrounding rock and shallow buried bias pressure at the portal of highway tunnel during operation are established. The relationship between the structural reduction coefficient and the structural state is studied using strength reduction method to continuously reduces the strength parameters of surrounding rock. Through the limit strain distribution law of lining structure, the structural safety state is studied, the typical structural mechanical characteristic indexes in different states are extracted, and the instability and failure modes of highway tunnel structure during operation are analyzed. It is concluded that under the condition of soft surrounding rock (non bias pressure), the lower part of sidewall, the middle part of inverted arch, and arch crown of tunnel lining structure are prone to deformation, cracking, and even damage, and the deformation and cracking of sidewall and arch crown occurs earlier than that of inverted arch. The monitoring should focus on the stage from the initial deformation of lining structure to the movable hinge of sidewall,  so as to prevent the sidewall from further invert damage due to movable hinge. Under the condition of shallow buried bias pressure at the portal, the arch waist and arch foot on both sides of the tunnel lining structure are the key parts of monitoring. The damage first occurs to the arch foot ahead of the arch waist, the free side, and the soil side. The stage from the initial deformation of the lining structure to the damage of the arch foot and arch waist on the free side is an important stage for monitoring hence the free side, should be prevented from damage.

Key words: soft surrounding rock tunnel, shallow buried bias section, strength reduction method, lining structure, service status, ultimate strain, deformation, strain