Abstract: The structural hazards of active subway tunnels are important factors affecting the safe operation of subways. The hazards such as segmental total and differential settlements, cracking, dis-alignment, and water leakage are particularly prominent in subway tunnels. Reasonable and scientific diagnosis are prerequisite for the identification and mitigation of these hazards. The shallow embedded tunnel of the entrance and exit line of an urban rail transit line No.2 in the Pearl River Delta is presented in this paper as a case history. Through the analysis of engineering geological conditions, surface structural loads, external disturbance and other aspects, the geological radar detection, three-dimensional laser scanning and other methods were used to detect the current conditions of tunnel structural hazards, and combined with a variety of monitoring methods, the deformation characteristics of the tunnel structure were comprehensively evaluated. The results show that the settlement is the main factor of the current tunnel hazard. The current tunnel is in an unstable state of continuous development. Through the comprehensive evaluation and empirical judgment, the vehicle load of municipal road above the tunnel and the poor characteristics of soft soils are the main causes of the hazards. Combined with these, mitigation measures such as steel ring reinforcement in the tunnel, upper sheet pile bridging structure and compaction grouting are recommended. This case history can provide a reference for hazard control and safety assurance of subway tunnels in similar soft soils during operation.

Key words: Soft Soil, Operating Subway, Tunnel Structure, Settlement, Monitoring