Behaviors of existing twin subway tunnels due to new subway station excavation below in close vicinity

This paper presents a case of Chongwenmen subway station of Beijing Subway Line 5 excavated below the existing Line 2, which was the first project of new subway construction below existing subway in Beijing. The minimum clearance between the new and the existing tunnels was 1.98 m. The layout, the geological and hydrogeological conditions, the construction process, and the monitoring measures are introduced. During construction, the maximum settlement of the existing tunnels reached 31.3 mm and the speed of trains run inside the existing tunnels was slowed down. In order to compensate the settlement of the existing tunnels and ensure the operational safety of the trains running inside the existing tunnels, compensation grouting was adopted. The pillar between the existing and the new tunnels was strengthened by grouting first. Then the pillar and the existing tunnels were uplifted together by specific controlled grouting pressure. The compensation grouting successfully uplift the existing tunnels. Moreover the subsequent settlements of the existing tunnels were small as the pillar had been sufficiently reinforced.