Analysis of the Impact of Shield Tunnel Construction on the Ancient City Wall in Loess Stratum

Abstract

Xi’an Metro is the first urban subway built in loess stratum in China. Loess is the continent unconsolidated sediments formed under the quaternary arid and semi-arid climatic conditions, when dry, the loess stratum has high strength, while mixed with water, its strength decreases dramatically as particle bonds vanish. Its special engineering geological properties have brought about serious technical challenges to the shield tunnel construction. Based on the project of Xi’an metro line 1 under-passing the Ancient City Wall, a three dimensional nonlinear finite element model was established to analyze the impact of shield tunnel construction on the city wall. Priority was given to the effect of shield tunneling in loess stratum on the ground surface’s transverse and vertical deformation and the principal stress change. Results show that, the maximum surface came after the shield tail passed the city wall, and the closer the monitoring section was to the tunnel, the larger the settlement would be. The extent of the city wall’s incline was mainly determined by disturbance of the soil and the jacking force, the jacking force was almost proportional to the incline. Besides due to the shield tunneling, stress concentration occured at the foot of the City Wall, and some reinforcement measurements should be taken into consideration.