Study On The Ground Surface Deformation Laws Induced By Metro Shield Tunnel Construction

Shield construction in Loess area to build subway tunnel is the first case in China. So many monuments make the ground surface subsidence and the impact on the surrounding environment due to the shield tunnel excavation more especially, there is important engineering application value to study it.This paper is based on the metro 2# line tunnel through the city wall in a city which is built by shield construction, use the manner of combine theoretical analysis, in-situ monitoring and FLAC^3D simulation to study the surface and the city wall foundation subsidence laws where the subway tunnel is built by shield construction in specific Loess areas, and suggested the matures how to control the distortion. The Main consequents are as follow:(1) Studied the main reasons and failure mechanism of the ground surface subsidence due to shield tunnel construction. The results show that the main reasons affecting the ground surface subsidence during the shield construction are ground movement formation, formation changes of the original stress state, the soil consolidation and secondary consolidation. The ground surface subsidence includes 5 parts as before the shield arrived, the shield arrived, the shield passed, the shield tail passed and follow-up subsidence.(2) Analyzed the surface subsidence data obtained, The result shows that the K of Peck formula changed from 0.44 to 0.50.Suggested the applicable conditions how to apply Peck formula in Loess area to estimate the ground surface subsidence.(3) Studied the process of the shield passing through the ancient city wall by designing a numerical model method with FLAC^3D.Suggested the best joint construction program which is formed by segment support and grouting behind the segment, chemical grouting in the soil within 3 city gates and piling bored piles closed to the ancient city wall south foundation by studying the calculation data with 3 conditions. The results show that as the loess cohesion and internal friction angle increased, the city wall foundation settlement and ground subsidence are slowed. But when the friction angle is over 21°, it has little effect on the city wall foundation formulation and the ground subsidence.(4) Designed the monitoring program of the city wall foundation and the ground surface subsidence, and completed the monitoring. Analyzed the calculation data and the monitoring data, the results show that the trend of the two settlement curves are basically consistent. This also prevented the model calculation was correct and credible.