## 6 Conclusions  
In order to relieve the increasing ground traffic pressure in the process of urbanization in China, it is inevitable to build more metro lines. The evaluation and control of the stratum displacement is a prominent problem during shield construction of city metro engineers. Based on the engineering project of an interval tunnel between Shizishan Station and Chuanshi Station in Chengdu Metro Line 7, China, this paper analyzed the action mechanism of stratum displacement induced by shield tunneling, and divided effect factors into two categories: ground loss factors and mechanical factors. Through the quantitative analysis of the effect factors, the following conclusions can be drawn:  
1. The calculation results of this paper are applicable for shield tunneling in urban soft soil layer, but the calculation methods and thoughts have a good universality for different working conditions, it just needs necessary parameter adjustments. Combining the advantages of Loganathan method and mirror source-sink method, the three-dimensional displacement induced by ground loss are calculated. Based on the elastic half-space Mindlin model, the displacement at any point induced by four mechanical effect factors through coordinate transformation and different integral region is deduced. The total displacement is the sum of the two, which is verified by field monitoring data. The difference is stable and the difference rate is about 5.3%. This is because that the elastic model is adopted in this paper, but in practice with the redistribution of stress, a pressure arch will be formed around the tunnel. The soil out of the arch is prevented from entering the loosening zone, which causes the field monitoring data to be slightly smaller than theoretical calculation result. It is a methodological flaw, not a statistical one, and needs to be solved in future.
2. The displacement induced by ground loss is over 95% of the total displacement, which plays a decisive role. The key to controlling stratum disturbance is decreasing the ground loss. When the outer diameter of shield and segment is determined, the most effective measure is decreasing the strain of grouting layer. As the stratum uplift caused by grouting pressure is much smaller than the settlement caused by grouting layer strain, the grouting pressure or grouting volume can be increased appropriately to ensure the grouting filling rate.
3. The ratio of displacement caused by mechanical effect factors to total displacement is too small (less than 5%), even far less than the error caused by various simplified assumptions in theoretical calculation. In practice, this part of the displacement cannot be separated from the total displacement, it is difficult to verify its accuracy and validity by numerical simulation or field monitoring. Therefore, it is suggested that in calculation, the mechanical effect factors can be neglected as appropriate, and only the displacement caused by ground loss is considered, which can not only reduce the computational complexity and make it easy to use, but also can meet the requirements of engineering application accuracy.  
[PERSON] wrote the article; [PERSON] provided the parameters of project; [PERSON] carried out the calculations and analyzed the calculation results; [PERSON] processed the field data; [PERSON] offered some useful suggestions for the preparation.  
The study was supported by the National Natural Science Foundation of China (NSFC) under Grant No. 51508037.  
We also highly appreciate the contribution of data collection from China Railway No. 9 Group Co., Ltd. Finally, the authors would like to thank reviewers for useful comments and editors for improving the manuscript.  
The authors declare no conflict of interest.