## 5 Conclusions  
In this study, based on the case of the Chengdu Metro Line 10 undercrossing Shuangliu Airport in a sandy cobble region, a three-dimensional numerical model was established to investigate the ground settlement during the shield tunnel construction procedure. Then, the development law of the surface settlement during the double-line shield tunnel construction period was investigated and the influence of runway pavement on ground settlement at different depths was analysed. Subsequently, according to the stress analysis of the pavement structure, the most unfavourable regions of the runway pavement structure under the influence of tunnel excavation was determined. Finally, the influence of the tunnel burial depth on the ground surface settlement was studied. The main conclusions are as follows:  
(1) When the double-line tunnel excavation was finished, the ground surface settlement curves at different sections induced by the double-line tunnels undercrossing were V-type settlement troughs instead of W-type settlement troughs. Among them, the maximum surface settlement at the pavement section was 1.96 mm and the maximum surface settlement at the sward section was 2.14 mm. (2) When the shield machine advanced to the 40 th loop, the settlement of the characteristic monitoring point at the sward surface reached the maximum value. When the shield machine advanced to the 60 th loop, the settlement of C1 reached the maximum value. (3) The existence of the runway pavement greatly limited the deformation of the surrounding soil; with increasing stratum depth, the effect degree of runway pavement on ground settlement decreased. (4) With increasing tunnel buried depth, the maximum settlement of the surface centre point decreased continuously and the development time of the ground surface settlement was extended. It is suggested that the tunnel buried depth of the shield tunnel in this project should be no greater than 22.5 m.  
(5) There were good linear relationships between the maximum tensile stress, the tensile stress area width and the tunnel buried depth. With decreasing tunnel buried depth, the tensile stress area width and the maximum tensile stress value of the pavement structure increased. Even if the maximum tensile stress of the asphalt pavement did not exceed the allowable tensile stress value, considering the influence of aircraft fatigue load after runway operation the safety detection of the tensile area should be considered. The effect of shield construction on surface buildings has always been a complex problem in the engineering. This study only discussed the influence of tunnel buried depth and surface roads on the ground settlement of the operating airport in a sandy pebble region. Figure 20: Influence of buried depth on ground surface settlement. (**a**) maximum stress vs. tunnel burial depth; (**b**) tensile stress area width vs. tunnel burial depth. * [PERSON] et al. (2017) [PERSON]; [PERSON]; [PERSON] Prediction methods on tunnel-excavation induced surface settlement around adjacent building. _Geomech. Eng._**2017**, _12_, 185-195. [CrossRef]
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* [PERSON] (1987) [PERSON] Analysis of Undrained Soil Deformation Due to Ground Loss. _Geotechnique_**1987**, _37_, 301-320. [CrossRef]
* [PERSON] (2015) [PERSON] A Complex Variable Solution for A Deforming Circular Tunnel in an Elastic Half-Plane. _Int. J. Numer. Anal. Methods Geomech._**2015**, _21_, 77-89. [CrossRef]
* [PERSON] (1998) [PERSON] Deformations of an elastic half plane with a circular cavity. _Int. J.