## 1 Introduction
[26; 27; 28; 29] investigated the shape of the settlement trough caused by tunnelling in cohesive ground by analytical solutions, empirical solutions and the finite element method. [PERSON] et al. [30] established a three-dimensional numerical model to analyse the effect of tunnel spacing, horizontal distance between tunnels and excavation sequence on surface settlement. [PERSON] et al. [31] simulated the shield tunnel mechanical excavation process including variable muck pressure, cutterhead overcut, shield conicity, installation of jointed segmental lining, annular gap grouting based on ABAQUS. Then, the development of ground deformations and internal forces of the lining were studied. In the relevant study of undercrossing the airport runway, [PERSON] et al. [32] analysed the stress and deformation of runway pavement induced by undercrossing tunnel excavation based on the mechanics method, and the calculation method of the soil settlement trough limit value and the vault control value were proposed. [PERSON] and [PERSON] [33] presented a novel deep learning model for real-time prediction of shield moving trajectory during tunnelling, and a tunnel case study at Bao'an International Airport, Shenzhen, was employed to demonstrate the accuracy of the proposed model. The field observation is the most popular and straightforward method for understanding the interaction between existing buildings and undercrossing tunnels. [PERSON] et al. [34] introduced a super-large diameter earth pressure balance (EPB) shield tunnel crossing below an operating airport in a soft soil region and the environmental effect of EPB shield tunnelling was investigated by monitoring the surface settlement. [PERSON] et al. [35] studied the control rules of tunnel shield construction parameters through field tests; the corresponding control measurements were given to reduce the settlement of airport taxiways, which provided guidance for construction parameter settings during shield advancing. Based on the observation data of runway surface settlements for more than 6 years, [PERSON] et al. [36] studied the settlement law of airport runways under aircraft take-off and landing loads. Different from the relatively mature technical conditions of shield tunnels undercrossing airports in coastal soft and clay areas, Chengdu Shuang-Liu Airport is located in a sandy cobble stratum with rich water and high mobility. The characteristics of this stratum are unstable mechanics, large gaps between particles, no cementation, sensitive response and ease of producing disturbance loss. When the cutter head is rotating and cutting, the stratum that is relatively stable or in equilibrium and is easily destroyed and then collapses, resulting in large stratum loss and surrounding rock disturbance, which has brought great challenges to the construction of metro shield tunnels that undercross airports. In addition, there are few reports of shield tunnels undercrossing operating airports in a high-flow sandy cobble stratum. Therefore, it is necessary to conduct in-depth research on ground settlement and discuss the feasibility of shield tunnel undercrossing airport in sandy cobble area. Considering above problems, based on the Chengdu Metro Line 10 project undercrossing Shuang-Liu Airport, the influence law of double-line shield tunnel construction on the ground settlement was investigated via a numerical simulation method. Finally, the vulnerable area of the pavement structure was proposed through stress analysis and the shallowest tunnel buried depth in this region was determined. It was expected to make a rational calculation and assessments of the safety of the airport runway and provide reference for similar projects in the future.