## 2 Background  
This study is based on the Chengdu subway project in China. The tunnel from Shuang-Liu West Station to Shuang-Liu Airport Terminal 2 of Chengdu Metro Line 10 is located in the Shuang-Liu District (Figure 1), which highlights the Chengdu Metro Line 10 Phase II Project undercrossing the Chengdu Shuang-Liu Airport flight area. The undercrossing area is mainly runways and sward, and the subway terminal station is Shuang-Liu Airport Terminal 2. The lithological log around the shield tunnels mostly consists of miscellaneous fill, silty clay, sandy cobble and weathered mudstone with increasing depth (Figure 2). Figure 1: The project location map. The plan view and spatial location of the project are shown in Figure 3. The distance between the left tunnel and right tunnel is 13.0 m. The airport runway is a double line with a width of 60 m, and the width of the sward between the runways is 40 m. The undercrossing tunnel constructed by the shield-driven method is a conventional circular section with single-layer reinforced concrete precast lining. The burial depth of the tunnels is approximately 46.5 m. The thickness of the segment is 300 mm, the inner diameter is 5.4 m and the outer diameter is 6.0 m.  
The typical soil profile at the site is presented in Figure 3b. The geological soil layer of the undercrossing of the airport runway section from top to bottom is pavement structure, miscellaneous fill, silty clay, pebble soil and strongly weathered mudstone. The shield tunnel is located in the weathered mudstone. The main physical and mechanical parameters of each layer are shown in Table 1.  
The pebble soil and weathered mudstone strata are widely distributed in the Chengdu area; the special engineering characteristics of these strata have caused great difficulties for shield construction, as shown in Figure 4. The characteristics of these two strata are introduced in this section. The pebble stratum is brown grey or grey yellow, mainly loose or medium dense. The pebbles are mainly slightly weathered magmatic rocks and metamorphic rocks. The shape of pebbles is subcircular. The pebble content in this stratum  
Figure 3: Illustration of excavated tunnels undercrossing airport runway: (**a**) plan view; (**b**) typical cross-section A-A\({}^{\prime}\). Figure 2: Geological profile. is generally 60-70%, the particle size is mainly 2-15 cm and the maximum particle size is approximately 40 cm. The filling materials are mainly fine and medium sand and round gravel. A pebble stratum is a typical mechanically unstable formation with a large gap between particles and almost no cohesion. Under anhydrous conditions, force is transmitted through point contact between particles and the stratum reaction is sensitive. During the construction of shield tunnels, the stable state of the stratum is easily destroyed and collapses, resulting in large stratum loss and disturbance to the surrounding rock. The weathered mudstone stratum is dark red or purple red. The development degree of the rock joint fissure is relatively great and the rock quality designation (RQD) is 75-90%. The natural single axis compressive strength is in the range 3.15-6.70 MPa, the saturated single axis compressive strength is in the range 1.76-3.91 MPa, the free expansion rate is in the range of 12-28% and the expansion force is in the range 5.8-25.1 kPa. In view of the particularity of sandy pebble strata, a project with a shield tunnel crossing an operating airport in a sandy pebble area has not been reported. Based on the actual project in Chengdu and the numerical simulation test, this paper discusses the feasibility of shield tunnels undercrossing airports in sandy pebble areas, which can provide some technical support for shield construction in other sandy pebble areas. The research scheme is shown in Figure 5.