Characteristics of Ground Surface Settlement of Double-Line Adjacent Metro Construction in Sandy Cobble Stratum: A Case Study of Beijing Airport Line

As a long-term urban transportation infrastructure, the stability of metro construction is very important. With the continuous expansion of urban metro network, metro adjacent construction is more and more frequently encountered. In this paper, taking the double-line adjacent construction problems encountered during excavation of Beijing New Airport Line metro in sandy cobble stratum as an example, the surface settlement caused by excavation of single-line metro and double-line adjacent metro is studied by computed tomography scan, electron microscope scanning, laboratory mechanical tests, orthogonal inversion and numerical simulation and other methods. The results show that 1) the sandy cobble particle size and sandy cobble distribution of Beijing sandy cobble samples are relatively dispersed, and the sandy cobble particle size is from 3 to 20 cm, of which the largest proportion is the sandy cobble particle size from 5 to 8 cm. 2) The internal friction angle of Beijing sandy cobble samples with different moisture content is from 27° to 40°, which is much larger than that of other soil samples. Even at the post-peak stage, the shear stress of some sandy cobble samples does not gradually tend to be gentle, but continuously increases or has secondary peaks. 3) For the excavation of double-line adjacent metro, the maximum settlement difference and the impact range of excavation are proposed as indicators to investigate the degree of interaction of the adjacent metro. 4) The results of numerical orthogonal analysis show that the width of surface settlement zone increases significantly with the increase of tunnel diameter and spacing when excavating adjacent metro in sandy cobble stratum, but the rate of increase with tunnel diameter (from 6 m to 14 m) almost reaches 2 times of the rate of increase with the spacing (from 3 m to 15 m). 5) The maximum settlement value of sandy cobble stratum increases with the increase of tunnel diameter and decreases with the increase of spacing. The orthogonal linear analysis shows that the influence of tunnel diameter on the maximum settlement value is much greater than the spacing. The maximum settlement increases with the increase of tunnel diameter, which is about 3.6 times of that with the decrease of spacing.