## 2 Background  
### Project Overview  
A double-track subway tunnel passes under the existing Beijing S1 medium-low-speed Maglev line. The tunnel has a standard horseshoe section. The outer contour size of the interval structure is 6.48 m \(\times\) 6.62 m, the initial lining thickness is 0.25 m, and the secondary lining thickness is 0.3 m. The new double-line subsurface excavation tunnel was excavated from north to south, and the step method was used for excavation. The tunnel curve section passes through the existing magnetic suspension elevated section at an angle of 80\({}^{\circ}\), and the tunnel overburden is about 16.0 m. The flat and cross-section positions of the new double-line subsurface excavation tunnel and the existing line are shown in Figures 1 and 2. The existing middle- and low-speed magnetic levitation elevated section is a double line, the structure foundations are piles, the pile length is 17 m, and the diameter is 1 m. The existing viaduct under the area of influence is a three-span continuous beam structure with a bridge span of 23.6 m + 39.2 m + 23.6 m. The new section crosses the middle span of the continuous beam bridge, and the crossing pier numbers are 72# and 73#. The minimum plane distance between the new right tunnel and the pier 72# bridge pile is about 5.9 m, and the minimum plane distance between the left tunnel and the pier 73# bridge pile is about 4.5 m. The bottom elevation of the new tunnel structure is about 1.0 m deeper than that of the existing bridge pile. The maximum depth of the exposed stratum is 70 m. According to the drilling data and indoor geotechnical test results, and according to the sedimentary age and genetic type of the stratum, the soil layer within the exploration scope of the project site is divided into four layers: artificial accumulation layer (Qml), newly deposited alluvial-diluvial layer (Q42 + 3 al + pl), Quaternary late Pleistocene alluvial-diluvial layer (Q3 al + pl), and Quaternary late Pleistocene slope diluvial layer (Q3 dl + pl). This section of the line tunnel through the surrounding rock mainly comprises a clay silt fill 1 layer, miscellaneous fill 1 layer, silty clay 21 layer, pebble 5 layer, pebble 7 layer, sandy silt clay silt 23 layer, pebble 81 layer, silty clay 8 layer, and pebble 9 layer. The groundwater depth is 10.0-25.0 m. The stratigraphic profile is shown in Figure 3.  
Figure 1: Plan of new subway excavated tunnel and existing viaduct. Figure 2: Elevation of new subway excavated tunnel and existing viaduct. Deformation Control Measures for Double-Track Excavated Tunnel Passing under a Magnetic Suspension Viuduct  
In order to control the influence of the tunnel passing underneath on the deformation of the existing viaduct and the Maglev track, it is necessary to take deformation control measures. In crossing engineering, deformation control measures are mainly divided into active control measures and passive control measures. The construction of the tunnel under the tunnel will have a certain impact on the existing structure, and the Maglev track structure has more stringent requirements for deformation control than the traditional wheel-rail line. Therefore, the project also adopts a combination of active and passive schemes:  
1. Deep-hole grouting is used in advance to reinforce the surrounding soil of the structure, and the cross-section is 2 m outside the initial support contour. The grouting slurry is a cement-water glass double slurry. The reinforced soil should have good uniformity and self-reliance, and the unconfined compressive strength of the reinforced soil should not be less than 0.8 MPa. 2. During the construction of the interval crossing bridge piles, the step method with a temporary inverted arch is adopted. The initial lining structure controls the excavation step of 0.5 m (the grid spacing is 0.5 m), and the temporary inverted arch is closed in time. The left and right lines of the tunnel are staggered 15 m, and the upper and lower steps are staggered 3 m.
3.