# Calculation Method for Settlement Deformation of Existing Tunnel Induced by Underpass Construction
## 4 Engineering Case Comparison  
### Engineering Measurements and Numerical Simulations  
Macau new port area sewage pipe new construction project: shield machine for the soil pressure balance shield machine, body length of 7.8 m, excavation radius of 3.38 m, pipe sheet width of 1 m, shield machine opening rate \(\xi=38\%\), the number of cutter plate chunks \(m=6\), the mass of about 240 t. The tunnel under the Lotus Square pedestrian tunnel is the object of study. Figure 6 illustrates the relative positional relationship between the two tunnels as well as the distribution of the strata in the area. The existing pedestrian tunnel is a single-width box culvert beneath the tunnel, with a cross-section size of \(7\times 4\) m, a top plate depth of 3 m, and a clear distance of 4.2 m between the top of the new tunnel and the bottom plate of the tunnel. Figure 5: Calculation flow chart. The shield tunnel mostly travels through the layer of powdery clay. The following characteristics are taken into consideration: the soil layer's average heaviness is 19.5 kN/m\({}^{3}\), its average elastic modulus \(E\) is 23.1 MPa, its Poisson's ratio is 0.3, and its angle of internal friction is 27.5\({}^{\circ}\). According to the parameters of the engineering case and the relevant formula above, the cutter plate thrust is 80 kPa, the friction force is 62 kPa, the synchronous grouting pressure is 0.15 MPa, the soil loss rate is 0.48%, and the equivalent bending stiffness \((EI)_{\text{eq}}\) = 7.786 \(\times\) 10\({}^{7}\) kN\(\cdot\)m\({}^{2}\). As illustrated in Figure 7, standard monitoring points were positioned in the ground surface and the passage structure in accordance with the relative positions of two tunnels in order to comprehend the ground and pedestrian passage deformation data over time and to gather dynamic information pertaining to the shield construction. Seven building settlement monitoring points are positioned along the longitudinal direction of the passageway at the axial line, numbered SB112-SB118 from bottom to top, and four surface settlement monitoring points, numbered SM76-SM79, are positioned along the shield tunneling direction. On-site monitoring is carried out by total station for fourth-class level measurement with a monitoring accuracy of 1 mm, and the monitoring frequency is 2 times/day within 20 m before and after the arrival of the shield and 1 time/day within 20 m-40 m before and after the arrival of the shield. Through numerical analysis, the construction process of the shield tunnel is simulated. The dimensions of the model are 45 m in length, 80 m in breadth, and 30 m in height. The existing tunnel is situated in the center of model. The shield machine starts at 40 m behind the existing tunnel and stops after boring to 40 m in front of the existing tunnel. The tunnel is excavated for 2 m per ring, and the gap between the tube sheet and the shield shell is replaced by an iso-surrogate layer. The excavation steps are as follows: first, the soil  
Figure 6: Diagrammatic representation of the project model. Figure 7: Arrangement of measurement points through the section. unit is to be excavated; then, the shield shell unit, the cutter thrust, and the shield shell friction are to be activated; finally, the tube sheet unit, the iso-surrogate layer unit, and the grouting pressure are to be activated. The model's boundary conditions are limited normal displacements on the left, right, front, and rear surfaces, free at the upper surface, and fixed at the bottom. Figure 8 displays the mesh model. The key physicochemical properties of each soil layer and supporting structure are given in Table 1.  
The settlement deformation of the old tunnel is retrieved and examined after the newly constructed tunnel is bored to a location of \(x=10\) m to confirm the accuracy of the theoretical formula findings. Figure 9 displays the results of the theoretical calculation, the numerical simulation, the on-site monitoring, and the comparison of the theoretical calculation findings from the literature [18].