# Analysis of Shield Tunnels Undercrossing an Existing Building and Tunnel Reinforcement Measures
## 9 Conclusions  
Taking the double shield tunnels of Changsha Metro Line 6 as an engineering background, this paper considered the building with a double basement and pile foundations and established the model for the shield tunnel to penetrate the existing building. Using the established model, the effects of the successive excavation, the parameter of grouting pressure, and the grouting reinforcement technology of shield tunnel on the mechanical behaviors of soil, segment, and building were researched. The conclusions were as follows. 1. The successive excavation of double shield tunnels led to the subsidence and uplift of the soils above and below the excavation face, respectively, and the outward lateral displacements of the soil on both sides. Grouting pressure had a significant impact on soil displacement. According to the calculation, the maximum subsidence value of the soil mass would decrease by 3.512 mm when the grouting pressure increased by 100 kPa. The subsidence of the soil above the tunnels was distributed in an \"M\" shape with the distance from the centerline because the soil at the midpoint of the connection between the two tunnels was compressed. 2. Due to the gravity effect of the upper building and the squeezing effect of the soil, the successive excavation of double shield tunnels caused the elliptical deformation of segments, and the maximum subsidence value of 7.19 mm, the maximum bulge value of 3.34 mm, and the maximum lateral displacement of 4.27 mm the segment were located at the vault, upset and waist, respectively. The subsidence of the segment of double shield tunnels was the most significant after the completion of the successive excavation, and the subsidence of arch segments should be monitored in real-time during excavation. 3. The grouting pressure had a nonlinear relationship with the segment bending moment and the segment circumferential axial force. The maximum positive bending moment and the maximum negative bending moment of the segments were located near the tunnel arch crown and arch bottom, respectively, and the maximum axial force and the minimum axial force of the segments were located near the arch waist and arch crown, respectively. When the grouting pressure was in the range of 50-250 kPa, the maximum longitudinal and transverse stress basically increased linearly with the increase of grouting pressure. The maximum longitudinal and transverse stress increased linearly with the change of grouting pressure when the grouting pressure was in the range of 250-550 kPa. 4. When the double shield tunnels passed through the building, tunnel excavation had a significant nonlinear impact on the lateral displacement of the pile along the excavation direction and the subsidence of the building but little impact on the tensile stress of the basement and the shear force of the pile. The increase of the grouting pressure can effectively reduce building subsidence and the lateral displacement of the pile. For every increase in grouting pressure of 100 kPa, the subsidence of the building decreased by approximately 2.138 mm, and the longitudinal displacement of the pile decreased by approximately 0.548 mm. (5) Increasing the radius of radial reinforcement within a certain range and adjusting the range of reinforcement before and after the building can effectively reduce the lateral displacement of the pile and the subsidence of the building. In this project, if the deep hole grouting reinforcement measures were taken to reduce the impact of shield tunnel construction and excavation on the building and piles, the radius of radial grouting reinforcement can be set as 2-3 m, and the reinforcement range before and after the buildings can be set as approximately two to six rings. Nowadays, with the rapid development of subway transportation, more and more subway tunnels are penetrating existing buildings.