# Effect of Soil Reinforcement on Tunnel Deformation as a Result of Stress Relief
## 1 Introduction  
In congested urban areas, the metro plays an important role in the transportation system. The safety and serviceability of existing metro tunnels are always under serious consideration. Owing to the rapid development of underground space in urban areas, the commercial demand for the construction of underground structures in close proximity to metro tunnel lines has been increasing. However, adjacent geotechnical engineering activities, such as deep excavation, may have adverse effects on or even cause damage to nearby existing tunnels. If the induced tunnel deformation and internal forces exceed the capacity of the tunnel structures, segment cracking, leakage, and even longitudinal distortion of the railway track may occur and seriously threaten the smooth travel and safety of the trains in operation. Many studies have investigated the effects of adjacent excavation on existing shield tunnels using various methods, including in situ monitoring [1; 2; 3], centrifuge model tests [4; 5; 6], numerical analysis [7; 8; 9; 10; 11; 12; 13; 14; 15], and semi-analytical methods [16; 17; 18; 19]. For example, the main objects of investigation have been the excavation dimension [13; 15], relative distance between the tunnel and excavation [4; 8; 9; 11; 13], construction and reinforcement methods [10; 12], tunnel dimension and physical parameters [9; 13], soil density and wall stiffness [6], different constitutive models [14], and influence zone [20].  
[PERSON] et al. [2] published an extensive report regarding the damage case history of a shield tunnel as the result of an adjacent deep excavation. In this case history, cracks in segmental linings and the distortion of connected blots were observed. Therefore, it is a major challenge for city designers and geotechnical engineers to evaluate and control shield tunnel responses associated with adjacent excavations. Although various construction and reinforcement methods of controlling tunnel deformation have been proposed by [PERSON] et al. [10] and [PERSON] et al. [12], the existing quantitative research on the evaluation of the bottom reinforcement is insufficient. Moreover, the effect of the soil reinforcement's mechanism on the tunnel deformation has not been clarified.  
On the basis of a centrifugal model test reported by [PERSON] et al. [4], the finite element simulation of the studied problem without ground reinforcement has already been done. Ground reinforcement before excavation may be a fresh approach to reduce tunnel heave as a result of stress relief. However, its influence law and deformation mechanism are not clear yet. On the basis of the centrifuge model test without ground reinforcement and the calibrated constitutive models reported by [PERSON] et al. [14], a finite element analysis was conducted to investigate the effect of ground reinforcement on tunnel deformation, both in the longitudinal and in the transverse directions. The effect of the reinforced soil's Young's modulus and the effect of the reinforcement depth under the basement bottom were investigated. The effect of the basement excavation on the deformation of an existing tunnel was analyzed. It is expected that this study may deepen the understanding of tunnel deformation control methods and provide an effective method to control tunnel deformation due to excavation. This study will help in taking effective measures to ensure the safety and serviceability of existing metro tunnels during adjacent excavation for engineering applications.