# Experimental Investigations of a Tunnel Lining Segment Strengthened by In Situ Spraying Mortar
## 1 Introduction  
In recent years, with the rapid development of China's economy, the population has gradually concentrated in large- and medium-sized cities, and the problem of population density has gradually become the primary problem that needs to be solved in major cities, which has further promoted the development and utilization of underground space. In particular, the construction of subways has become the focus of the development of major cities. With the continuous development of underground space technology, there are more and more urban subways. Considering the particularity of urban subway construction, it is necessary to minimize the impact on the surrounding environment, so shield tunneling is often used. At present, China has become the fastest-developing country in in the world in terms of urban rail transit. During the construction or operation of subway shield tunnels, considerable deformation of tunnel lining structure is caused by surcharge on the top of the tunnel [1]. Considerable deformation of the tunnel causes many structural issues, such as concrete spalling, cracks, leakage and so on, which seriously endanger the safe operation of thesubway shield tunnel. For shield tunnels with serious issues, there are two methods to improve the stress condition of the tunnel. One is to strengthen the stratum externally, and the other is to strengthen the lining structure internally, including enhancing the structural bearing capacity, controlling the structural deformation, controlling the development of cracks and preventing seepage and plugging [2]. The external strengthening method involves strengthen the tunnel lining structure by grouting outside the tunnel. This method effectively reduces the lateral deformation of the tunnel and solves the problem of water leakage of tunnel segments and has been applied in Shenzhen and Shanghai subway tunnels [3; 4]. However, the process of this method is complex, and there are many uncertainties in construction. Internal strengthening methods mainly refer to improving the performance of the lining structure by adding reinforcement layers on the inner surface of the lining structure to form a composite structural system. This strategy an easy method to form a standardized reinforcement technology, and the influencing factors are relatively few, mainly affected by reinforcement materials. Internal tunnel-strengthening methods include strengthening by bonding fiber cloth, internal steel tension rings, and composite cavities. The advantages of strengthening by bonding fiber cloth include lightweight reinforcement materials and convenient construction. However, its fire resistance is very poor, and the control of structural deformation and an anti-seepage plugging effect are not obvious [5; 6; 7]. Strengthening with internal steel tension rings effectively improves the stiffness of the structure, which is conducive to controlling structural deformation. However, the installation of steel plates requires special lifting equipment, with many welds and large fluctuation of welding quality [8]. The composite cavity strengthening method makes up for the deficiency of steel-ring reinforcement in construction, but the structure shows brittle failure, poor fire resistance, and does not have the effect of anti-seepage and plugging after reinforcement [9; 10]. The in situ spraying method is a repair method by which high-performance composite mortar material is uniformly coated on the surface of the structure to form a lining by pressure spraying. An American sewer operator found that this method can structurally repair concrete pipes and has the advantage of fast construction speed [11]. The in situ spraying method has been widely used to renew riveted steel pipes since the early 1900s [12]. With the continuous progress and development of materials and technology, \"in situ cement mortar spraying\" technology has become a mature method of sewer pipeline repair.