Abstract : This study investigates and analyzes the causes and characteristics of structural deformation during the construction and operation of urban rail transit lines in several cities. It summarizes deformation patterns, predicts deformation trends, identifies prominent deformation areas and key monitoring points, providing guidance for subway operation monitoring and aiming to raise awareness among subway operators regarding the importance of structural deformation monitoring. Conclusion : Structural deformation is a common problem in urban rail transit projects during operation. Monitoring structural deformation is necessary to dynamically understand its status. Selecting representative areas for settlement, horizontal displacement, and convergence monitoring, and taking timely remedial measures for areas with significant deformation, are crucial for ensuring operational safety and should be given sufficient attention by relevant departments. Keywords : Urban rail transit; operation; main structure; deformation monitoring; necessity Currently, many urban rail transit lines (referred to as "urban rail" or "subway") have been built and are in operation in China. Through investigations and research on some operating lines in cities such as Tianjin, Shanghai, and Beijing, it has been found that during the construction and operation periods, deformation occurs in the main structures, including tunnels, viaducts, U-shaped structures, and roadbed retaining walls, leading to line settlement and track deformation, which in severe cases affects the operational safety of the urban rail. To promptly grasp the deformation of the subway's main structure and eliminate potential safety hazards, appropriate deformation monitoring of the main structure during operation is essential. Based on the deformation monitoring results, timely remediation plans can be proposed to ensure the operational safety of the urban rail. 1. Necessity of Main Structure Monitoring for Subways 1.1 Changes in Subway Structure Due to Ground Subsidence The changes in the main structure of urban rail during construction are mainly due to ground subsidence. For example, Tianjin is located in an alluvial plain area with local soft soil layers and seismic liquefaction layers, resulting in significant overall settlement. The Tianjin Metro Line 1 project employs open-cut or shield tunneling methods for its newly constructed underground sections. This presents challenges: construction of the retaining structure; changes in groundwater levels due to dewatering; and unloading of the foundation soil during excavation, leading to pit bottom settlement. The main structure construction and backfilling will re-load the foundation soil, causing further settlement. After the main structure is completed, changes in the groundwater level will generate buoyancy, reducing the tendency for structural settlement; however, excessive buoyancy can cause the structure to float. Urban rail transit structures themselves experience structural deformation and settlement due to foundation deformation, internal stress, and changes in external loads. If structural deformation and settlement exceed allowable values, it will affect the operation of the urban rail transit system, potentially causing operational interruptions. Monitoring the structure, understanding deformation, analyzing its causes, and taking effective measures are crucial for preventing accidents and ensuring the normal operation of the urban rail transit system. During construction, in addition to monitoring the ground surface during pit excavation and main structure construction, monitoring of surrounding buildings and groundwater level changes is also necessary. Practice has shown that these monitored items all exhibit uneven settlement changes. For example, in Tianjin, since 1923, with the development of groundwater, land subsidence has been ongoing, initially only a few millimeters per year. After liberation, with the development of industry and agriculture, groundwater extraction gradually increased, and land subsidence became increasingly severe. The maximum cumulative subsidence value reached 2.85m between 1959 and 2000. Along the Tianjin Metro line, the area passing through Xiaowangzhuang Jingjin Bridge in Hebei District, with a cumulative subsidence of 2.0–2.5m, has reached 37 km². In summary, monitoring the main structure of the metro during construction and operation is essential. 1.2 Differences in Structural Deformation Due to Different Line Laying Methods Urban rail projects are linearly distributed over a long range. Due to different line laying methods, uneven settlement may exist throughout the entire project area. A typical urban rail line includes different laying methods such as underground lines, elevated lines, and ground lines, as shown in Figure 1. The underground structures include circular tunnels constructed using shield tunneling, rectangular structures constructed using open-cut methods, and horseshoe-shaped structures constructed using cut-and-cover methods. The elevated sections feature various structural forms such as continuous beams, simply supported beams, and steel-concrete composite beams. The ground-level track subgrade involves filling and excavation. The deformation of these different structural forms is complex, and there are differences in deformation among the various substructures. It is necessary to promptly understand the deformation of all parts of the entire line, especially at the joints. 1.3 Differences in Structural Deformation Between Existing Lines and New Projects Some subway projects involve the renovation of existing lines. With the rapid development of urban scale and economic level, the subways built in the early stages can no longer meet the needs of modern life and need to be renovated in terms of scale and standards. For example, the seven existing stations from West Station to Xinhua Road on Tianjin Metro Line 1 were renovated and expanded using existing subway tunnels. The existing section was initially built in 1970 as the "7047" civil defense project and opened to traffic in 1984, having been in operation for 20 years. The reconstruction project made full use of the existing section structure, only demolishing and reconstructing the stations. Each reconstructed station is connected to the new structure at both ends, and there are differential settlement issues between the old and new structures at the connection points. The amount of differential settlement will inevitably affect the stability of the structure and may even affect the normal operation of the subway line. Therefore, it is necessary to monitor the deformation of the subway structure and track structure during operation, to understand the trend of structural changes in a timely and accurate manner, and to take necessary remedial measures to ensure the normal operation of the subway. The Harbin subway under construction utilizes an existing civil defense project, and deformation monitoring should also be emphasized during construction. 1.4 Subway operation can induce structural deformation. The repeated vibrations and unbalanced centrifugal forces on curves during subway operation can induce deformation of the tunnel body and changes in the properties of the surrounding soil, which is also an important reason for subway operation monitoring. 1.5 Changes in the surrounding environment of the subway can also cause structural deformation. The subway line passes through many bustling urban areas, and some high-rise commercial buildings are under construction or about to be constructed. These buildings close to the subway are very likely to cause deformation of the subway structure during construction. Therefore, it is essential to develop a suitable monitoring plan for the subway before the commencement of surrounding projects, and to conduct full- process deformation monitoring of the subway as the surrounding projects are under construction. 2 Key Points of Subway Main Structure Monitoring 2.1 To grasp the overall situation of subway structural deformation at any time through monitoring Monitoring can dynamically collect information on subway structural deformation, grasp the situation of structural deformation, ensure operational safety, and ensure the reliability of the project. The deformation caused by different underground structures and elevated bridge structures is not the same. Through monitoring, the correctness and reliability of settlement deformation theory can be verified, the actual stress state of the structure can be understood, and the safe bearing capacity and service conditions of the structure can be judged. Various technical data are collected through the monitoring system, and a database is established to better grasp the overall situation of structural deformation at any time. The current deformation status and development trend can be discovered in a timely manner, and contingency plans for handling measures can be taken. 2.2 To attach importance to accumulating monitoring data and provide reliable basis for disease treatment With the continuous development of cities, the scale of construction of subway and other rail transit will inevitably continue to expand and will become an important pillar of urban public transportation. The safe operation of urban rail transit has become a window image of the city. Through monitoring the main structure of the subway, collecting monitoring data, recording treatment plans, systematically organizing and accumulating data, it is possible to grasp the operational deformation of existing completed subway projects in a timely manner. By monitoring the main structure, we can promptly and accurately grasp the operational deformation of existing subway projects, continuously summarize relevant experiences and lessons learned, provide a reliable basis for disease control, and also serve as a reference for future related engineering design, construction, operation and maintenance. 2.3 Key Monitoring Locations Based on theoretical analysis and past experience, the following main parts of the subway are generally monitored to understand the structural deformation of key locations: (1) Differential settlement at the connection between the station and the section; (2) Special settlement of tunnel sections where urban rail transit crosses rivers or areas with poor geological conditions; (3) Differential settlement at the connection between existing tunnels and newly built tunnels; (4) Differential settlement at the connection near the connecting passage between sections; (5) The impact of sections with tall buildings or projects under construction along the urban rail transit line on the tunnel; (6) The impact of the section near the intersection of this line and the urban rail transit line constructed later on the tunnel of this line; (7) Settlement of piers and abutments and flexural deformation of beams in the elevated bridge section; (8) Differential settlement of the transition section between the tunnel, the elevated bridge and the roadbed; (9) The impact of urban rail transit crossing the existing national railway on the tunnel. 2.4 Support and Analysis of Ground Settlement Theory For ground settlement of the main structure during and after the construction of urban rail transit, commonly used theories such as the Peck method, finite element method, and Peck modified formula are generally adopted to estimate the surface settlement. The Peck method assumes that ground loss is uniformly distributed along the tunnel length and that ground settlement is normally distributed in the direction perpendicular to the tunnel. An estimation formula is proposed for the ground settlement of the transversely distributed surface settlement trough above the tunnel. The calculation results should be modified according to the specific geological conditions and soil characteristics of the project, and verified by monitoring. 2.5 Theoretical Basis for Calculating Foundation Deformation of Important Buildings For the impact of the construction of important buildings attached to the subway and adjacent tall buildings on the main structure of the subway, it is necessary to first understand the stress changes caused by the building load in the foundation soil layer, and secondly, to understand the distribution of the foundation soil layer and its stress-strain relationship characteristics. This allows for the pre-calculation of the deformation values ​​that will occur. For buildings, the most important factor under normal circumstances is the vertical compressive deformation of the foundation, which manifests as the settlement of the building foundation. Therefore, foundation deformation calculation usually refers to foundation settlement calculation. From the start of subway construction, newly constructed buildings within the subway protection zone must be monitored until they are deemed stable, or their deformation values ​​and rates are within normal ranges. On the one hand, the deformation of the building's foundation pit retaining structure must be monitored; on the other hand, the subway structure in adjacent building areas should be closely monitored. Appropriate monitoring schemes should be adopted based on the project conditions and deformation, and if necessary, on-site probes and sensors should be installed, data transmitted via fiber optic cables, and remote monitoring should be conducted in real time. 3. Survey Results According to the survey, many operational rail transit lines have varying degrees of structural deformation. Surveys of operational rail transit lines in cities such as Shenzhen, Guangzhou, Nanjing, Shanghai, and Tianjin revealed that some lines have severe deformation, especially Shanghai's operational Lines 1, 2, and 3, which have experienced significant and ongoing deformation. In particular, on Shanghai Line 1, the tunnel subsidence reached 27 mm in some sections, and the tunnel cross-section deformation reached 11 mm. Moreover, the deformation continues to develop, and various measures are being taken to ensure operational safety. During the construction of the Tianjin Metro Line 1 project, the maximum settlement in the river-crossing section at the West Station reached 480 mm, indicating severe encroachment. Adjusting the track alignment could not fundamentally solve the problem. After research, the track was adjusted to achieve a more uniform encroachment, and a special track design was adopted to reduce the thickness of the track bed, thus solving the problem and avoiding the need for demolition and reconstruction. Currently, some urban rail transit projects in China have been completed and opened to traffic. Some cities have monitored the structural deformation of these projects, while others have not yet given it sufficient attention, especially missing the initial data collection opportunity, which is extremely detrimental to the analysis and processing of subsequent deformation monitoring data. Relevant national standards also have clear provisions for such major projects. The "Code for Measurement of Building Deformation" (JGJ/T8—97) and the "Code for Measurement of Underground Railway and Rail Transit Engineering" (GB 50308—1999) both contain explicit clauses stipulating that deformation measurement is essential throughout the entire construction and operation phases of metro projects. After analyzing the monitoring data and assessing that the deformation has stabilized, observation can be discontinued. 4 Conclusions It is recommended that during the construction period and after the completion and operation of subway and other urban rail projects, the monitoring of the settlement, horizontal displacement and convergence of the main structure should be put on the agenda, and the data should be sorted out and accumulated. The deformation of the main structure and the future development trend of the deformation of the main structure should be given, and a rectification plan should be proposed. 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