Abstract: This paper focuses on the research and application of tunnel boring machines (TBMs) for rectangular underground passages in urban transportation by the Shanghai Tunnel Construction Technology Research Institute. This information is intended to provide a reference for the research, design, and construction of tunnels with irregular cross-sections. Keywords: Urban subway, rectangular underground passage, TBM. With the rapid pace of urban development, transportation plays an increasingly prominent role in urban construction. The advancement of development and construction demands more underground pedestrian passages in cities, such as pedestrian tunnels at subway station entrances and exits, and underground utility tunnels for urban pipelines. Rectangular tunnels are the most economical type of underground tunnel. Therefore, the research and application of TBMs for rectangular underground passages in urban transportation are essential. 1. Development and Application of Rectangular Tunnels The world's earliest shield tunnel was the London Underground Tunnel, which began construction in 1826. Its cross-section was 11.4m x 6.8m rectangular. Due to manual excavation and a water inrush accident during construction, the 458m long rectangular tunnel took 18 years to complete. Since the 1970s, with economic development, shield tunneling machine construction technology has made new leaps. In particular, Japan's demand for underground space development and utilization spurred the further development of shield tunneling technology. Following the steel mill era of the 1980s, a global surge in the development of irregularly shaped shield tunneling machines ensued, with experimental research and engineering applications of rectangular, elliptical, double-circular, and multi-circular tunnel shield tunneling machines and construction technologies. From the perspective of tunnel function, rectangular cross-sections are most suitable and economical for urban pedestrian underpasses, underground utility tunnels, and subway tunnels. Therefore, the re-research, development, and application of rectangular shield tunneling machines are of great significance. Japan has conducted research on large-section rectangular shield tunneling methods, primarily addressing the construction of vehicular underpasses crossing railways. This involves assembling steel segments and then pouring concrete lining, achieving a minimum overburden depth of only 3 meters. In 1981, Nagoya and Tokyo used 4.29m x 3.09m hand-dug rectangular shield tunnels to excavate two utility tunnels, 534m and 298m long respectively. Nagoya also used a 5.23m x 4.38m hand-dug rectangular shield tunneling machine to excavate a 374m long rectangular tunnel. In short, the application of rectangular tunnels and rectangular shield tunneling technology is booming, its advantages are becoming increasingly apparent, and its technology is maturing. The Shanghai Tunnel Construction Technology Research Institute began research on rectangular tunnels in 1995, and through a research project, completed the design of a 2.5m x 2.5m variable grid rectangular pipe jacking machine, a rectangular tunnel test engineering scheme, and engineering design in 1995. In April 1999, the construction of the rectangular passage at Exit 5 of Shanghai Metro Line 3 adopted a 3.8m x 3.8m rectangular cutterhead earth pressure balance pipe jacking machine independently developed by the Shanghai Tunnel Construction Technology Research Institute. Rectangular tunneling began in mid-April, and the second rectangular tunnel project was completed in early June. The project quality was excellent, ensuring the normal operation of the Shanghai Yan'an East Road Tunnel and the safety of underground pipelines along Lujiazui Road. The first domestic rectangular shield tunnel construction completed the advancement of two tunnels in just 40 days, demonstrating the success of the research and application of rectangular tunnels. 2. Research on Tunnel Boring Machines for Rectangular Underground Passages in Urban Transportation 2.1 The Economic Advantages of Rectangular Tunnels: Compared to circular cross-sections, rectangular tunnels offer over 20% more usable area. Urban pedestrian underpasses require shallow burial depths, making rectangular tunnels more suitable for their construction. With the increasing use of urban pedestrian underpasses as subway station entrances and exits, and the development of underground utility tunnels in my country, rectangular tunnels are the most economical option for such projects. Therefore, the research and application of rectangular tunnels directly serve the needs of engineering construction and have broad application prospects. 2.2 Research Methods for Rectangular Tunnels The feasibility study and technical route for rectangular tunnels are as follows: (1) Digestion and absorption of foreign rectangular shield tunneling and rectangular tunnel engineering; (2) Conception and design of rectangular pipe jacking test projects; (3) Technical and economic comparison of rectangular pipe jacking machine models, model design and selection; (4) Development of test rectangular pipe jacking machines, and development of engineering rectangular pipe jacking machines based on the test machines; (5) Understanding the stress distribution of rectangular reinforced concrete pipe sections through structural tests, and providing a basis for improving pipe section design and optimizing section design; (6) Understanding the construction parameters and mastering the laws of rectangular tunnel jacking through a 2.5m x 2.5m rectangular tunnel test project, and providing a basis for engineering application; (7) Designing engineering application schemes and construction designs, completing engineering applications, and conducting research on construction methods. 2.3 Development of Rectangular Pipe Jacking Machines Since variable grid rectangular pipe jacking machines have the advantages of relatively simple processing, low cost, and quick start-up, and can also obtain valuable data in experiments, this scheme was selected. The rectangular grid-type pipe jacking machine utilizes a grid to cut the soil and effectively prevent soil collapse at the excavation face, allowing for manual excavation. The main jacking unit propels the machine head forward. The machine head is divided into front and rear sections connected by a correction cylinder. Correction devices are installed on both sides of the casing, and variable-angle cutters are installed at the cutting ring to allow for a certain amount of over-excavation, which is beneficial for controlling the machine head's attitude and ensuring that the tunnel axis deviation remains within the design range. The grid contains four variable grids, which can adjust the amount of soil entering the machine head, thus helping to control the stability of the soil at the face. To ensure a certain gap between the pipe section and the soil, facilitating the formation of a mud ring, the cross-sectional dimensions of the machine head are designed to be larger than those of the pipe section. The main body of the pipe jacking machine can be divided into front and rear sections connected by a correction cylinder. The seal between the front and rear sections uses a lip seal and a supporting rubber ring, and variable-angle cutters are installed at the cutting ring. The grid contains variable grids with adjustable opening ratios, and correction devices are installed on both sides of the casing. The aforementioned device controls the attitude of the machine head. The main jacking device consists of 8 hydraulic cylinders, a U-shaped jacking iron, a jacking ring, shims, a base frame, and a steel backrest. The 8 hydraulic cylinders are divided into two groups of 4, stacked symmetrically and fixed with a split-structure support. The working stroke is 1450mm. Each hydraulic cylinder can be controlled independently. The correction device is mainly used to control the left-right and up-down axis deviation of the machine head. The total correction force is 752t, and the correction angle is ±2 degrees. The grouting and correction system (finned plates + grouting) is mainly used for correction after the machine head rotates, with a correction torque of 210x2-420kN. 2.4 Rectangular Tunnel Engineering Test Project Overview: The test project is located in Hangtou area, Nanhui County, Shanghai, with a jacking distance of 60m and a soil cover depth of 6.45m. There is a small river 10m north of the jacking axis and a reinforced concrete main road 10m south of it (see Figure 1). The soil layers traversed by the pipe jacking machine are: gray silty clay and gray silty clay silt in the entry and exit sections; and gray silty clay and gray sandy silt in the intermediate sections. Through the engineering test, the design selection of the rectangular pipe jacking machine, the selection of rectangular pipe sections, the joint type, and the design selection of the waterstop were verified. The rectangular pipe jacking construction method was studied by collecting various construction parameters and working condition records. The engineering test completed the experimental research on the key technologies of the rectangular pipe jacking machine, collected first-hand data and information, and accumulated practical construction experience in rectangular section tunnel excavation. 3. Application of rectangles in underground pedestrian passages of urban subways In February 1998, the research team proposed a rectangular pipe jacking construction scheme for the No. 5 entrance/exit tunnel of Lujiazui Station on Shanghai Metro Line 2. The No. 2 entrance/exit passage of Lujiazui Station on Shanghai Metro Line 2 required the construction of two 62m rectangular tunnels, each with an internal net dimension of 3m x 3m. 3.1 Development of the Combined Cutterhead Earth Pressure Balance Pipe Jacking Machine: The 3.8m x 3.8m combined cutterhead earth pressure balance pipe jacking machine was developed based on the successful development and testing of the 2.5m x 2.5m rectangular pipe jacking machine, specifically for the No. 5 entrance/exit underground passage project of Lujiazui Station on Shanghai Metro Line 2. The selection of the rectangular underground passage tunnel boring machine was based on the project conditions. Through scheme comparison, considering the advantages of a large cutterhead plus contour cutter, such as compact structure, high reliability, and simple operation, it was unanimously agreed that a full-section cutting earth pressure balance pipe jacking machine should be used for construction. The combined cutterhead earth pressure balance pipe jacking machine uses a large cutterhead and contour cutter to cut the soil. It also blocks the soil at the excavation face, effectively preventing soil collapse. The earth pressure in the soil chamber is controlled by adjusting the speed of the screw conveyor and the jacking speed to maintain the stability of the excavation face. To ensure a certain gap between the pipe section and the soil, which is beneficial for the mud to form a ring, the cross-sectional dimensions of the machine head are designed to be larger than those of the pipe section. (The outer cross-sectional dimensions of the machine head are 3.828m x 3.828m, and the outer cross-sectional dimensions of the pipe section are 3.8m x 3.8m). The combined cutterhead type earth pressure balance rectangular pipe jacking machine features a main unit that can be divided into front and rear sections, connected by 16 hydraulic cylinders. The seal between the front and rear sections uses two lip-shaped rubber sealing rings. The front face is cut across the entire section of the soil using a large cutterhead and four contour cutters. Soil is discharged by a screw conveyor; adjusting the speed of the screw conveyor maintains the earth pressure balance within the soil chamber, thus maintaining the stability of the excavation face. The problem of full-section cutting in rectangular pipe jacking machines: If a rectangular pipe jacking machine only has one large cutterhead for rotary cutting, it can only achieve a section cutting rate of about 90%, leaving the four corners of the rectangular pipe section uncut. Considering the complex geological conditions, pipelines, environmental protection, and the need for the jacking head to pass through the SMW reinforcement layer when entering and exiting the tunnel in the Lujiazui area, a large cutterhead is used to cut most of the front soil, while contour cutters located behind the cutterhead cut the soil at the four corners. The problem of jacking head rotation in rectangular pipe jacking machines: To address the rotation phenomenon of the rectangular pipe jacking machine head, grouting correction technology is used, with the disc rotating clockwise or counterclockwise to achieve correction. The method for controlling the axial deviation of the rectangular pipe jacking machine head: Based on the direction and amount of axial deviation, the correction cylinders are grouped and their extension and retraction controlled to create an angle between the front and rear housings, thereby changing the direction of the jacking head to achieve the correction purpose. In addition, grouting correction can also be used to achieve the correction purpose, or a combination of both methods can be used for correction. 3.3 Rectangular Tunnel Construction Project: The Shanghai Metro Line 2 Lujiazui Exit 5 Pipe Jacking Project is located in the Lujiazui Financial and Trade Center Area of ​​Pudong. Exit 5 serves as the starting shaft, and Exit 4 as the receiving shaft, located on the north and south sides of the approach road section of the Yan'an East Road Tunnel. The tunnel consists of parallel pipes, each 62.25m long, with a net distance of 2.2m between the two pipes. The slope of both pipes is 0.2%, and the average overburden thickness is approximately 5.3m. The entire tunnel structure utilizes precast rectangular reinforced concrete pipe sections. The pipe section dimensions are 3800 x 3800 mm, with a wall thickness of 40cm and a length of 2m. A total of 64 pipe sections were used in the project. Controlling the jacking axis is a major challenge in rectangular pipe jacking. During normal jacking construction, close attention must be paid to controlling the jacking axis. After each pipe section is jacked, the attitude of the jacking machine must be measured and corrected as needed, with the correction amount not being too large to avoid significant soil disturbance and angular separation between pipe sections. Environmental protection and settlement control are crucial because the project will traverse Lujiazui Road, the Pudong approach road of the Yan'an East Road Tunnel, and various pipelines including water pipes, gas pipes, rainwater pipes, sewage pipes, telephone lines, and power lines. The net distance between the top of the pipe and the bottom of the 450mm sewage pipe, the 1000mm under-pipe, and the 800mm rainwater pipe is 1m, and the net distance between the top of the pipe and the bottom of the structure of the Yan'an East Road Tunnel approach road is 1.564m (see Figure 5). Therefore, ground settlement control and environmental protection during the jacking process are extremely important. When the pipe jacking method causes surface settlement around the tunnel, a contour cutter device is used: the soil in the four dead corners of the rectangular pipe jacking machine is cut, combined with the large cutterhead to fully cut the soil in front. Settlement monitoring is implemented, data feedback is collected, construction parameters are adjusted, and information-based construction is adopted. Ground settlement has been controlled and environmental protection has been achieved. However, the settlement of the approach section of the Yan'an East Road Tunnel in this project is to be controlled between +10mm and -30mm. Therefore, it is necessary to take measures to control the settlement and ensure the safety of the approach section of the tunnel under specific conditions. Control technology in the jacking of rectangular pipe jacking machine (1) Strictly control the construction parameters of the pipe jacking to prevent over-excavation; (2) Strictly control the amount of correction of the pipe jacking, and implement the principle of "frequent correction and slow correction" to control the jacking line throughout the entire jacking process; (3) The jacking speed should not be too fast, and the construction should be balanced as much as possible. The jacking speed should be controlled at about 15mm/min. (4) During the jacking construction, continuous and uniform grouting must be ensured so that the resulting building gaps can be filled quickly and the stability of the soil above the pipe jacking pipe can be ensured. (5) To overcome the "backing soil" phenomenon, in addition to injecting thixotropic mud at the machine head to avoid the "backing soil" phenomenon, it is also necessary to grout the upper part of the exit section of the pipe section during the jacking process to fill the construction gaps caused by the "backing soil" of the pipe section at any time. (6) Monitor and control the "sinking" or "floating" of the existing pipeline behind the machine head. When the pipeline sinks seriously, the bottom of the sinking part should be grouted to prevent ground subsidence. 4 Conclusion Rectangular shield tunnels are a novel shield tunnel technology developed and applied abroad in the 1990s. Due to its advantages such as large cross-sectional utilization, shallow overburden, and low construction cost, this technology can be used for urban traffic pedestrian tunnels, vehicular tunnels, underground pipeline trenches, water diversion and drainage pipeline projects. The successful rectangular tunnel engineering test marks the substantial launch of this technology application in my country. It fills the gap in my country's rectangular pipe jacking construction technology, provides design basis and construction experience for the engineering application of this technology, and will surely have broad application prospects.