Basic Principles
　　The immersed tube method, also known as the precast tube segment immersion method, simply put, is to first precast large concrete box-shaped components or composite box-shaped components of concrete and steel in a dry dock or on a slipway, and close both ends with temporary bulkheads, outfit them with equipment for towing and positioning, then float and sink these components into a pre-dredged trench on the riverbed and connect them, and finally backfill with sand and gravel and remove the bulkheads to form a tunnel.
　　According to the 1993 report of the Immersed and Floating Tunnels Working Group of the International Tunnelling Association (ITA), a currently widely accepted definition of an immersed tunnel is: An immersed tunnel is a tunnel-type carrier connecting land on both ends of a water body, composed of several precast tube segments, each floated to the site, sunk and installed one after another, connected underwater and correctly positioned in an excavated underwater trench, followed by related engineering construction to make these tube segments into a combined body.
　　Construction Technical Characteristics
　　1) The immersed tube construction has strong adaptability to geology and hydrology due to shallow trench excavation depth and small foundation load.
　　2) Compared with shield tunnels with larger burial depth, the road surface elevation of immersed tube tunnels can be raised, making it easier to connect with onshore roads.
　　3) The immersed tube tunnel has longer tube segments, fewer joints, and good waterproof performance.
　　4) Precast tube segments and trench excavation can be carried out simultaneously, with less construction interference and short construction period. 　　Classification of Immersed Tube Tunnels
　　By cross-section: circular and rectangular.
　　By tube segment production method: slipway type and dry dock type.
　　By tube segment material: steel structure and reinforced concrete structure.
　　At the same time, there is a special form of immersed tube tunnel—the submerged floating tunnel, whose special feature is that the immersed tube segments are not buried in the riverbed trench but suspended in water, with the tunnel anchored to seabed anchorages at certain intervals by anchor cables, and the other end of the anchor cables connected to the main tunnel structure through rings fixed on the tunnel. 　　Historical Development

　　1) Foreign Development
　　Origin of the Immersed Tube Method (Early 20th Century to 1930s)
　　The first successful immersed tube construction was the Shirley Drainage Tunnel in Boston, USA, completed in 1894, with a diameter of 2.6 meters and a length of 96 meters, connected by 6 steel-shell brick-lined tube segments.
　　In 1910, the United States used the immersed tube method to build the Detroit River Railway Tunnel, with the underwater section consisting of 10 steel-shell tube segments each 78.2 m long. In 1928, the first underwater road tunnel built by the immersed tube method was the Posey Tunnel between Oakland and Alameda, California, USA, with an underwater section 744 meters long, using 12 tube segments each 62 meters long. It was a reinforced concrete circular structure with an outer diameter of 11.3 meters. The tunnel adopted many important features such as a circular two-lane cross-section, which became a model for later immersed tube methods in the United States. However, starting from the Detroit-Windsor Tunnel built in 1930, steel-shell tube segments were again used, and the cross-sectional shape was changed to octagonal. From 1910 to 1942, the United States built a total of 7 immersed tube tunnels.
　　In 1927, Germany built an underwater pedestrian tunnel with a total length of 120 meters in Berlin using the immersed tube method, with a circular steel shell structure.
　　Development of the Immersed Tube Method (1940s to 1960s)
In 1943, the Netherlands built a road tunnel under the Maas River in Rotterdam using the immersed tube method, with 4 lanes and dedicated bicycle and pedestrian passages. The tube segment cross-section was 24.8×8.4 meters. The tunnel structure was rectangular reinforced concrete tube segments, waterproofed with steel plates outside and protected with concrete anti-corrosion layer. It also solved the joint waterproofing of tube segments, inventing rubber waterstops for initial sealing and rubber waterstops for permanent sealing. Due to the large width of the tube segments, a sand jetting cushion foundation treatment method was created. In Europe, due to the development towards multi-lane cross-sections, this rectangular reinforced concrete tube segment was adopted, laying the foundation for the second generation of immersed tube tunnels.
　　Japan built its first immersed tube tunnel in 1944, with most of the structure being rectangular reinforced concrete tube segments and a small part being circular steel-shell concrete tube segments.
　　Before the 1950s, for steel-shell tube segments, underwater concrete placement was used for underwater connection. After the 1950s, due to the breakthrough in underwater connection technology—the hydraulic coupling method, and the application of rubber gaskets as waterstop joints, the immersed tube method was widely adopted and developed rapidly. In the late 1960s, a third-generation immersed tube tunnel without ventilation ducts and ventilation rooms appeared. As the tube segment cross-section was correspondingly reduced, it was beneficial to improve the construction efficiency of the immersed tube method. The Limfjord Tunnel in Denmark, completed in 1969, was the first application of this type. The immersed tube method is also applied to the construction of subway tunnels. The Rotterdam subway tunnel project in the Netherlands, which started construction in 1960, is an example.
　　Long-distance Development of the Immersed Tube Method (1970s to Present)
　　The Tokyo Port Highway Tunnel in Japan, completed in 1976, has 6 lanes, an immersed tube tunnel length of 1035 m, a cross-section of 37.4 m×8.8 m, 9 tube segments each 115 m long, and a grouting foundation.
　　The Øresund Link between Denmark and Sweden, completed in 2006, has a total length of 16 km, including a 4050 m undersea tunnel on the west side, 38.8 m wide and 8.6 m high, located 10 m below the seabed, consisting of 5 tubes: two railway tracks, two dual-lane highways, and one evacuation passage. It is currently the world's most spacious undersea tunnel; the artificial island in the middle is 4055 m long, connecting the projects on both sides; the east side cross-sea bridge is 7845 m long, with a 4-lane expressway on top and a double-track railway below, with 51 piers, and a cable-stayed bridge in the middle with a span of 490 meters and a height of 55 meters, which is currently the world's largest load-bearing cable-stayed bridge completed.
　　In 2008, the Bosphorus Tunnel in Turkey was completed. The immersed tube tunnel is 1.4 km long, consisting of 11 tube segments each 15.3 m wide, 8.6 m high, and 98.5–135 m long, with a central partition wall. The tube segments are in the form of three sides wrapped with steel plates, concrete poured inside, and a rubber waterproof membrane added on top.
　　The Busan-Geoje Expressway project in South Korea, completed in 2010, includes a 3240 m immersed tube tunnel—one of the world's longest immersed tube tunnels, with a cross-section of 26.46 m×9.97 m, 18 tube segments each 180 m long, and a pre-laid gravel foundation.
　　2) Domestic Development
　　Exploration of the Immersed Tube Method (1960s to 1980s)
　　In the 1960s, Shanghai, China began experimental research on immersed tube tunnels, focusing on trench excavation technology and back-silting problems, tube segment structure design and waterproofing, towing and immersion construction technology and its sinking speed, stability, external waterproofing technology of tube segments, design and construction technology of sand jetting foundation, and the impact of immersed tunnel construction on shipping and waterways. In 1972, Hong Kong built its first cross-harbor immersed tube tunnel. In 1973, it was implemented in the outlet crossing embankment and pipe project of the Jinshan Petrochemical Plant. Through six tube segments with outer dimensions of 6.4 m×3.2 m×30 m, according to the immersion construction process requirements, a scaled test process of a 30 m×10 m×150 m (4-6 lane tunnel) tube segment for a river-crossing tunnel was simulated. The tube segments were produced using the launching layer by the Puxi riverside of the Dapu Road Tunnel as a slipway and launched. After the tube segments were produced and floated, they were towed to the waters of the Jinshan project site in Hangzhou Bay, with a towing distance of up to 140 nautical miles. The immersion construction of the entire outlet project not only completed the outlet crossing embankment pipe but also conducted the entire process experimental research of the river-crossing immersed tunnel, obtaining actual exploration of main design parameters and construction process flow. In 1984, Taiwan's Kaohsiung Port also built an immersed tube road tunnel.
　　Development of the Immersed Tube Method (1990s to Present)
　　In the 1990s, some tunnel engineering technicians from Shanghai participated in the construction of the Guangzhou Pearl River Tunnel and the Ningbo Yongjiang Tunnel, undertaking projects such as external waterproofing of tube segments and foundation treatment. The Hong Kong Eastern Harbour Crossing, completed in 1990, has 6 lanes, an immersed tube tunnel length of 1859 m, a cross-section of 35 m×9.8 m, 15 tube segments each 122 m-128 m long, and a sand flow foundation. In 1993, Guangzhou completed the Pearl River immersed tube tunnel shared by roads and subway, and the Ningbo Yongjiang road immersed tube tunnel was completed in the same period. In January 2002, the Ningbo Changhong Tunnel was completed and opened to traffic. It was a two-way four-lane road immersed tunnel project completed by the Tunnel Design Institute and Tunnel Company in cooperation with Shanghai Salvage Bureau, Waterway Bureau, Third Harbor Engineering Bureau, etc. The river section of the Changhong Tunnel was constructed by the immersed tube method. Design started in December 1999, opened to traffic in March 2002, and won the 4th Zhan Tianyou Award for Chinese Civil Engineering in October 2003. It is the first immersed tube tunnel with pile foundation in China. The dry dock is set on the tunnel axis, and an outfitting groove is set at the axis position, for which there is no similar design scheme in China. During tube segment immersion, three-point temporary support was used, with the front end resting on a nose support, and the temporary support points at the rear with two vertical jacks using large-diameter (φ1200 mm) steel pipe piles, which was the first in the world tunnel history. It was the first in China to use reinforced concrete structure self-waterproofing as the premise, with no external waterproofing on the tube segment bottom plate.
　　The Shanghai Outer Ring Tunnel, completed in 2003, is the largest immersed tube tunnel after these tunnels. The Shanghai Outer Ring Tunnel is the first large immersed tube tunnel with two-way eight lanes constructed by the immersed tube method in Shanghai. The project has a total length of 2882.83 m. The immersed tube section is 736 m long, divided into seven tube segments of varying lengths from 100 to 108 m. The tube segment cross-section is three holes and two pipe galleries, with corresponding lane numbers of 3+2+3. The tube segment is 9.55 m high and 43.0 m wide. The vehicle clearance net height: side holes 5.0 m, middle hole 5.5 m. The maximum burial depth of the tunnel is 33.0 m, and its scale is the first in China and Asia. Since the start of construction on December 28, 1999, it took more than 3 years, experienced and overcame many key technical challenges, and was successfully opened to traffic on June 21, 2003. The completion of the Outer Ring Tunnel marks that the design of large tube segments and tube segment immersion of immersed tube tunnels have reached the international advanced level.
　　At present, immersed tube tunnels such as the Guangzhou-Foshan High-speed Railway Tunnel, Nanchang Honggu Tunnel, and the immersed tube section of the Hong Kong-Zhuhai-Macao Island and Tunnel Project are under construction, and the development of immersed tube tunnels is steadily advancing.