Ningbo, February 26, CCTV News (Reporter Yu Ye Correspondent Ma Jingbo **On February 26, a cutterhead painted with a "Ruilong" pattern of the super-large diameter shield machine "Yongzhou" in Changsha, Hunan Province, with a cutterhead excavation diameter of 1457 meters, will be used in the world's longest submarine high-speed rail tunnel - Yongzhou Railway Jintang Submarine Tunnel Ningbo end of the project. The "Yongzhou" shield machine is jointly developed by China Railway 14th Bureau Group and China Railway Construction Heavy Industry Group, with a total length of 135 meters, a total weight of 4,350 tons, and 308 special tools, reaching the world-class level. The cutterhead of the shield machine is coated with the patterns of "Ruilong" and "auspicious clouds", which means that the builders took advantage of the auspicious spirit of the dragon and the heroic courage to break through all difficulties to start the journey of the shield through the sea. 
Yongzhou" shield machine (photo by Fan Shaowen from CCTV) Jintang undersea tunnel is one of the key control projects of the whole line of Yongzhou Railway, the tunnel starts from Yinzhou District, Ningbo City in the west, and reaches Jintang Town, Zhoushan City in the east, with a total length of 1618 kilometers, of which the shield section is 112 kilometers, the maximum buried depth of 78 meters, the tunnel through the 100,000-ton Jintang main channel, the use of two shield machines from Ningbo, Zhoushan respectively from the opposite direction of tunneling, and finally in the seabed docking, the accuracy requirements of the error does not exceed 20 mm, less than the diameter of a penny coin, will achieve the world's first super-large diameter shield machine submarine docking and dismantling. It is reported that the "Yongzhou" shield machine starting from the Ningbo end of the working well will excavate 4,940 meters in a single head, not only to penetrate 23 oil pipelines, various ground buildings, seawalls, wharves, main channels and other 42 risk sources, but also to face the complex structure of the stratum and sea environment, hard rock and soft and hard uneven strata account for nearly seventy, the highest strength of 200 megapascals.
There will be 14 alternating conversions of soft and hard strata in the interval, and the long-distance crossing with great differences requires frequent high-pressure tool change operations on the seabed at a depth of 70 meters, and the complexity, difficulty and risk of construction are unique in the world. 
According to Hu Hao, project manager of the Yongzhou Railway of China Railway 14th Bureau, in view of the complex construction characteristics of the submarine tunnel, the project team carried out 6 innovative designs for the shield machine, using a heavy-duty cutterhead design, increasing the weight of 70 disc cutters and increasing the weight by more than 100 tons, reducing the knife spacing and increasing the number of disc cutters. "The shield machine applies the guidance system with the highest control accuracy and the most advanced technology as the 'eye' of the equipment tunneling, and is equipped with special equipment such as advanced geological prediction and advanced grouting reinforcement, which can detect and strengthen the bad geology in front of the shield machine in advance, so as to ensure the accuracy and safety of docking in the sea. Zhao Dabin, deputy manager of the Yongzhou Railway Project of the 14th Bureau of China Railway, said. The shield machine also adopts a double-layer shell, which dismantles the shield from the inside like a "golden cicada shell" after completion, and at the same time plays a supporting role in the stratum to cope with the in-situ dismantling of high pressure and narrow space in the sea. 
Yongzhou" shield machine off the assembly line ceremony (photo by Fan Shaowen from CCTV).
It is understood that around the construction of the Jintang submarine tunnel, the construction team has set up a research team to sort out more than 10 scientific research topics such as "research on the complete set of process and construction method for the whole process of docking in the submarine of ultra-large diameter shield tunnel", and comprehensively promote the technical research on equipment configuration, difficult technical research, high-quality tunnels and other aspects, so as to make technical reserves for the construction of more strait tunnels in China in the future. With a total length of about 77 kilometers and a design speed of 250 kilometers per hour, the Yongzhou Railway is a passenger dedicated railway. After the completion and opening to traffic, it will make up for the last short board of Zhejiang Province's "provincial one-hour high-speed rail traffic circle" and "city-to-city high-speed rail", which is of great significance for improving the layout of the railway network, promoting the development of tourism along the line, realizing the integration and development of Ningbo and Zhoushan, and accelerating the integration of Zhoushan and Ningbo into the "Belt and Road" and the Yangtze River Economic Belt.
*: CNR.