New Year Grassroots Visit
　　On February 7, in Suzhou, there were still 8 days before the 2018 New Year's Eve. The world's largest UHV GIL comprehensive utility tunnel project, namely the construction of the Sutong GIL Yangtze River Tunnel, has reached an important milestone, safely passing 78 meters underwater.
　　“The water and soil pressure at this depth exceeds 0.8 MPa, equivalent to 9.8 kilograms per square centimeter, making it the tunnel with the highest water pressure and deepest burial depth in China at present.” At 78 meters below the riverbed, accompanied by the continuous excavation of the massive shield machine, Chen Peng, project manager of China Railway 14th Bureau, told the Science and Technology Daily reporter.
　　The Sutong GIL Yangtze River Tunnel has a total length of 5468.5 meters, with a shield machine excavation diameter of 12.07 meters, making it the longest shield tunnel crossing the Yangtze River to date. Since the shield machine began excavation on June 28, 2017, in addition to the maximum water depth of 78 meters, the project has continuously overcome safety risk points such as the Jiangnan levee, 1800 meters of harmful gas strata, riverbed scour channels, and a 5% uphill section, with a cumulative excavation of 2836 meters, more than half of the total.
　　GIL stands for Gas Insulated Metal-enclosed Transmission Line. The Sutong GIL comprehensive utility tunnel project is part of the Huainan-Nanjing-Shanghai 1000 kV UHV AC power transmission and transformation project. According to State Grid data, this is the world's first project to use UHV GIL in an important power transmission corridor. It is an innovative ultra-long-distance GIL project with the highest voltage level, largest transmission capacity, and most advanced technology. It is also the “throat” and controlling project for the closed-loop operation of the East China UHV AC ring network, requiring a series of key technical research and breakthroughs in engineering design, equipment development, tunnel construction, GIL installation, and operation and maintenance.
　　Exploration found that in addition to the burial depth challenge, the strata where the Sutong GIL Yangtze River Tunnel is located are rich in harmful gases such as methane. Among them, the 1800-meter stratum not only has high concentrations of harmful gases but also high pressure, which is rare in the world.
　　To overcome these unprecedented construction challenges, China Railway 14th Bureau Group, in collaboration with shield machine manufacturers, utilized global resources for joint research and development, design, procurement, and manufacturing. In just seven and a half months, the “Zhuoyue” shield machine was successfully rolled off the production line, setting a world record for the shortest manufacturing cycle of a large-diameter shield machine.
　　“The entire process was designed for explosion protection, including the main components of the shield machine and the entire photoelectric system, all specially customized according to manufacturing needs.” Chen Peng said.
　　Due to the high concentration of toxic gases in the tunnel, the airtight standards for the shield are extremely high. “If the air cushion chamber leaks, not only may harmful gases leak into the tunnel, but the compressed air in the air cushion chamber may also leak into the excavation chamber, causing instability of the tunnel face.” said Sun Maozhou, project chief engineer.
　　To address this challenge, the project department innovatively developed a set of air duct storage devices, enabling the airtightness of the slurry shield to reach the highest safety standards, greatly reducing the tunnel working temperature and the potential hazards of harmful gases.
　　Although the New Year is approaching, except for a very small number of special needs to return home for family visits, more than 300 builders remain at the construction site.
　　“The tunnel excavation uses a slurry balance shield machine. Once excavation starts, it must continue 24 hours a day, moving forward until the tunnel is completed.” Chen Peng said.
　　It turns out that using shield construction under the Yangtze River bed, long-term shutdowns pose huge risks. Especially when it is in a full-face fine sand stratum, once the 1300-ton main machine stops, it will experience sinking and instability, with risks of collapse, roof fall, and cutterhead jamming, causing unpredictable losses.
　　The Sutong GIL comprehensive utility tunnel project has a total investment of 4.763 billion yuan. Construction started in August 2016 and is planned to be completed and put into use in October 2019. By then, East China will have an additional 35 million kilowatts of power receiving capacity, reducing coal used for power generation by 200 million tons per year, and reducing sulfur dioxide emissions by 960,000 tons, nitrogen oxides by 530,000 tons, and soot by 110,000 tons.