On the morning of June 28, the "Yongxing" shield machine began boring the Zhanjiang Bay undersea tunnel of the Guangzhou-Zhanjiang High-Speed Railway, marking the start of main construction on the shield-driven section.

The Guangzhou-Zhanjiang High-Speed Railway is using a single-tube, double-track undersea high-speed railway tunnel to cross beneath Zhanjiang Bay for the first time. "The Zhanjiang Bay undersea tunnel has a total length of 9,640 meters, including a 1,148-meter open-cut section, a 1,140-meter U-shaped trough section, a 7,352-meter shield-driven section and about 2,500 meters under the sea. The excavation diameter is 14.33 meters, the segment outer diameter is 13.8 meters and the inner diameter is 12.6 meters," said Chang Yong, project manager of the Major Shield Tunneling Company of China Railway 14th Bureau Group. He said the project is a controlling works on the entire Guangzhou-Zhanjiang High-Speed Railway and the first time a large-diameter shield machine has crossed Zhanjiang Bay into the city's central urban area. It will take on the longest single-head boring drive of any undersea tunnel in China and is another landmark project for China's high-speed railway.

"The Zhanjiang Bay undersea tunnel is characterized by a large excavation cross-section, poor geological conditions, large volumes of waste slurry, a tendency to form mud cakes, high construction standards and many key technologies," said Wang Hao, shield manager of the project. Starting from the launch shaft, the shield machine will mainly pass through silty clay and sand layers. The large excavation diameter, long boring distance and dense underground pipe networks make construction difficult and impose strict requirements on shield machine operation, parameter control and equipment performance.

Construction drawings for the Zhanjiang Bay undersea tunnel show that the tunnel will pass successively beneath Dongwang Avenue, Dongcheng West Road and the Zhanjiang Olympic Sports Center, then beneath Longwangwan Road into the undersea section running parallel to the Zhanjiang Bay Bridge. After emerging from Zhanjiang Bay at the Sino-Australian Friendship Garden in Haixi, it will run for a long distance beneath Leshan Road, a main artery of Zhanjiang with dense buildings and underground pipe networks on both sides. It will pass alongside the pile foundations of two bridge piers on the ramp of the Zhanjiang Bay Bridge, with a minimum clear distance of only 1.20 meters and 1.3 meters.

In response to the geological conditions of the Zhanjiang Bay undersea tunnel, China Railway 14th Bureau Group drew on its rich experience in shield tunneling beneath rivers and seas over the years to carry out targeted research and design on the cutterhead structure, cutter types, slurry circulation and slag discharge, and pressure and corrosion resistance of the sea-crossing shield machine. The "Yongxing" shield machine began manufacturing in September 2020 and completed production and debugging with high efficiency in more than eight months. The machine is 147 meters long and weighs 3,600 tonnes. The minimum horizontal curve radius of the shield tunnel alignment is 3,000 meters, the maximum longitudinal gradient is 13.18‰, the total installed power is 7,800 kilowatts, the maximum burial depth is 50 meters and the maximum water pressure is 6 bar. The cutterhead is equipped with 112 fixed scraper cutters, 59 atmospheric-pressure replaceable scraper cutters, 32 fixed leading cutters and 12 peripheral scraper cutters, totaling 227 cutters. It adopts a telescopic main drive with a maximum torque of 43,156 kilonewton-meters, and the enhanced design achieves the optimal configuration to improve boring efficiency.

"In view of the long-distance single-head boring drive of this project, the cutterhead of the 'Yongxing' shield machine uses an atmospheric-pressure cutterhead. To adapt to efficient boring in silty clay strata, a cutterhead opening ratio of 45%, larger than that of ordinary shield machines, was selected. At the same time, to address the difficulty of mud cakes forming on the cutterhead, the central flushing system was optimized and strengthened to prevent muck accumulation in the central area of the cutterhead and at the bottom of the excavation chamber," Wang Hao said. At the slag discharge outlet below the air cushion chamber, a "stirrer + roller crusher" device is installed, the first of its kind used in China, to more effectively crush the blocky clay cut by the cutterhead, facilitate slag removal and improve slurry circulation and slag discharge efficiency.

Technical staff said that during the subsequent boring process, managers will carry out round-the-clock uninterrupted construction operations and conduct comprehensive control through a smart command center, with real-time intelligent monitoring of shield machine parameters, measurement and monitoring of ground settlement and detection of staff conditions. Boring parameters will be strictly controlled and relevant data grasped in real time to ensure the safe and smooth advance of the shield machine.

The main line of the Guangzhou-Zhanjiang High-Speed Railway is about 401 kilometers long, and construction began along the entire line on June 30, 2020. Designed for a operating speed of 350 kilometers per hour, it is the high-speed railway project with the longest construction mileage and the largest investment in Guangdong Province. Once open to traffic, downtown Guangzhou and the urban area of Zhanjiang will be reachable from each other in 90 minutes, enabling Zhanjiang to integrate at high speed into the coordinated development of the Guangdong-Hong Kong-Macao Greater Bay Area and enhancing the regional economic status of western Guangdong.