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The second phase of the cross-river tunnel of Nanjing Metro Line 4 crosses the main channel of the Yangtze River, with a total length of 3,062 meters. “The cross-river tunnel boring machine was assembled and debugged underground from the middle wind shaft of the Jiangxin Island, and then excavated to the north and south of the river respectively.” Shan Xiaobo, the technical person in charge of the project of China Railway 14th Bureau, introduced that the one that started to the north of the river was the “New Journey” slurry balance shield machine with an excavation diameter of 11.64 meters.
In January this year, a slurry shield machine named “New Journey” was successfully rolled off the production line in Changsha, and was then transported to the Jiangxinzhou intermediate ventilation shaft of the second phase of Nanjing Metro Line 4, and began to be assembled in the shaft in April.
At present, the assembly of the shield machine has been completed, and all the people on the project are making every effort to carry out the final inspection and debugging of the shield machine. The ground-based mud-water separation equipment and double-liquid slurry mixing station are also being debugged simultaneously to prepare for the final sprint of the shield machine.
It is understood that the work of the slurry shield machine is mainly to promote excavation and segment assembly. There are two channels on the cutter head at the front of the shield machine, one in and one out. During the excavation process, the excavation face will inject the mud prepared in advance into the soil to be excavated through pressure to form a mud film; while the mud film is formed, the cutter head will continue to rotate and excavate the soil, and mix the excavated soil with the mud to form a high-concentration mud, which will be transported out of the shield machine through the mud discharge pipeline and enter the mud-water separation system on the ground for treatment.
Every time the shield machine advances 2 meters, it will stop and assemble prefabricated segments. The entire tunnel is formed by a series of ring-shaped prefabricated segments connected together.
As the most critical construction section of the second phase of Line 4, the “New Journey” slurry shield machine will complete the most technically difficult part of the entire line. The shield construction passes through the silty clay, fine sand, medium-coarse sand, and round gravel formations of the Yangtze River floodplain. The biggest challenge is that it needs to pass through a medium-weathered mudstone formation of about 700 meters.
This domestically-produced slurry shield machine has a total weight of 2,100 tons and a total length of 120 meters. The “simultaneous dual-liquid slurry injection technology” it uses is the first application in domestic cross-river subway tunnel construction, which can greatly improve the efficiency and safety of the shield.
It is reported that when the shield machine started from Jiangxinzhou, the thickness of the top cover was about 26.8 meters thick. When crossing the Yangtze River, the deepest cover thickness was about 55 meters. The cover thickness at the receiving end was 34 meters, making it the deepest cross-river subway line in the Yangtze River Delta region.
First of all, it is necessary to complete the construction of corresponding supporting facilities on the ground, such as material warehouses, machine processing workshops, double-liquid slurry mixing stations, air compressor rooms, cooling towers, segment and box culvert prefabricated parts storage sites, mud and water separation systems, etc., and the assembly and debugging of the shield machine must be completed underground.
Secondly, in view of the situation of the cross-river tunnel, a special construction plan should be formulated in advance, strict safety education and technical explanation should be given to the construction workers, the possible risks of the shield machine should be analyzed in advance, and risk pre-control and corresponding emergency plans should be prepared. In addition, the burial of ground monitoring points, prefabrication of pipe segments and box culverts, etc. should be carried out.
“Only if we do a solid job in the early stages,
Only in this way can a solid foundation be laid for subsequent construction.”
Looking forward to the “New Journey” shield machine
Early departure
Looking forward to the second phase of Line 4
Open to traffic as soon as possible
