At 11 a.m. on August 18, the first shield machine “Lvtou” for the Wuhan Dadonghu Core Area Sewage Transmission System Project was successfully launched, marking the official entry of the deep sewage transmission tunnel into the construction and excavation stage. It is understood that ordinary sewage pipes are 3 to 4 meters below the ground, while this deep tunnel is buried about 30 meters deep, with the deepest point reaching 51 meters. Why bury it so deep?
The project has a total length of 17.5 kilometers and a maximum burial depth of 51.5 meters, equivalent to a “special subway.” Through this deep tunnel system, sewage from Wuhan's main urban area is collected and centrally transmitted to the Beihu Sewage Treatment Plant outside the Fourth Ring Road for centralized treatment. The reason for such deep burial is mainly that sewage is corrosive. To avoid affecting the strata, the tunnel must be buried in stable rock formations, thus ensuring the safety and stability of the sewage pipeline.
The construction of this deep sewage tunnel also makes Wuhan the first city in China to build a deep sewage tunnel. It not only greatly improves sewage treatment efficiency, but more importantly, the tunnel also reserves rainwater treatment channels and will become a drainage channel for urban waterlogging in the future.
The deep sewage tunnel is planned to be completed by the end of 2019. By then, sewage from the area north of Wuluo Road in Wuhan's Jiangnan district and around East Lake will quickly enter the Beihu Sewage Treatment Plant via the “deep dedicated tunnel,” with a daily sewage treatment capacity of 1.5 million tons and serving a population of 3 million. The main tunnel of the project has a total length of 17.5 kilometers, spanning four administrative districts: Wuchang, Hongshan, Qingshan, and East Lake Scenic Area. It is currently the domestic deep sewage tunnel with the largest transmission flow, the longest transmission distance, the first to adopt swirl-type inflow, and the first to use fully underground sewage pretreatment stations in the main urban area. It features “the longest deep tunnel project in China,” “the most complete construction techniques,” “the most complex shield construction conditions in Central China,” and “the deepest shield construction shaft in Wuhan.” The construction manager introduced that this ultra-long deep tunnel is equipped with 9 shafts and 9 shield sections. The shafts are all over 32 meters deep, with a maximum depth of 51.5 meters, equivalent to a “special subway,” directly transporting sewage to the treatment plant for centralized treatment.
The No. 3 shaft, where the shield was first launched, is 34.9 meters deep, and the line is 3.16 kilometers long. Since the sewage deep tunnel system has a tunnel diameter of about 3 to 3.4 meters, only a quarter of the area of an ordinary subway tunnel, while the burial depth is twice that of a subway, traditional shield machines are useless. To this end, they jointly developed with shield machine manufacturers and boldly created the country's most advanced small shield equipment “tailor-made” for the Dadonghu deep tunnel. The 18-section small shield machine, with a total length of more than 120 meters, was disassembled and connected via pipelines, distributed underground and on the surface, gradually lowered into the shaft for excavation, and converged underground to achieve whole-machine excavation. It is reported that in future operation, the tunnel will bear both groundwater pressure and internal sewage pressure, imposing extremely high requirements on tunnel construction: no gaps or cracks. “During high-pressure transmission, it must be like a completely sealed water pipe; the 17.5-kilometer structure allows no pores or tiny cracks. We call this building the ‘Swiss watch’ of the construction industry.”
To this end, they used ultra-long-distance vertical and horizontal concrete pumping during construction to ensure concrete forming quality; for the first time in China, they carried out synchronous construction of small-diameter shield segments and secondary lining concrete structures. The inner side of the secondary lining is coated with a surface coating to achieve corrosion resistance, ensuring that water inside and outside the tunnel does not interfere with each other. This coating can guarantee no corrosion for 100 years.