Key technologies in the central route's tunnel project
The construction of the upstream tunnel crossing the Yellow River is a key challenge for the central route of the South-to-North Water Diversion Project.
The South-to-North Water Diversion is the world's largest water transfer project that pumps water from the Yangtze River in the south to feed the draught-prone north.
An exhibition in 2011 gave visitors a vivid presentation of water from the Han River, a tributary of Yangtze River, being diverted northwards, crossing transversally the Yellow River and running all the way to Beijing.
Exhibition illustrators explained that the Han River water crosses the Yellow River via a shield tunnel placed to the west of the Huayuankou town in Zhengzhou city, central China's Henan province. The tunnel is arranged in two lines of 4,250 meters in length. They are excavated 40 meters under the riverbed of the Yellow River, subjected to high water pressure.
The construction has set several records in China: It was the first big-diameter tunnel to cross the Yellow River, the first application of the state-of-the-art slurry balanced shield machine, and the first tunnel to adopt composite lining consisting of primary segment lining and pre-stressed cast-in-situ reinforced secondary lining.
Niu Xinqiang, president of the Changjiang Institute of Survey, Planning, Design and Research and chief designer of the project, said the engineers were able to conquer a series of technical challenges since the river crossing was launched on Sept. 27, 2005. The tunnel needed to pass through seismic zone and wandering river with complex geological conditions. Researchers obtained favorable results in development of anti-seismic technique, and tested the seismic response of the tunnel under the three dimensional seismic power. Models for assembled segment joints, and models for interactions between the primary lining and the external soil, were built for the composite lining structure. A full-scaled tunnel simulation experiment model was built to study the external water and soil environment and the mechanical conditions.
Shaft work started on the north bank of the Yellow River. The sand layer of the river floodplain, characterized by high groundwater levels and permeability, posed challenges to constructors. According to Gao Bihua, an official from the Construction and Administration Bureau of the central route of the South-to-North Water Diversion Project, engineers utilized a top-down method by installing the diaphragm walls first as the retaining structure. Advanced hydraulic double-wheel trench cutter was used for installation of the diaphragm walls, which were of 1.5-meter in thickness and 76.6-meter in depth, both topping the corresponding national records. Based on field tests, the unconventional single-tube high pressure spun jet method was adopted for completion of the base slab, effectively improving pouring quality and construction efficiency.
The shield machine started tunneling in the north bank at 50 meters deep underground, and drove under the river to reach the south bank. An underwater hub was developed for optimizing the connection methods to ensure accurate data transmission; a straight-sided circumferential anchor dynamometer for the thin secondary lining was also developed to monitor pre-stress of the secondary lining with circular anchored tendons. Both inventions were granted patents by the State Intellectual Property Office.
Gao said the sand, cobble and calcareous concretions in the strata resulted in high wear of the machine's cutterhead tools, while maintenance was difficult due to the high groundwater level and soft formation conditions. Successful application of the tri-axial mixing pile facilitated cutter replacement for the technical workers.
"Tunneling parameters of a shield machine, such as thrust force, torque, advance speed, penetration rate vary with different geological conditions," Guo said. "We've basically got to know the temperament of the strata and grasped the tunneling rules of the machine."
In view of the problems such as segment fragmentation and dislocation in the initial stage of tunneling, constructors optimized the original segment design scheme after months of research and practice to meet the demands of shield tunnel construction and enhance cost-effectiveness.
Hailed by the industry as a brilliant engineering feat due to its application of the world's most sophisticated technologies to meet the numerous challenges, the tunneling project can provide guidelines for similar water conveyance projects under difficult geological conditions.