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Large diameter PCCP helps divert water to Beijing

China.org.cnUpdated: April 4, 2018

The Prestressed Concrete Cylinder Pipe (PCCP) is a composite pipe structure made up of a concrete core and a thin steel cylinder, compressed using high-tensile-strength prestressed wires and protected with a mortar coating.

The Beijing section of the South-to-North Water Diversion Project is 80.4 kilometers in length. With high land costs and a large population, Beijing is covered by high-density urban facilities like pipes and communication and power networks. Given the infrastructure's need to travel along varieties of complicated landforms like hills, farmland and residential areas, it is important to find a suitable vehicle to divert water resources to Beijing. The solution: A PCCP construction project that connects the Huinanzhuang Pump Station and the Daning Surge Tank, stretching 56.359 kilometers and adopting a double-rowed PCCP that is four meters in inner diameter.

"The PCCP is almost tailor made for Beijing's situation," said Shi Weixin, vice dean of the Beijing Institute of Water. As the chief designer of the Beijing section of the South-to-North Water Diversion Project, Shi participated in the planning of the project since 1989 and witnessed the transition of vehicle adoption in the water diversion plan, from open channel to the PCCP. As a new form of pipe, the PCCP has various advantages like wide applicability, long service life, excellent shock resistance, easy installation, low operating cost and great sealing.

There were a total of 21,533 PCCP tube sections used during the Beijing section of the South-to-North Water Diversion Project. Such a big-scale installation of the large diameter PCCP tubes (4 meters in inner diameter) had never been done in China. The challenges were unprecedented in terms of design, manufacturing, transportation and installation.

To meet this challenge, China listed the research project on the vital technologies of large diameter PCCP in the National Key Technology R&D Programs for the 11th Five-Year Plan. The Beijing Institute of Water, Tsinghua University, the China Institute of Water Resources and Hydropower Research, and the Nanjing Hydraulic Research Institute joined their efforts to set up a research group in order to innovate the technologies and solve the problems all through the project.

Technological innovation highlighted in the project

The Beijing section covers mountainous areas, with the buried depth of PCCP reaching over 20 meters; hence, countering the huge pressure from the earth depends strictly on the quality of the raw materials these pipes are made of. Even worse, the high alkaline level of the sandstone aggregate in Beijing degraded the durability of local concrete. After rigorous investigation and research, the research group decided to mix coal ash with concrete and keep the alkaline level within 2.5 kilograms per cubic meter.

The manufacturing of large diameter PCCP needs specialized equipment and effective erosion prevention. The manufacturer organized engineers to develop solvent-free coal tar epoxy materials and an automatic spray facility, which would ensure the quality of spray and save time. The method not only improves the PCCP's anti-erosion capacity, it also eliminates risks to people and the environment, as the materials are neither volatile nor odorous.

"This technological innovation effectively improves the anti-erosion capacity of the pipe by adopting aseptic technique -- cathodic protection," said Guo Yongfeng from the construction and administration bureau of the South-to-North Water Diversion Project's central route. As the pipe must bear huge internal pressure, it needs to be supported by circumferential prestressed steel wires. The steel tends to generate chemical reaction when mixed with alkali in wet soil, causing erosion to the pipe. A solid solution is to lay metal strips of magnesium and zinc paralleling with the pipes, then let positive ion in the metal to generate chemical reaction with alkali in place of the pipe, which would help to avoid erosion of the steel wire brought by chloridion in the underground water.

To monitor the pipe's erosion and take preventive measures, the research group developed cathodic protection test probes, which also help to lower the risks of erosion.

A single section of PCCP is four meters in inner diameter, 4.8 meters in external diameter, and 78 tonnes in weight. Transporting such giant pipes was also a problem. The research group designed special trucks for carrying the pipes, as well as a new form of tunnel pipe-building truck that combines the functions of transportation, installation and correction to solve the problem.

The primary plan requires the distance between the pipe and the cave wall to be only 30 centimeters along the section of the Xiganchi and Congqing Tunnels, and it was hard to install the pipe in such a narrow space. The construction authority of the Beijing section of the South-to-North Water Diversion Project worked with the Taiyuan Heavy Machinery Factory to design a new form of tunnel pipe-building truck. The truck uses a hydraulic device to load up, then uses a power-driven device to transport pipes into the tunnel on fixed tracks, and adopts a hydraulic crossbeam device to adjust the position of connectors. As engineers kept improving the technique, it finally just took two days to connect each two PCCP tubes, a significant reduction from the four days. "This new truck performed a vital role, not only saving time, but also maintaining outstanding precision," said Feng Qi, deputy director of the construction authority of the Beijing section of the South-to-North Water Diversion Project.

According to the initial plan, construction workers had to use a 350-tonne German-made crawler crane as a sling cart. It was quickly apparent that the machine would take half a year to finish ground preparations, not to mention that the heavy machinery could not be used at construction sites filled with high voltage cables and high pressure towers. In the end, the engineers designed an operation method with big and small gantry cranes, which reduced the cost and worked much more nimbly on construction sites.

Having solved the problem of hoisting and moving materials and equipment, the engineers went on to tackle another difficulty in installation - it was hard to connect the pipes. The project uses a specific kind of large-caliber connector shaped for PCCP, and the connector is sealed by slidable O-rings rubber gasket, because the connector's groove is filled with the sealed rubber gasket, and it requires tight seals between the pipes. The engineers conducted over a hundred tests on the circular degree of the PCCP's connecting steel ring. After the analysis, they found that installation was difficult if the ellipticity error of the connector's steel ring exceeded 9 millimeters. The research team was eventually able to narrow the gap down to an acceptable range, down from 12.7 to 6 millimeters. The new methods of making the connectors allowed the installation of pipes to process much more smoothly.

Finding value in technological innovation

There are 12 key technologies developed during the large diameter PCCP construction project, ranging from new material, techniques to devices, which obtained seven patents from the State Intellectual Property Office. The South-to-North Water Diversion Project contributed to the industrialization and timely application of new scientific achievements, producing significant economic benefits.

The PCCP construction project of the Beijing section of the South-to-North Water Diversion Project adopted international bidding to recruit contractors, attracting a number of competitors like Ameron, a famed American corporation who put in a bid of 3.1 billion yuan. The winner was a joint bid of 1.25 billion yuan by the Beijing Heshan Water Conduit Corp., the Shandong Electric Power and the Xinjiang Guo Tong Pipeline Co., Ltd.

The Beijing Heshan Water Conduit Corp. advanced the manufacturing capacity of PCCP through participation in the project. "Our company has changed the name to Beijing Hanjian Heshan Water Conduit Corp.," Tian Yubo, board chair, said. "We were encouraged by the great feedback from our customers, and we expanded our business to more provinces like Heilongjiang, Shanxi and Anhui."

"The successful application of this technology will certainly foster the development of the domestic PCCP industry, and it also offers valuable experience for construction of the second phase and the western route of the South-to-North Water Diversion Project," Beijing Institute of Water's Shi said.