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Why is the large aqueduct stable like a mountain?

China.org.cnUpdated: April 3, 2018

Interpretation of design and construction of aqueducts with large flow prestress

A girder-type aqueduct body with U-shaped bidirectional prestressed structures has been on the on-site prefabrication stage recently, in the construction site of the Shahe Aqueduct, the largest control project of the central route of the South-to-North Water Diversion Project. With the extensive application of the large-flow prestressed aqueducts in the project, the large aqueducts cluster on the central route will be "stable like a mountain," according to the engineers.

The vulnerabilities of large aqueducts

Aqueducts are water transmission structures that cross river channels. The total length of the main canals in the central route of the South-to-North Water Diversion Project is 1,276 kilometers, and 49 aqueducts are planned, accounting for about 7 percent of the total length. One of the aqueducts that is located at the Tuanhe River in Nanyang, Henan province, has a normal water flow of 350 cubic meters per second, and it can be ramped up to 420 cubic meters per second. The aqueduct, with a span of 40 meters, is about 30 meters wide and 8.8 meters tall. It has a load of more than 12,000 tons per span. At present, the largest aqueduct in the world is in India, with a normal water flow of 357 cubic meters per second, a water depth of 6.7 meters, and a water surface width of 12.8 meters. The size and flow capacity of the aqueducts of the South-to-North Water Diversion Project will surpass it.

One of the important indicators for measuring aqueducts is its loading capacity. The load of highway and railway bridges is usually 6 to 7 tons per meter. In comparison, the Tuanhe River aqueduct has a load of more than 400 tons of water per meter. There is no precedent for a design with such a large-scale aqueduct cluster in the world, which has brought new issues to the design and construction of the large-scale aqueducts.

This project carries with it two risks. One is the structural integrity. Impregnability and impermeability are the basic requirements for the quality of the aqueduct. The other is the stability. Both the foundation and the body should be stable with good anti-seismic capabilities. If the aqueduct collapses, it would affect the water supply across the entire route. The stability and safety of the aqueducts are the crucial prerequisites for construction, as well as the main goal for scientific research.

The key in scientific and technological research

The structural integrity in the large aqueducts is first to be addressed in terms of scientific and technological research. There could be many causes for structural cracks in aqueducts, of which temperature is one of the most important factors.

Temperature stress is one of the common causes for cracks in aquedusts. Wang Changde, a professor at Wuhan University and one of the project research leaders, said that it is also called "thermal stress," which is the stress caused by the fact that an object cannot freely expand or contract due to temperature rise or fall, or that the temperatures in various parts of the object are different. For example, remaining gaps at the connecting parts at the railway tracks are to avoid or reduce possible temperature stress. Similarly, temperature effect also needs consideration in the design and construction of an aqueduct, but the continuous water flow makes it much more complicated and difficult than laying railway tracks.

In the winter, the outside of the aqueduct is cold and the inside is hot; in the summer, the situation reverses. Without accurate parameters and calculation methods, the aqueduct concrete will crack and deform. By observing and studying the aqueduct temperature changes in the complex environment, including the local air convection, daily radiation and sudden temperature drop, engineers could determine the boundary conditions and the main influencing factors. Then, using water temperature models and other methods of experiment, they can calculate the stress limits and propose temperature control measures for different construction periods of the aqueduct.

In every 10 aqueducts, nine leaks. Analysis shows that sealing failure of the connecting sections, such as the expansion joints, is the main reason for aqueduct leakage. The expansion joints of the aqueduct must be caulked and sealed to allow it to flex freely without leaking. The key point of this research is to study the mechanical properties and impermeability of high-performance sealing materials, and to provide a high-reliability sealing material suitable for large aqueduct expansion joints.

Second, the problem of the stability of large aqueducts should be solved.

Under the condition of huge loads, choosing a type of aqueduct structure that is suitable for the upper and lower structures of large aqueducts not only affects the success of the project, but also has an impact on project quality and schedule. Wang Changde said that the research team has done a lot of analysis on multi-box girder aqueducts. By changing the structural dimensions of the girders, the team found a new type of aqueduct structure with small deadweight and large bearing capacity and its design calculation methods.

Benefits of the research results

Cao Weimin, one of the project leaders and deputy head of the Construction and Management Bureau of the central route of the South-to-North Water Diversion Project, said that the engineers have tackled the problem and proposed an optimized structure and design method for the new multi-box girder aqueduct with large flow. They have also provided the temperature load calculation method and discovered the natural vibration characteristics and dynamic structural response of the aqueduct structure. The engineers proposed a method for calculation and analysis of pile-soil interactions in large-scale aqueducts, and formulated the concrete curing and temperature control measures, as well as the control methods for early cracks in concrete. He said the revised design technical regulations for the beam aqueducts are completed, and the design and construction guidelines for large beam aqueducts have been issued to relevant design, construction and management units.

Wang said, "The earthquake fortification intensity in the area, where the central route of the South-to-North Water Diversion Project passes through, ranges from 6 to 8 degrees. The load on the piers is huge, and the aqueduct is 'top-heavy.' The structure seismic problem cannot be underestimated. Now this problem has been solved."

According to the study on the seismic performance of the aqueduct and its pillars, and the simulation tests, the optimal damping scheme for the upper structure of the aqueduct was determined, and the effective anti-seismic measure of the aqueduct pile foundation was proposed, Wang said. According to the results of this study, the design of the pile foundation of the Minghe Aqueduct was optimized, and good results were achieved. The shock-absorbing bearings proposed in the study of shock absorption measures have been used throughout the central route.

Compared with the traditional aqueduct, the new type of multi-box girder aqueduct can save 10 to 20 percent of the cost while fulfilling the project quality requirements. New materials for large aqueducts, new structures and the research results on the durability and reliability of large-scale aqueducts have improved the safety and reliability of large-scale aqueducts and reduced the cost of project operation management and maintenance.

National patents are pending for the aqueduct's heat preservation materials and preparation methods developed during construction, which could be widely used for similar projects. The large-scale aqueduct, as a controlling project of the central route of the South-to-North Water Diversion Project, has a special status. The research team of the project has successfully solved the technical difficulties of crossing rivers and valleys in large-scale channels. It has provided guarantees for accelerating the construction of the central route project and diverting water supply as scheduled.