In 2010, the Danjiangkou Dam played a significant role in the flood season, shielding water flood and saving properties worth 390 million yuan (US$ 62 million) from flood damage. In March 2010, a few months before that year's flood season, an elevation project was implemented to increase the capacity of the Danjiangkou Dam, which turned out to be truly significant. As a key component of the South-to-North Water Diversion Project, the Danjiangkou Dam, originally built in 1973, was bulked up using technologies developed by the Changjiang Institute of Survey, Planning, Design and Research.
Elevating the dam
The Danjiangkou Dam, located at the juncture of Hubei and Henan provinces, is the water source of the first aqueduct of the central route of the South-to-North Water Diversion Project.
The project increased the elevation of the dam from 163 meters to 176.6 meters, and the normal water level from 157 meters to 170 meters, adding 11.6 billion cubic meters of capacity, two and a half times the size of Beijing's Miyun Reservoir. The dam has functioned well in flood control, hydroelectricity and navigation. Now, the elevation project is crucial for it to undertake another mission — to divert clear water to the north.
During the elevation work, the toughest technological problem was the blending of new concrete with existing concrete, which some called a "dressing up" of the 40-year-old dam. Temperature and water pressure could affect the mixture and compromise the dam's architectural integrity.
Capacity expansion was sure to cause gigantic pressure on the dam's foundation. For starters, the back scarp of the dam needed to thicken to withstand enhanced water pressure, and then technicians would pour concrete over the top of the dam to increase its height. New concrete poured on the back scarp of the dam was not only restrained by the bedrock located at the dam's bottom, but also by the existing concrete at the dam's base and slope. The contraction of the newly poured concrete would create stress over the existing dam body, particularly on the dam heel, where intense stress could rip apart the junction surface. The mixture of new and existing concrete was therefore the top concern for ensuring the safety of the dam.
Chen Zhikang, the person in charge of the Key Technologies Research for Elevation Project of the Danjiangkou Dam, said the project was unprecedented in China and rarely seen worldwide. To guarantee success, the Changjiang Institute of Survey, Planning, Design and Research previously conducted multiple experiments on mixing new and existing concrete and learned from successful projects in other countries. The institute eventually came up with a master plan: Direct pouring would be the main method, while combining chiseling keyways in the vertical mating face and trench backfilling on the surface of the overflow dam section as auxiliary means.
Dismantling the existing concrete and controlled blasting, detection and removal of the charring layers, construction of artificial keyways, and temperature control of mass concrete were all the technical challenges conquered thanks to the above-mentioned research. Research fellows at the Changjiang Institute have also carried out studies on materials, construction techniques and temperature control, structure of the junction surface, change of stress before and after the elevation project, etc.
Strengthening the dam
After the elevation, the Danjiangkou Dam had a normal water level of 170 meters, amounting to an increase of 11.6 billion cubic meters of water storage, adding more than half of its original capacity.
The increased capacity brought enormous stress to the foundation of the dam. It was difficult to find out whether the existing anti-seepage curtain at depth can continue to endure prolonged stress. The rock formation at the dam foundation had relatively low permeability, and its structure can go through minor changes over time. If the water pressure was too high and broke down the anti-seepage curtain, the Danjiangkou Dam will collapse in a very short time, threatening the lives of 700,000peopledownstream from the Han River.
To address this, technicians needed to find a way to detect the seepage-control's effectiveness and the durability of the existing anti-seepage curtain. It was the first time in China such a type of detection was conducted on a high head dam. After a careful examination of the dam, technicians decided to dig inspection holes in different sections of the dam, and use cameras, ultrasonic equipment and CT scans to collect data of the anti-seepage curtain. Based on information gathered, technicians found it necessary to reinforce some parts of the curtain by grouting, and they came up with a solution. Their work not only led to discovering the probable longevity of the anti-seepage curtain, but also to strengthening and evolution to the studies on the durability of polymer grouting materials, offering valuable insights for durability detection of other kinds of curtains.
As a result, technicians could now determine the precise sections of the dam that needed reinforcement. Their solution includes grouting methods and technology, grouting materials and matching, and grouting pressure. They also helped to establish an industrial standard regarding acrylate grouting materials.
Technological achievements bringing in remarkable efficacy
The elevation project began in 2005, and it has since completed elevatingall54 sections without a single incident in quality control.
Research and application of new technologies and methods also helped to save more than 20 million yuan from the budget. Specifically, 12 million yuan was saved through arrangement optimization of anchor bars, 3 million yuan was saved through improving ways of concrete casting on the dam’s slope, and 5 million yuan was saved in the reinforcement of the anti-seepage curtain.
Those key technological achievements helped to produce remarkable results for the Danjiangkou Dam's capabilities. By matching new concrete with existing concrete, the highest water level could increase to 152 meters in the early stage of concrete casting on the dam's slope, therefore gaining a greater water head for power generation. The solution of direct concrete casting on the dam's slope cut down the usage of anchor bars and shortened the schedule of construction, ensuring the completion of the project before the flood season. Persistent heavy rainfalls eventually come in 2010, but the Danjiangkou Dam withstood the increased water flow rushing downstream the Han River. It shielded the middle and lower reaches, without the assistance from the nearby Dujiatai flood diversion and storage area. The elevated Danjiangkou Dam, standing taller, bigger and stronger, is now playing a more important role in protecting and benefiting people living in the middle and lower reaches of the Han River.