Ring Tendons for Tunnel in World's largest Dam Project
Xiaolangdi Multipurpose Dam Project is located at the exit of the last gorge of the middle reach of the Yellow River Huang He, Chinas second largest river. The main objectives of this mega project are flood and ice control, siltation control, irrigation, water supply, and hydroelectric power.
The project consists of 10 inlet towers, 9 flood and sediment tunnels, one underground power plant tunnels, and one graded earth and rock fill dam with a 154m inclined impervious core and a 1,317m long crest. The power plant features six 300MW turbine and generator units, totaling 1800 MW of installed capacity and an annual energy output of 5.16 GWh.
Three sediment tunnels will be used for siltation control. Post-tensioned concrete was chosen to be the lining material over a steel lining because the high content of silt and sand in the river water would be too abrasive for a steel lining. According to the bid documents, bonded ring tendons were to be used for the post-tensioning.
The designer finished a model test 10 years before the start of the projects. Nevertheless the proposal to use unbonded ring tendons for the post-tensioning of the sediment tunnels was set forward by the Lot 2 sponsor company Zublin at the bidding stage in 1994. Compared with the bonded solution, the unbonded ring tendon technology resulted in significant savings and proved to be more reliable in the field. Although the unbonded system was thought to be superior over a bonded system, the client and the designer had reservations because unbonded technology for tunnel lining has never been used in China before and there were only few international reference projects. The client requested field tests for both bonded and unbonded systems in March 1997. DSI conducted both tests and satisfied the client with excellent results of the unbonded ring tendon system.
In January of 1998, DSI was awarded the supply contract to deliver over 4400 DYWIDAG-ring tendons with type M anchorages that were installed under DSI field supervision. The project started in July 1994 and was successfully completed in July 2001. The post-tensioning on the sediment tunnels was conducted between February and August of 1998.