Development and application of a new ballast water system for immersed tunnel installation: A case study of the Yuliangzhou tunnel in Xiangyang, China

Ballast water systems are an indispensable part of the installation of immersed tunnels and are utilized to increase or reduce the total weight of immersed tunnel elements in the processes of floating transportation, mooring and sinking. Traditional ballast water systems generally utilize rigid tanks, which consist of retaining walls made of steel-wood parts, for water supply and discharge. However, there are many drawbacks to the traditional ballast water system based on rigid tanks. For example, the construction of rigid ballast tanks is a complex process that is not very efficient and but is very costly. In addition, the processes of installing and dismantling the steel panels of rigid ballast tanks involve substantial welding work and many fire operations, which result in a substantial amount of smoke and dust inside the tunnel elements, resulting in a construction environment that is harmful to human health. Based on the Yuliangzhou Immersed Tunnel, which extends across the Hanjiang River in Xiangyang, China, a new type of ballast water system with flexible water-filled containers was developed to address the abovementioned problems. The equivalent volume method was proposed to determine the geometric size of the flexible water-filled containers. Field stability tests of the flexible water-filled containers were conducted on a 4.8% slope. The experimental results showed that the designed flexible container could meet the actual water demand of tunnel element immersion, without sliding or rolling during water injection, and that the relationship between the water amount and water pressure was approximately parabolic. In view of the specific functions in different installation stages of tunnel elements, the equipment layout of the new type of ballast water system was determined. Based on the processes of tunnel element installation, water supply and discharge technologies were designed. This system has been successfully applied in the Yuliangzhou Immersed Tunnel project, which supports the use of these technologies in immersed tunnel construction.