User:Angelaz00/sandbox/Nanjing Yangtze River Tunnel

The Nanjing Yangtze River Tunnel is a shield tunnel that was designed and built by the Nanjing University from 2005 to 2009 with an overall length of 5859 meters and an external diameter of 14.5 meters. The tunnel was opened for service in May 2010 and has received many awards due to its complex design since then.

Nanjing Yangtze River Tunnel was built under the Yangtze River in Nanjing, China and connects the Pukou district with Jiangxin Island. Geographically, the Yangtze River divides the city of Nanjing into two parts, and the division causes a big demand for efficient transportation between the two areas of the city. Nanjing Yangtze River Tunnel was built to solve this issue and was extremely successful in alleviating the heavy traffic in the city of Nanjing.

Due to the uncommon soil properties and difficult working condition of the tunnel, the construction team had to improve the Tunnel Boring Machines and open the pressure chamber with filter cake, the residue left over on the permeable medium after filtration,  in order to complete the construction project.

The usage of large diameter TBM tools, underwater tunneling performance and the soft soil geological condition make the Nanjing Yangtze River Tunnel stands out among the other construction projects in China.

Geology
At the construction cite by Yangtze River, the subsurface soils are mostly made up of clay and silty clay, a clay soil that contains up to 50 to 70 percent of silt. As a result, a total of sixty-two bores of soil samples were drilled to analyze the precise soil property of the site.

At the location of the tunnel, ground is mainly composed of fine sand and coarse sand that are permeable and low in clay content. Due to the large amount of quartz presented in the soil, the equipment used in the project needed extra maintenance to prevent bluntness.

Technology
Since the Yangtze River has particularly shallow cover, tunneling underwater was not an easy task to complete. Many designers and engineers worked together to analyze the technical difficulties arose during the construction due to complex sandy ground and high water pressure of the Yangtze river.

TBM Tools
During the project, engineers redesigned the slurry shield TBM (Tunnel Boring Machine) several times to improve the overall functionality of the machine. The original design of the cutting machine used in the construction is planned without any replacement, meaning that there would be only one tool involved in cutting the tunnel. However, the large amount of quartz presents on the ground causes the cutters to abrade easily.

In order to solve this issue, the construction team analyzed the materials and techniques of the current TBM tools and upgraded the machine with a larger hard alloy cross-section, a wider cutting tool mono-block alloy, and a better welding technology. The improved TBM tool was immediately placed into work and demonstrated its adaptability to the complicated geological condition.

The breakthrough in improving the Tunnel Boring Machines (TBM) tools used in the project played a big role in developing the technology of slurry shield with large size in China.

Maintenance
One main obstacle the construction team had to overcome was how to perform maintenance task on the TBM machine while it is completely below the water level.

Problem
The large-scaled TBM tool used in the project needs constant maintenance in order to function properly. However, since the machine is completely in the tunnel (below the water level), it was difficult for the construction team to perform maintenance operations on it.

Under such condition, opening the pressure chamber under high pressure seems to be the obvious and only solution. But such abnormal repairing process could be extremely dangerous. The explosion caused by the welding process might happen at any time.

Solution
Engineers worked closely together to analyze the current obstacle. After carefully examine the condition, they proposed that open the chamber and safely perform the maintenance process is feasible as long as the the internal and external pressures stay in equilibrium. Because compressed cannot resist the pressure in a highly permeable ground, an impermeable filter cake is required to balance the effective pressure with the help of compressed air. Filter cake is the residue left over on the permeable medium, and its thickness is directly proportional to its flow resistance.

For thus, filter cake was introduced to the maintenance process and successfully formed a dense and air proof surface on the excavation face to balance the internal and external air pressure.

With the help of filter cakes, the engineers were able to maintain a stable air pressure in the tunnel and to finish the maintenance on the TBM tool.

Awards Received
The Nanjing Yangtze River Tunnel was awarded The Luban Prize for Construction Project, which is the highest prize in building construction quality in China in 2013, making the tunnel the first river tunnel project to receive this prestigious national award.

Moreover, it won the National Quality Engineering Award in December, 2015 for its creative design and quality construction work.

In March, 2016, Nanjing Yangtze River Tunnel was awarded the Tien-yow Jeme Civil Engineering Prize due to the technological improvements made in the project.