How was the cross-harbour tunnel built? Can you share the science with us?

There are about 20 subsea tunnels in the world, the deepest of which is more than 100 meters from the sea surface. Among them, the Qinghai Tunnel in Tokyo, Japan is the longest undersea tunnel in the world, with a total length of about 54 kilometers. However, the Hong Kong-Zhuhai-Macao Cross-Harbour Tunnel in China is the most difficult and technologically advanced immersed tunnel in the world. So how was the cross-harbour tunnel built tens of meters above the sea? How to ensure no leakage?

The biggest problem in the construction of subsea tunnel is how to avoid the interference of seawater. At present, there are two most commonly used methods, one is shield method and the other is immersed tube method. Compared with immersed tube method, shield method is safer and more efficient, and it also has its own waterproof function. The excavation work will start a few kilometers away from the sea, and the route usually starts along the rock layer below the seagoing ship. The shield machine will automatically fill the annular segment during excavation. The error is less than 5 mm, and the most important thing is that special waterproof gaskets are installed between sections to prevent water leakage. These ring sections are all made of reinforced concrete, which has strong resistance to compression and subsidence and can effectively prevent tunnel collapse. The head of the shield machine is a huge knife card, and the blades on it can be replaced according to the different soil conditions. Driven by the power system, a large amount of soil can be removed every time the knife row rotates, and most of the soil will be squeezed to the inner wall of the tunnel by the shield machine. The soil and large stones used to reinforce the soil layer will be transported directly to the rear through the transportation system. Another advantage of electric construction machinery is that it is not affected by the weather. You know, offshore construction depends on God's face.

Compared with shield method, immersed tube method is a traditional way. Before the appearance of shield machine, most subsea tunnels in the world were built by immersed tube method. As the name implies, immersed tube method is a tunnel formed by sinking sealed pipes into the seabed and connecting them. However, due to the uneven seabed, some places are too soft to bear the gravity of the tunnel, so it is necessary to reinforce the seabed under the jurisdiction of the tunnel before sinking the pipeline into the seabed. The pipes used to assemble the tunnel are specially made, especially the waterproof effect at the joint of the pipes is more important. Transport the pipeline from the seaside factory to the place where it needs to be sunk. Because both ends of the pipeline are sealed and filled with air, it is very easy to transport. After reaching the designated location, the pipeline will sink to the seabed and be spliced together.

In order to prevent water leakage in the tunnel, special rubber sealing rings are installed at the joints, but because of this, the air in the pipeline will generate buoyancy. In order to prevent the submarine tunnel from floating, a large amount of gravel will be used to cover the upper and both sides of the tunnel. In China's Hong Kong-Zhuhai-Macao subsea tunnel, Class 33 pipelines are spliced at the seabed by immersed tube method, which requires very high environmental requirements due to the need for transportation, storage and splicing in seawater. But its advantages are also obvious, such as short construction period and low cost. So it is welcomed by all countries.