CRI听力:Main tunnel of Hong Kong-Zhuhai-Macao Bridge to be completed by end of Nov

The Gongbei Tunnel has two storeys, each with three lanes of traffic. It is the world's largest highway tunnel in terms of cross-section area, and is over 30 meters under the ground. 

The tunnel's cross-section area is roughly the size of a basketball court, so workers separated the section into three parts while digging. One of the parts passes right beneath a customs clearance point at the Gongbei border crossing linking Zhuhai and Macao. And that made its construction a lot more difficult as workers had to avoid disrupting the running of the customs office, which clears nearly 300,000 inbound and outbound travelers a day. 

In the early stage of tunnel construction, 36 special steel pipes with a length of 255 meters and a diameter of 1.6 meters were installed. Saline water at minus 24 degrees Celsius was poured into the pipes to freeze the moist, sandy soil around the pipes. This enabled the engineers to carry out excavation safely. 

Later, workers had to thaw the artificially-frozen silt to facilitate further construction work inside the tunnel. 

Li Gang is an engineer with the Gongbei Tunnel Project.

"The freezing mission has already been completed. Now we are using hot saline water of over 30 degrees Celsius to fill the tubes to accelerate the thawing. We adopted the artificial forced thawing method with a hot saline water cycle. In this way, we can forcibly thaw the frozen soil in a very short period," said Li Gang.

Meanwhile, engineers have to monitor closely the sedimentation situation after unfreezing the tunnel. To ensure the soil surrounding the tunnel is stable, they will strengthen the soil body through grouting, which involves the pouring of a fluid form of concrete to fill gaps. 

Li Gang says that monitoring shows the current average sedimentation rate of the soil body per day is within two millimeters, which is basically under control.

"The soil body swells somewhat when frozen, but its volume would shrink in the unfreezing process and then the body would subside. If there is excessive sedimentation, we will do grouting at designated points. And the grout will expand the size of the unfrozen soil to keep its sedimentation under our control," said Li Gang.

Currently, the tunnel's civil construction work has been completed, while mechanical-electric facilities and pavement inside the tunnel are still under construction.

The tunnel is to be fully operational by the end of November.

The main structure of world's longest sea bridge linking Hong Kong, Zhuhai and Macao was completed in July.

It took seven years to build the bridge, which will be open at the end of the year.

The Y-shaped bridge will cut travel time between Hong Kong and Zhuhai from three hours to just 30 minutes, further integrating cities in the Pearl River Delta.