On February 23, the ninth day of the Lunar New Year, to welcome the opening of the Hong Kong-Zhuhai-Macao Bridge, the Gongbei Tunnel on the Zhuhai Link Road of the bridge had been thawed for over three months. Five meters above it is China's largest land port—Zhuhai Gongbei Port.
Why did this double-deck highway tunnel, with the world's largest cross-section, need to be thawed? After thawing, has there been any settlement in the strata? What impact has it had on the Gongbei Customs above? On February 23, a reporter from Science and Technology Daily visited the tunnel site.
"After the tunnel was thawed, we conducted continuous thaw settlement monitoring and maintenance. The monitoring data shows that the stratum settlement of the tunnel is controlled within the allowable range, with basically no settlement," said Pan Jianli, project manager of the Gongbei Tunnel Project Department of China Railway 18th Bureau Group.
Data from Gongbei Customs shows that on the 23rd, the number of people passing through exceeded 400,000. "Three months after the tunnel was thawed, there has been no impact on Gongbei Customs."
The Gongbei Tunnel, with a total length of 2,741 meters, is a core control project of the Zhuhai Link Road of the Hong Kong-Zhuhai-Macao Bridge and the only channel connecting the main project of the bridge to Zhuhai.
The 255-meter-long mined section of the tunnel beneath Gongbei Port is extremely challenging, with a maximum cross-section height of over 20 meters. Its strata are weak and rich in groundwater, and it is less than 5 meters from the ground. The slightest mistake could cause a significant negative impact on customs clearance at Gongbei Port.
"The 255-meter mined section of the Gongbei Tunnel concentrates all the difficulties of tunnel construction encountered by mankind to date," said Wang Mengshu, an academician of the Chinese Academy of Engineering.
Tunnel construction is extremely challenging. In the soft geology, the world's largest curved pipe roof was erected, giving the tunnel an "iron shirt"; later, using the curved pipe roof, a super "large ice bucket" with a diameter of 23 meters, a thickness of 2.6 meters, and a longitudinal length of 255 meters was built beneath the port; finally, the tunnel was excavated under the protection of this super "large ice bucket".
The Gongbei Tunnel of the Hong Kong-Zhuhai-Macao Bridge started construction in August 2012 and was completed in April 2017.
"After the tunnel's own structure was completed, the freezing circle used in construction was powered off, and all thawing work was completed by the end of October 2017," said Pan Jianli.
"The deepest part of the Gongbei Tunnel is more than 30 meters underground, where the temperature is different from the ground. If it were to thaw completely under natural conditions, it might take several years, which would greatly extend the risk period of the project," Pan Jianli told the reporter. To coordinate with the full opening of the Hong Kong-Zhuhai-Macao Bridge, the tunnel adopted forced thawing to accelerate the thawing process, and then, based on monitoring data, real-time thaw settlement grouting was carried out to reduce the impact of thawing on surface buildings and port customs clearance.
This year is the fifth consecutive Spring Festival that Pan Jianli has spent on the construction site, on duty together with most of the builders in the project department. "As an engineer, it is a great honor to encounter a world-class project like the Gongbei Tunnel. It pioneered the first 'curved pipe roof + freezing method' construction method in China, and its pipe roof length, pipe roof area, and freezing scale have all set world records." Although somewhat apologetic, Pan Jianli and all the builders are very proud.
Since the construction of the Gongbei Tunnel of the Hong Kong-Zhuhai-Macao Bridge started in August 2012, overtime has become the norm for Pan Jianli and his team, and the Spring Festival holiday has also become their "special" working day.
Since the construction of the Gongbei Tunnel began in 2012, within five years, more than 600 million person-times and nearly 160 million vehicle-times have safely and smoothly entered and exited the border. "These numbers are the best reward for us," said Pan Jianli.