In June 2016, construction of the main structure of the Xiamen Haicang Tunnel began. Five years later, in June 2021, the Haicang Tunnel will officially open to traffic.

The Haicang Tunnel is the third undersea highway tunnel in mainland China and the second, more challenging undersea tunnel in Xiamen after the Xiang'an Undersea Tunnel. The Haicang Tunnel project features high technical content, a massive scale, and a complex construction environment, making it one of the most complex and difficult underground engineering projects in China today.

With the vast ocean overhead and the constant risk of water inrush, any slight negligence could ruin the project; it includes a rare double-arch tunnel worldwide; and the thinnest point between the tunnel crown and the road surface is only 5 meters... Since construction began, one challenge after another has confronted the builders.

Bringing together the essence of drill-and-blast undersea tunnel construction technology, overcoming the challenges of large-section water-rich strata and shallow-buried excavation, and establishing a new set of underground engineering construction technologies under complex urban conditions, the Haicang Tunnel construction is akin to a museum of underground engineering, representing the highest level of undersea and urban tunnel construction technology in China, with its techniques and methods reaching world-leading standards.

Recently, reporters interviewed the project construction unit, Xiamen Road and Bridge Engineering Investment Development Co., Ltd., to uncover the technical "roadblocks" overcome and world-class challenges tackled by the Haicang Tunnel over the past five years. The Haicang Tunnel inherits the "never give up" spirit of the Xiang'an Tunnel from Xiamen Road and Bridge, forging a new brand for Xiamen Road and Bridge in the new era and writing a new chapter for Xiamen's cross-island development.

Tenacious Efforts

Passing through a 160-meter undersea weathered trough in 465 days

Overcoming the greatest safety risk

160 meters, 465 days—these figures are deeply etched in the minds of the road and bridge builders.

The undersea section of the tunnel is 2.8 kilometers long, with the deepest point 71.2 meters below sea level. During the undersea crossing, it must pass through four fully and strongly weathered trough zones, the longest affecting 160 meters. Sun Zhipeng, section manager of Xiamen Road and Bridge Engineering Company, explained: "The rock mass in the weathered trough is fractured and weak, with fissures connected to seawater. Construction is like digging a hole in debris. If the weathered zone is not well reinforced, it can easily cause tunnel collapse and seawater inflow, posing extremely high safety risks."

Making such a large tunnel "solid" in debris is no easy task. The construction adopted the CRD method, combined with precise grouting reinforcement technology of full-hole one-time drilling and injection separation, using chemical grout to fill the fractures in the broken rock mass, sealing the water in the fissures and reinforcing the rock mass into a whole, "as if shattered ice refreezes into a block of ice," Sun Zhipeng vividly compared. At the same time, advanced drilling with borehole imaging technology was used to perform a "colonoscopy" on the reinforced rock mass, using cameras to investigate the rock conditions before drill-and-blast excavation.

"With the vast ocean overhead, we must be extremely cautious," Sun Zhipeng said. During construction, every detail of grouting was repeatedly studied and verified, and the equipment configuration and grouting technology for curtain grouting in different surrounding rocks were continuously explored, ensuring grouting effectiveness and shortening the cycle operation period from 2 months to 3 weeks, improving construction efficiency.

On September 10, 2020, the 160-meter undersea weathered trough—the greatest safety risk of the undersea tunnel—was safely and smoothly passed, taking 465 days from excavation to structural completion.

Ingenuity

World's largest cross-section double-arch tunnel

Construction technology world-leading

As Xiamen's second undersea tunnel, the Haicang Tunnel differs from its "big brother"—the Xiang'an Tunnel—as it is China's first project combining an undersea tunnel with an urban tunnel.

The island section of the Haicang Tunnel project mainly runs along Xinghu Road, a major traffic artery in the city. The project has successively overcome a series of technical challenges, including the service tunnel passing beneath the ultra-shallow buried weak surrounding rock section of Chang'an Road, the C ramp passing beneath the Yuehua Hotel, the world's largest cross-section double-arch tunnel, and the excavation of an ultra-large cross-section small-clearance tunnel beneath a major urban road in a complex environment.

Under Xinghu Road, there are double-arch tunnel sections of 160 meters and 240 meters at the C ramp bifurcation and near the Shigu Mountain Interchange, respectively. The double-arch tunnel is about 10 lanes wide, with an excavation cross-section of 550 square meters, the largest of its kind. The boundary is only 6 meters from surrounding buildings, and the thinnest point between the tunnel crown and the road surface is only 5 meters, making construction extremely difficult—another major challenge of the project. Sun Lei, project technical director of Xiamen Road and Bridge Engineering Company, explained: "We adopted a construction method of breaking the whole into parts and dividing into blocks. Simply put, during excavation, we divided the large excavation face into 4 to 6 smaller faces and advanced them sequentially, reinforcing before excavation and promptly supporting after excavation, strictly controlling deformation at each step."

A concise summary yet containing extremely intricate processes. With the efforts of all units, the double-arch tunnel was successfully holed through. The complex conversion of construction procedures and the extremely intricate processes in double-arch construction, with precision and order throughout, best demonstrate the craftsmanship spirit of the Haicang Tunnel, with its techniques and methods reaching world-leading standards.

Daring Innovation

Gathering the wisdom of academicians and experts

Blasting vibration controlled within 0.3 cm/s

Another 203-meter shallow-buried excavated tunnel on Xinghu Road was the final roadblock in the project construction. Lian Yichuan, section manager of Xiamen Road and Bridge Engineering Company, introduced: "This shallow-buried tunnel faces complex geological conditions, with dense surrounding residential areas and pipelines, and drill-and-blast construction requires blasting operations, so a balance must be struck between blasting efficiency and surrounding vibration impact."

To ensure construction progress and surrounding safety, and to effectively minimize the impact on the living environment of nearby residents, the project invited academicians of the Chinese Academy of Engineering, national engineering survey and design masters, and numerous industry experts to discuss construction plans and formulate blasting control measures. Through measures such as partial excavation, middle pipe roof shock absorption + shock-reduction holes, use of advanced blasting materials, and optimization of initiation sequence and charge quantity, the blasting vibration during construction was strictly controlled within 0.3 cm/s, less than one-fifth of the allowable value in national standards. The actual ground vibration felt by citizens was only equivalent to that caused by an ordinary freight vehicle passing by.

Striving for Excellence

Drainage system benchmarked against the Hong Kong-Zhuhai-Macao Bridge

Daily seepage less than 5,000 cubic meters

"Those in tunnel engineering all pay attention to one indicator—drainage volume," said Wang Xuebin, deputy general manager of Xiamen Road and Bridge Engineering Company. Drainage volume represents the technical level of tunnel engineering and affects the operational quality and service life of the tunnel.

The drainage system design parameters of the Haicang Tunnel are an upgraded version of the Xiang'an Tunnel, benchmarked against the Hong Kong-Zhuhai-Macao Bridge. It is understood that a daily seepage volume of less than 10,000 tons in an undersea tunnel is considered normal. Based on the Xiang'an Tunnel and incorporating green, energy-saving, and environmental protection concepts, the Haicang Tunnel has reconstructed a complete set of scientific and effective comprehensive management and control systems for undersea tunnel waterproofing and drainage from the design stage, implementing "water control thinking" in every aspect of construction. After completion, the daily seepage volume is less than 5,000 cubic meters, far less than the designed expected seepage, only one-third of underwater tunnels of the same scale, achieving high-level quality control throughout the construction process of the Haicang Tunnel, greatly reducing the burden on the drainage system during tunnel operation, and saving 5 million yuan annually in pumping and drainage electricity costs.

According to reports, the Haicang Tunnel follows the waterproofing and drainage policy of "blocking as the main measure, combining prevention and drainage, limited discharge, and comprehensive treatment," and has made a series of technical improvements and process breakthroughs based on traditional underwater tunnel experience. In water-rich surrounding rock sections, a fully enclosed scheme of pre-grouting + fully wrapped waterproof board is adopted; for sections with good geological conditions of Class II and III surrounding rock, a limited discharge scheme of post-grouting + semi-wrapped waterproof board is used; C50/P12 self-compacting high-performance concrete is used to improve the structural self-waterproofing capacity... It can be said that every process of waterproofing in the Haicang Tunnel has been taken to the extreme.

Facing Difficulties

Building a large-section undersea pump room at the lowest point of the tunnel

Equivalent to a five-story underground building

At 3 a.m. on December 6, 2020, at the lowest point of the Haicang Tunnel, 70 meters below sea level, the main structure of the undersea drainage pump room of the A1 section of the Xiamen Haicang Tunnel was successfully poured, marking the completion of all main works of the undersea section of the Haicang Tunnel.

The undersea drainage pump room is located below the normal tunnel structure and was constructed by re-expanding and blasting the completed service tunnel horizontally and vertically. It is reported that the drainage pump room has a transverse span of 17.82 meters, a longitudinal length of 100 meters, a maximum depth of 19.15 meters, and a capacity of 6,000 cubic meters. It can collect and discharge wastewater from the entire line's traffic tunnel and service tunnel to meet the operational needs of the tunnel.

Yang Zeyi, section manager of Xiamen Road and Bridge Engineering Company, stated that the excavation space of the undersea drainage pump room is equivalent to building a five-story, four-basketball-court-sized underground structure below the tunnel, with enormous construction difficulty and high risk.

It is understood that constructing such a drainage pump room requires re-expanding the previously completed service tunnel. The undersea drainage pump room is located at the lowest point of the tunnel, where expansion construction is affected by atmospheric pressure, making ventilation difficult, the environment humid, and construction conditions harsh. Moreover, to ensure construction quality and safety, the main structure needed to be poured in five stages.

Overcoming numerous difficulties, the undersea drainage pump room was successfully completed and put into use, providing another safety guarantee for tunnel operation. The "Undersea Wastewater Pump Room Structure" of the Xiamen Haicang Tunnel has been certified with a national utility model patent.