Amid the mountains and rivers, construction sites at key nodes of the Yuxiang (Chongqing-Hunan) Double-Track Expressway are bustling with activity. The Xianglong Tunnel has surpassed the 4,000-meter mark, the Daxi River Grand Bridge has been fully connected, and the cap beam hoisting operation of the Shuangbao Grand Bridge has been successfully completed... Since the project was launched in 2020, this mountainous expressway—with a total length of 280 kilometers and an average bridge-tunnel ratio as high as 80%—has been continuously updating its "progress bar."

According to reports, the Yuxiang Double-Track Expressway project was built with a joint investment of more than 64 billion yuan from over 30 central and state-owned enterprises. During design and construction, the project team carried out a series of major technical breakthroughs and innovatively applied multiple advanced techniques, successfully setting three world records in the field of expressway bridge construction—the world's largest span for a double-arch bridge, the world's largest span for a cable crane with independent pylons, and the world's largest lifting capacity for a cable crane of the same structure—and achieving two national firsts: the first domestic use of an intelligent cable active balance control system and the first comprehensive application of 3D laser scanning digital pre-assembly technology, providing "Chinese experience" for the construction of long-mileage, high bridge-tunnel ratio expressways worldwide.

Currently, this "giant dragon" in the deep mountains has taken shape. Let us explore its "growth code" together.

Complex bridge-tunnel structures and crisscrossing ecologically sensitive areas—apart from not being a plateau highway, it has "every problem imaginable"

Opening Beidou Navigation, one will find that from downtown Chongqing, driving on the expressway to Hunan currently mainly relies on the existing Baotou-Maoming Expressway's Chongqing-Hunan section (Yuxiang Expressway).

"First heading east through Wulong and Pengshui to Qianjiang District, then south through Huayuan in Hunan to enter the province. Overall, it requires a big detour—long mileage and prone to congestion," said Zhou Wuzhao, chairman of Chongqing Yuxiang Double-Track Expressway Co., Ltd. The Yuxiang Double-Track Expressway is a capacity-expansion route of the Yuxiang Expressway.

The reporter learned that the Yuxiang Double-Track Expressway project starts from downtown Chongqing and runs southeast through Banan, Nanchuan, Wulong, Pengshui, and Youyang, finally connecting to the Youyang-Yanhe Expressway to leave Chongqing and enter Hunan. After the entire line opens to traffic, vehicles will no longer need to detour through Qianjiang District, shortening the distance from downtown Chongqing to Hunan by 47.8 kilometers compared with the existing Yuxiang Expressway, and shortening the distance to Yanhe, Guizhou by 79 kilometers. This is of great significance for further enhancing traffic capacity in the southwestern region, promoting integrated development among Chongqing, Hunan, and Guizhou, and accelerating the building of an international comprehensive transportation hub.

However, turning the blueprint into reality is not easy.

"Among existing or under-construction expressways nationwide, some have long mileage but low bridge-tunnel ratios, while others have high bridge-tunnel ratios but short mileage. The Yuxiang Double-Track Expressway has the 'dual difficulty' of long mileage and a high bridge-tunnel ratio," Zhou Wuzhao introduced.

The bridge-tunnel ratio refers to the proportion of bridges and tunnels in the total mileage in highway and railway construction. The higher the bridge-tunnel ratio, the greater the engineering difficulty. It is understood that the bridge-tunnel ratio of ordinary expressways is about 20% to 30%. In southwestern China, affected by terrain, expressway bridge-tunnel ratios are relatively high, usually between 60% and 70%. However, the average bridge-tunnel ratio of the entire Yuxiang Double-Track Expressway is as high as 80%, with the section from Wulong, Chongqing to Daozhen, Guizhou reaching 92%—the difficulty is imaginable.

"Apart from not being a plateau highway and thus not having permafrost or oxygen deficiency issues, it has almost every problem that may be encountered in high-difficulty expressway construction—water erosion, steep slopes and deep valleys, natural chasms and cliffs, karst geology, toxic gases—and every one of them is difficult," Zhou Wuzhao said.

Chongqing was built along rivers and prospered beside mountains. The great rivers and mountains give this city unique scenery and regional characteristics, while the natural barriers formed by layered peaks and surrounding waters also bring enormous challenges to expressway construction.

The Yuxiang Double-Track Expressway has 139 bridges and 62 tunnels along its entire route, including 17 grand bridges with a total multi-span length greater than 1,000 meters or a single span greater than 150 meters, and as many as 18 extra-long tunnels over 3,000 meters. The route crisscrosses multiple ecologically sensitive areas such as the Wulong Karst World Natural Heritage Site, Wulong Karst National Geopark, Furong River National Key Scenic Area, Youyang National Geopark, Maoyun Mountain Nature Reserve, and the Wujiang Hundred-Li Gallery. Along the route are distributed various unfavorable geological conditions such as karst, dangerous rock, and landslides, making the geological landforms and ecological environment extremely sensitive and complex.

According to statistics, the Yuxiang Double-Track Expressway crosses or intersects various roads and rivers 128 times; there are 21 gas tunnels, including 5 high-gas tunnels, where a slight lapse could cause unimaginable consequences.

No matter how rugged the journey, it cannot stop those determined to move forward. Facing extremely complex bridge-tunnel structures and numerous unfavorable factors, the Yuxiang Double-Track Expressway project team actively explored and innovated, making every effort to overcome the many difficulties of mountainous expressway construction and ecological environmental protection.

Independent innovation successively cracked the hardest "nuts"—multiple major scientific and technological breakthroughs made construction more efficient and more precise

In Wulong, Chongqing, above the "double-gully valley" of Dadong River and Xiaohegou, two red continuous arches stand out even more against the verdant landscape. The successful completion of the cap beam hoisting operation of the Shuangbao Grand Bridge, known as the "Rainbow Bridge," means that all substructure construction of the main bridge has been completed, entering the superstructure construction phase.

This is a key control project of the Yuxiang Double-Track Expressway and also the world's largest-span deck-type double-span continuous concrete-filled steel tube arch bridge. According to reports, the arch bridge has a total length of 1,620 meters, six lanes in both directions, a single deck width of 16 meters, and a design speed of 80 kilometers per hour. Its typical "W-shaped canyon terrain + karst geology" posed enormous challenges to the design team.

Not just the Shuangbao Grand Bridge. The high bridge-tunnel ratio determines that bridges and tunnels—especially grand bridges and extra-long tunnels—are the "crux" of the entire Yuxiang Double-Track Expressway construction. To overcome engineering construction difficulties such as extra-large continuous arch bridges under complex terrain conditions and high-gas karst large-section tunnels, the Yuxiang Double-Track Expressway project team launched a "brainstorm," carrying out multiple major scientific and technological breakthroughs including research on key technologies for the design and construction of canyon large-span deck-type double-span continuous concrete-filled steel tube arch bridges, providing reference for high-difficulty expressway bridge-tunnel construction nationwide and worldwide.

Taking the Shuangbao Grand Bridge as an example, based on the terrain and landforms at the bridge site, after repeated on-site exploration and measurement, and comprehensively considering various complex conditions such as stress under special geological conditions, the design team ultimately determined the structure of the Shuangbao Grand Bridge as a deck-type double-span continuous concrete-filled steel tube truss arch bridge.

However, the 2×405-meter double span brought numerous difficulties to the bridge's builders.

"We often say build a bridge when encountering a river, but little do people know that before building a bridge, one must first build pylons," introduced Yin Tao, chief engineer of the Shuangbao Grand Bridge project. During the construction of large-span suspension bridges and arch bridges, steel wire cables are usually threaded between pylons on both banks to form a cable crane for hoisting and transferring important load-bearing components of the bridge. Suspension bridges usually use the main towers on both banks as pylons, but arch bridges have no main towers on either bank, so new pylons must be built. However, the Shuangbao Grand Bridge has a large span and requires hoisting many steel arch rib segments of large volume, which ordinary pylons cannot meet. A kilometer-class cable crane system must be specially built for it.

But the terrain around the bridge is narrow with limited construction space—where to place the pylons? After repeated demonstration, the project team boldly decided to place the west bank pylon directly on top of Pier No. 8, and the east bank pylon on the open ground between Pier No. 12 and Pier No. 13.

"Pier No. 8 is one of the main piers of the Shuangbao Grand Bridge, with a height of 97.5 meters above ground. By erecting a temporary tower frame on top of the pier, a west bank pylon composed of 'pier + temporary tower frame' can be formed, which ensures load-bearing capacity while saving project cost," Yin Tao said.

The project team pre-divided the pylons into individual tower frame components, then assembled them upward like "building blocks." In this way, with the 167-meter-high pylons on the east and west banks as side supports, cables could be extended to carry out hoisting operations across the "double-gully valley."

"With a span of 900 meters and a maximum lifting capacity of 210 tons, it set two world records in the field of expressway bridge construction—the world's largest span for a cable crane with independent pylons and the world's largest lifting capacity for a cable crane of the same structure," Yin Tao introduced.

Although the cable crane was built, this was not the only construction challenge.

The curved arch ring, as the main load-bearing component of the Shuangbao Grand Bridge, can transfer loads to the piers, abutments, and foundations. The arch ribs, as the "skeleton" of the main arch ring, are composed of individual high-strength steel components, also known as arch rib segments. The hoisting of arch rib segments must be completed to form the curved arch ring.

Pier No. 9, located on the middle hill of the "W-shaped canyon terrain," is the connection point of the Shuangbao Grand Bridge's double arches and the most suitable "starting point" for arch rib installation. However, what troubled the project team was that the arch rib segments were too heavy to be lifted simultaneously on both sides of Pier No. 9—they could only be hoisted sequentially. But this would cause Pier No. 9 to experience load deviation between the left and right sides, and poor control could affect the overall stability of the bridge. "It's like a balance scale—if the two sides are unbalanced, the middle pier will shift, driving the displacement of the buckle cables fixed on the pier, and the arch rib segments connected to the buckle cables will also shift, bringing safety hazards to the bridge," Yin Tao said.

To crack this "hard nut," the project team carried out technical research and developed an intelligent cable active balance control system integrating multiple cutting-edge technologies such as the Internet, cloud computing, and automatic control, and applied it domestically for the first time to solve the problem of unbalanced buckling of the middle pier. At the same time, it was the first nationwide comprehensive application of 3D laser scanning digital pre-assembly technology in the fabrication and installation of steel arch ribs, solving various problems that might arise during physical assembly in advance and successfully controlling assembly precision to the millimeter level.

Using these innovative technologies, the project team completed the assembly of all arch rib segments of the Shuangbao Grand Bridge in only 8 months, more than a month ahead of schedule, with assembly precision far exceeding design requirements.

Construction does not damage native vegetation—expressway construction and environmental protection go hand in hand

Not long ago, accompanied by the last blasting at the tunnel face, the 2,200-meter Qiaopingshan Tunnel successfully achieved double-tube breakthrough.

The "front-mounted portal" construction method is a highlight of this tunnel's construction. This is an environmentally friendly tunnel-entry method. Advancing tunnel construction in this way was an important consideration by the project team to protect surrounding native vegetation and reduce ecological damage from tunnel excavation.

According to reports, the surrounding rock grade at the exit of the Qiaopingshan Tunnel reaches Grade IV and V, distributed in horizontal thin layers. The portal section is located in a landslide body and is prone to collapse. In addition, there is a high-voltage tower above the tunnel, and it crosses over the Chongqing Railway East Loop Line and the Yuxiang High-Speed Railway, with small clear distance and large cross-section. Using the traditional slope-cutting tunnel-entry method could also solve the construction difficulties and would be lower in cost, but due to the large amount of earthwork excavation that would significantly impact vegetation, the project team ultimately decided to choose the "front-mounted portal" construction method. That is, entering the tunnel in a "no slope cutting, zero excavation" manner, using temporary lining as protection for micro-excavated side and face slopes before open-cut tunnel construction, significantly reducing the excavation of side and face slopes at the tunnel exit and achieving a "silent tunnel entry" without damaging native vegetation.

"Simply put, before formally entering the tunnel, a semicircular passage is manually set up to naturally 'kiss' the mountain body. Compared with the slope-cutting tunnel-entry method that 'cuts away' part of the mountain, this can better prevent side and face slope hazards and maximize the protection of native vegetation and mountain balance," said Yang Yu, project manager of the Bapeng Road section of Chongqing Yuxiang Double-Track Expressway Co., Ltd.

Carrying the concept of green development through to the end has been the persistent commitment of the entire Yuxiang Double-Track Expressway construction from beginning to end. To implement this important concept, the project team insisted on formulating the "greenest plan" for every bridge and every tunnel.

In 2022, a video of a concrete mixer truck "walking a tightrope" in the air went viral. This "tightrope," with a span of 245 meters, a width of 4.3 meters, and a maximum vehicle load of 45 tons, is the cableway bridge built by the Yuxiang Double-Track Expressway project team to solve the material supply problem for the construction of Pier No. 9 of the Shuangbao Grand Bridge—a veritable green temporary "aerial corridor."

Pier No. 9 of the Shuangbao Grand Bridge is surrounded by canyon cliffs front, back, left, and right. The mountain road leading to the bridge site is narrow, like an "isolated island." The construction of Pier No. 9 requires more than 2,300 tons of steel rebar and 39,000 cubic meters of concrete, plus a large amount of spoil to be transported out. Using conventional methods would require land acquisition of about 15 mu and the special construction of a mixing plant and a steel processing plant to meet such enormous material demand. But the "isolated island" site is too narrow to accommodate additional mixing and steel processing plants.

After surveying the terrain, the project technical team decided to use lightweight, high-strength ultra-thin steel box structures as the bridge deck and 33 strands of high-strength low-relaxation unbonded prestressed steel strands as deck cables and stability cables, building a cableway bridge between Pier No. 8 and Pier No. 9 for concrete mixer trucks and other large vehicles to transport construction materials. With this cableway bridge, the construction of Pier No. 9 no longer required additional land acquisition for mixing and steel processing plants, saving 2,800 forest trees from being cut down, while reducing diesel consumption for material transportation by about 2,300 liters and reducing exhaust emissions by more than 20,000 cubic meters.

Adhering to the parallel approach of road construction and environmental protection, the project team also adopted measures such as centralized storage of stripped topsoil, mud purification and sewage treatment, fully enclosed intelligent CNC equipment for crushed stone processing, and environmentally friendly intelligent multifunctional sweepers, effectively reducing mud sand content and sediment at the bottom of holes, pile foundation settlement, and mud transport volume, achieving the cleaning of reusable mixer trucks and the separation, collection, and reuse of sand, stone, and mud from waste, and reducing or eliminating the impact on the surrounding environment from pile foundation construction and discarded tunnel spoil.