Driving through the fully connected Muzhailing Extra-Long Tunnel on the Weiyuan-Wudu Expressway, one can see tunnel construction workers doing the finishing work of laying tiles in the tunnel. This tunnel, which runs through the belly of the mountain and took a full eight years to complete, has a total length of 15.226 kilometers, and its breakthrough marks a major technological breakthrough and conceptual innovation in China's tunnel construction.

Recently, reporters entered the Muzhailing Tunnel and, through exchanges with expert academicians and frontline workers, explored the hardships and twists, the glory and dreams of these eight years.

A Key Juncture on a Thousand-Mile Transportation Artery

Muzhailing, known in ancient times as Pululing, was named after the wooden fence fort (Mulan Zhai) established by the Minzhou Guard. As the watershed between the Zhang River and the Tao River, Muzhailing is nestled on the northern foot of the Minshan Mountains at an altitude of 3,200 meters, and is the main barrier on the route southward through Gansu and into Sichuan.

For a long time, vehicles crossing Muzhailing had to wind up the mountain along National Highway 212, then pass through the 1.71-kilometer National Highway Muzhailing Tunnel at an altitude of 3,216 meters, and continue downhill for several kilometers before reaching the foot of the mountain.

In rainy or snowy weather, with sharp curves and steep slopes, traffic accidents occurred constantly on this section, threatening the safety of passing vehicles. Accelerating expressway construction and building a new extra-long expressway tunnel through Muzhailing became the common aspiration of drivers and passengers alike.

In 2016, as the Weiyuan-Wudu Expressway invested and constructed by Gansu Provincial Highway Aviation Tourism Investment Group broke ground, the Weiyuan-Wudu Expressway entered the construction phase.

"Once the Muzhailing Tunnel is connected, the driving time between Weiyuan and Wudu in Gansu will be shortened from 4 hours to 2.5 hours," introduced Wang Zhicai, Party Secretary and Chairman of Changda Company under Gansu Provincial Highway Aviation Tourism Investment Group.

"By then, the G75 Lanzhou-Haikou National Expressway, one of the main north-south trunk lines of China's expressway network, will also be fully open to traffic. With the opening of this north-south artery, regional traffic conditions and the investment environment will be greatly improved, further boosting the high-quality development of regional economies and cultural tourism in western provinces and municipalities along the route such as Chongqing, Sichuan, and Gansu, and bringing historic opportunities for inland development and construction."

Given China's current infrastructure capabilities, 15.226 kilometers could generally be completed in three to four years. No one expected it would take a full eight years.

"I got married in May 2016, and after my honeymoon I went to the construction site. When I left, I told my wife, 'I expect three years, four at most, and I'll be back in Lanzhou with you.' I never imagined the project would run into a 'roadblock.' And clearing that 'roadblock' took eight years," said Gao Zexi, who stayed on the front line of the Muzhailing Extra-Long Tunnel construction site, with a touch of emotion.

"Boring a Hole Through Tofu Dregs"

In May 2016, construction of the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel fully began, and the participating units, full of confidence, started excavation from their designated positions.

To speed up tunnel excavation, in addition to tunneling from both ends of the entrance and exit, the construction units also excavated inclined shafts at multiple locations in the mountain, carrying out operations after reaching different positions along the tunnel.

"I came here as a technician right after graduation. When I followed the engineering team into the tunnel, I realized the portal was easy to enter, but excavation was extremely difficult," recalled Yang Baodong, chief engineer of Inclined Shaft No. 7 of the Muzhailing Extra-Long Tunnel project of China Railway 21st Bureau.

To resist tunnel deformation, the builders had to first support the rock mass with steel arches after each excavation segment, then spray concrete to form the primary lining, and after the concrete set, apply the secondary lining, thereby stabilizing the entire tunnel.

However, as the project advanced to Inclined Shaft No. 3, under strong geostress, the rock mass exerted enormous pressure on the steel arches. The daily convergence rate of the tunnel reached 19 centimeters, and the total deformation reached 0.7 to 2.05 meters.

"It really felt like boring a hole through tofu dregs—like using steel rebar and cement to hold up a mountain," Yang Baodong said. The steel arches would twist into the shape of fried dough twists within a few days.

Ordinary steel arches could not bear such weight, so the construction unit replaced them with higher-strength arches. Yet the result was still severe deformation. With no other choice, the builders could only advance bit by bit while continuously replacing the deformed steel arches behind them. Some steel arches even had to be replaced three times.

"At that time, it really was two steps forward, one step back," said Wang Zhijiao, former deputy director of the Dingxi Project Office of the Weiyuan-Wudu Expressway. Frontline construction workers kept leaving, and supervisors and client management personnel all fell into difficulty. During the hardest period, monthly excavation was less than 20 meters—less than the length of tunnel whose steel arches had to be removed and replaced—and construction progress once fell into "negative growth," becoming the "stranglehold" on the Weiyuan-Wudu Expressway.

Academicians Come to "Take the Pulse"

Replacing "I"-shaped steel arches with higher-strength H-shaped steel arches, adopting asymmetric double-layer arch support technology... To cope with large deformation in soft rock, the builders of the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel adopted every measure they could think of.

The plans were constantly being explored, adjusted, and improved, but with little effect. The slow progress and high costs sharply increased pressure on all participating parties. By mid-2017, some builders even began to think of withdrawing and prepared to transfer away from the Muzhailing Extra-Long Tunnel project.

Everything else had been built, but this was the bottleneck—what was to be done?

In September 2017, Changda Road Industry organized and invited top experts in geotechnical mechanics and tunneling, including Sun Jun and He Manchao, academicians of the Chinese Academy of Sciences, and Zheng Yingren, academician of the Chinese Academy of Engineering, to convene an expert consultation meeting on support technology, to diagnose the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel and pool efforts for a breakthrough.

The academicians visited the frontline construction site many times and ultimately identified the root of the problem.

Academician Sun Jun of the Chinese Academy of Sciences believed that the magnitude of deformation encountered during construction of the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel broke through the theories and levels of understanding followed in the underground engineering field. There was no mature experience in survey and design or engineering construction to draw on, making it, so to speak, a forbidden zone for tunnel engineering.

"Its difficulty lies not in its length, but in its complex and variable geological conditions, the concentration of carbonaceous slate strata and large fault fracture zones, and the extremely strong geological tectonic action," said He Manchao, academician of the Chinese Academy of Sciences and chairman of the Chinese Society for Rock Mechanics and Engineering, describing the difficulties and challenges faced in constructing the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel.

What exactly did the conclusions of the two academicians mean?

"It can be viewed from two different angles. One is that the rock mass encountered during tunnel excavation is very poor; the other is that the geological geostress is too strong," said Wang Zhicai, Party Secretary and Chairman of Changda Company under the Provincial Highway Aviation Tourism Investment Group, who was responsible for construction of the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel.

From the perspective of the rock mass, the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel passes entirely through carbonaceous slate. This carbonaceous slate is extremely fragile, turning to mud upon contact with water, with poor stability and a tendency to fall in blocks and collapse.

The tunnel route passes through 12 large fault fracture zones and carbonaceous slate strata. The geological tectonic action is strong, and the magnitude of deformation has broken through the theories and levels of understanding followed in the underground engineering field. There is no mature experience in survey and design to draw on, and it is difficult to predict the variation patterns of the surrounding rock. "Weak carbonaceous slate + extremely strong geostress" thus became a rare worldwide problem in the history of tunnel construction—large deformation in soft rock.

A Shift in Thinking Solves the Problem

"Faced with natural phenomena that inspire awe in us, it can evoke a new, more inclusive sense of wonder, stimulate our collective imagination, and prepare us to explore new ways to adapt to nature's call, making us more resilient."

At the "Innovation China · Smart Hui Lanzhou" forum on the Muzhailing Extra-Long Tunnel, the "bottleneck" project of the National Lanhai Expressway, Academician He Manchao quoted these words.

Through a series of on-site investigations and theoretical studies, Academicians Sun Jun and He Manchao broke with traditional passive support schemes and separately proposed two active support technical solutions—a yielding support system and an NPR anchor cable support system—transforming passive support into active support.

"Put simply, it is like 'sole-stitching': first reinforce the mountain mass, then use anchor cables to pull and reinforce the mountain mass so that the mountain itself bears part of the weight," explained Li Bin, deputy director of the Dingxi Section Project Office of the Weiyuan-Wudu Expressway. Test results showed that it met design requirements and constituted normal deformation.

Just as Academician He quoted, in the face of such complex geological conditions and loose rock structure, following the previous approach of "hard resistance" with reinforced concrete would lead to the dilemma of "two steps forward, one step back." By seeking more reasonable solutions with "awe" in mind, and ultimately using the "mountain" to hold up the mountain, this worldwide problem was finally solved.

In September 2018, an academician workstation led by Academicians Sun Jun and He Manchao was officially established, and technical research on the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel entered an accelerated phase.

"The speed suddenly increased to 45 meters per month, and we finally entered the rapid construction phase," Yang Baodong said.

In December 2022, the exit section was holed through on both lines;

In February 2023, the middle section was holed through on both lines;

On July 6, 2023, the entire line was holed through.

After eight years, the final breakthrough of the Weiyuan-Wudu Expressway Muzhailing Extra-Long Tunnel marks the conquest of large deformation in soft rock, a problem that has plagued tunnel construction worldwide. Its many scientific research and innovation achievements pioneered the treatment of large deformation in soft rock in China's highway tunnels, and a number of research results have been promoted and applied in more than 10 tunnel projects across the country.

From the moment daylight is seen again after a 12-minute drive, looking back at the long tunnel beneath the verdant Muzhailing, it is bound to leave a significant mark in the history of tunnel construction.