Desheng Tunnel exit.

On May 11, deep in the mountains at an altitude of 2,800 meters, when the last section of surrounding rock in the Desheng Tunnel of the Sichuan-Qinghai Railway was cut through, construction workers in work uniforms of different colors ran from both ends toward the middle to meet, embracing each other excitedly.

After the railway opens to traffic, a high-speed train will take only 7 minutes to pass through this nearly 23-kilometer-long tunnel, but tens of thousands of construction workers struggled for 10 years to build it. "I came here right after graduation, and now it's finally open—I'm truly thrilled!" Recalling the hardships of construction over the years, Li Yinan, head of the engineering department of the Chengdu-Lanzhou Railway project command headquarters of China Railway 16th Bureau, was filled with emotion.

With the breakthrough of the Desheng Tunnel, the construction of the Sichuan-Qinghai Railway has scored another victory. Within the year, high-speed trains will speed across the northwestern Sichuan plateau at an altitude of more than 3,000 meters for the ** time. How was this longest single-tube double-track railway tunnel in China cut through?

Multiple adverse geological conditions superimposed

More than 300 emergencies encountered

The Sichuan-Qinghai Railway runs from the Chengdu Plain to the high mountain plateau region of western Sichuan, with a maximum elevation difference of more than 2,000 meters. The geology along the route is extremely complex, with multiple adverse geological conditions superimposed, and the construction difficulty is unprecedented. The Desheng Tunnel is located in the area with the most complex geology along the entire line.

Schematic diagram of the section of the Sichuan-Qinghai Railway opening to traffic within the year.

"During the 10 years of construction, there were more than 300 emergencies that affected the construction schedule, including tunnel face collapses, mud bursts, water inrushes, and large deformations. It can be said that all the adverse geological problems that can be encountered in tunnel construction were encountered here," said Yin Houji, technical director of the Desheng Tunnel of the Chengdu-Lanzhou Railway project of China Railway Eryuan Engineering Group.

Why was it so difficult? He showed a geological structure map, on which it can be seen that the Desheng Tunnel is located in the zone of abrupt topographic change between the Sichuan Basin and the eastern side of the Qinghai-Tibet Plateau, in an "A"-shaped triangular fault block region formed by three active fault zones. It passes through 17 secondary folds and 1 fault, and is adjacent to the Minjiang active fault zone, with the closest distance being only 500 meters.

Construction of the Desheng Tunnel started in 2014, but survey and design work began as early as 2008. Lu Jinlin, geological director of the Desheng Tunnel of the Sichuan-Qinghai Railway of China Railway Eryuan Engineering Group, participated in the entire preliminary route selection process. "Over 6 years, we conducted geological exploration of 80 square kilometers of surface along the tunnel route," he said. A total of 8,000 meters of geological drilling and 30,000 meters of geophysical prospecting were carried out—cumulatively longer than the tunnel itself—yet no safe tunnel portal could be found through which to cut into the mountain. Finally, by "selecting the best among narrow options," the originally planned two short tunnels were merged into one long tunnel, and a single-tube double-track combined construction method was adopted, completely avoiding the impact of adverse geology on the construction and operational safety of the tunnel portal. As a result, the Desheng Tunnel started construction about a year later than the entire Chengdu-Huanglong section of the Sichuan-Qinghai Railway (formerly the Chengdu-Lanzhou Railway).

There are many adverse geological conditions along the Sichuan-Qinghai Railway. Materials used in the construction of the Desheng Tunnel were transported from the plain area 300 kilometers away, and every year the construction was affected by mudslides, landslides, dangerous rockfalls, heavy rainfall, and other disasters. Li Yinan recalled that one year, a bridge on National Highway 213 collapsed, large trucks were restricted from passing, and material transport vehicles had to detour 500 kilometers.

Deformation rate 14 times that of conventional tunnels

Customized technology and customized materials to help

The biggest trouble brought by adverse geology is large deformation of soft rock.

Walking into the newly breakthrough Desheng Tunnel, construction workers were carrying out support operations. Under the steel frame, a protruding mass of cement stood out conspicuously; on closer inspection, it also contained twisted steel frames. "This was originally the completed initial support, located near the rock wall on the outer periphery of the tunnel, but because of tunnel deformation, it was squeezed to here," Tan Yong, deputy chief engineer of the Chengdu-Lanzhou Railway project command headquarters of China Railway 16th Bureau, told reporters.

Comparison of the size of the support steel frame used in the Desheng Tunnel and that of a conventional tunnel; the outer layer is the support steel frame of the Desheng Tunnel.

The fault zone adjacent to the Desheng Tunnel is extremely active, bringing extremely strong in-situ stress to the tunnel. In some areas, the maximum pressure value reached 25.8 MPa, equivalent to an area the size of a fingernail bearing more than 500 jin of weight. The surrounding rock in the tunnel is mainly phyllite and slate, which crumbles at a pinch and turns to mud when exposed to water. High in-situ stress plus extremely poor stability of the surrounding rock caused large deformation of soft rock in a section of more than 7,400 meters of the Desheng Tunnel, with the deformed section accounting for as much as 32.6%.

Large deformation of soft rock is relatively common on the Chengdu-Lanzhou Railway, but the deformation in the Desheng Tunnel is more prominent than elsewhere along the line. In July 2023, when only the last 160 meters of tunneling remained, large deformation was encountered again. Monitoring found that the deformation rate was 14 times that of a conventional tunnel. The original 18-meter circular cross-section was squeezed into an irregular shape, with the narrowest point only 10 meters.

Thickened and lengthened specially made anchor pipes used in the construction of the Desheng Tunnel.

After several failures, the builders developed an "active control" technology: 10-meter-long hollow anchor pipes were driven into the rock formation, grout was injected inside to let the anchor pipes "take root" in the rock formation, and double-layer HW200 steel arches were installed to "lock and grip" the surrounding rock formation, forming a hard protective layer like a "turtle shell," after which lining construction was carried out. Because the deformation was extremely prominent, the materials used in the Desheng Tunnel were all specially made. The anchor pipes in the section with the largest deformation have a diameter of 51 millimeters, and the steel arches weigh as much as 50 kilograms per meter, 2.5 times that of conventional tunnels.

Rescue station built inside the tunnel

Evacuation at the ** time in case of danger

Construction was fraught with difficulties. Can the safety of the tunnel be guaranteed after it is completed and opened to traffic?

"Yes!" said Li Qun, Party branch secretary of the Chengdu-Lanzhou Construction Command of the Chengdu-Lanzhou Railway Company. The Chengdu-Huanglong section of the Sichuan-Qinghai Railway is also a lifeline connecting the Chengdu Plain and the northwestern Sichuan plateau. During construction, the Chengdu-Lanzhou Railway Company joined forces with top research teams from multiple fields across the country to carry out research on 14 technical topics, and some of the experience has already been promoted and applied in railways nationwide. Among them, the Desheng Tunnel was built to seismic intensity 8 requirements and has high safety. During construction, it has already withstood multiple earthquake tests, including the 2017 Jiuzhaigou earthquake. At that time, the straight-line distance between the epicenter and the Desheng Tunnel was only more than 60 kilometers, and the tunnel remained safe and sound.

Sign of the disaster prevention and rescue station inside the Desheng Tunnel.

Inside the Desheng Tunnel, a large disaster prevention and rescue station is also "hidden." This is the first of its kind among single-tube double-track combined railway tunnels in China and even in Asia.

Pedestrian evacuation passage of the disaster prevention and rescue station in the Desheng Tunnel.

Arriving at the middle section of the tunnel, a mileage section of about 500 meters long, there are several openings in the tunnel walls on both sides. Stepping into one and walking along the indicated direction, one will reach outside the tunnel in about 10 minutes. "These are escape passages, one every 50 meters on average. Once danger is detected, they can ensure the ** time safe evacuation of personnel," said Chen Fei, deputy manager of the Chengdu-Lanzhou Railway engineering command headquarters of China Railway Second Bureau. He also pointed overhead: "Look, there is also a smoke exhaust passage here to prevent smoke from becoming too dense and causing secondary injuries."

Signs on the disaster prevention and rescue passage of the Desheng Tunnel.

Yin Houji introduced that after a fire or other emergency occurs on a high-speed train, it can travel at most another 20 kilometers. Therefore, rescue stations are set up in tunnel groups and in railway tunnels longer than 20 kilometers. However, rescue stations are generally set up outside the tunnel, and most railways are built with two separate tubes, which facilitates the design of escape routes. The Desheng Tunnel, however, is a single-tube double-track tunnel, and given the special geographical conditions, the rescue station could only be set up inside the tunnel, which brought considerable difficulty to design and construction.

Schematic diagram of the disaster prevention and rescue station in the Desheng Tunnel.

The disaster prevention and rescue station has three levels—upper, middle, and lower, and left, middle, and right—with a total of 35 branch tunnels, forming an extremely complex cavern group around the main railway line. Its main functions are smoke exhaust on the upper level, evacuation on the middle level, and escape and withdrawal on the bottom level.

"Some branch tunnels pass under the main railway line, with the shortest distance being only 28 centimeters," Chen Fei said. Because there were too many branch tunnels and they were too close together, a group-cavern effect occurred during construction. Some tunnels that had already been completed developed deformation or cracks again when nearby tunnels were under construction. The construction adopted controlled blasting technology, zero-approach (crossing cavern) structure construction technology, and others, overcoming many difficulties and taking ten years to complete the rescue station.