Opening the traffic map of Sichuan Province, a 'long dragon' traverses mountains and ridges, winding among the clouds, bringing the expressway to the heart of the Xiaoliang Mountains. This is the 123-kilometer-long Emei-Hanyuan Expressway. Currently, the world's deepest highway tunnel is embedded in this artery, namely the Grand Canyon Tunnel on the Emei-Hanyuan Expressway, with a maximum depth of 1944 meters.

Recently, after more than 6 years of arduous construction, the main works of the Grand Canyon Tunnel have been successfully completed, laying a solid foundation for the full-line opening and operation of the Emei-Hanyuan Expressway by the end of this year.

▲Image source: Jinguan News

What difficulties and challenges were overcome in building such a 'world's first'? How can operational safety be ensured after opening? Let us approach this world's deepest highway tunnel and explore the traffic 'dragon' hidden deep underground.

Adverse geology such as high ground temperature, gas, and karst caves accompany the project

Excavating at 'maximum depth' is high-risk and fraught with difficulties

With crisscrossing rivers and high mountains and deep valleys, the Emei-Hanyuan Expressway is located in the heart of the Xiaoliang Mountains at an altitude of up to 4000 meters. The special geographical environment has given rise to the 'world's deepest highway tunnel'.

The Emei-Hanyuan Expressway project starts in Emeishan City, Sichuan Province, passes through Ebian Yi Autonomous County, Jinkouhe District of Leshan City, and Ganluo County of Liangshan Prefecture, and ends in Hanyuan County of Ya'an City. The total investment is estimated at 20.521 billion yuan. It is one of the mountainous expressways under construction in Sichuan Province with the greatest construction difficulty and the highest cost per kilometer, with a bridge-tunnel ratio of 87.4%. Among them, the Grand Canyon Tunnel is located in the canyon and river valley section. The tunnel site has rugged terrain and complex geological structures, and is a controlling project for the entire line. It is invested by Shudao Expressway Group under Shudao Group and constructed by Sichuan Road & Bridge.

▲Image source: Jinguan News

'Depth refers to the vertical distance from the top of the tunnel excavation section to the natural ground surface,' said Li Huaming, chief engineer of Lehan Company under Shudao Expressway Group. 'The maximum depth of the Grand Canyon Tunnel on the Emei-Hanyuan Expressway is 1944 meters. This 'world's deepest' was not deliberately chosen, but is controlled by the elevations of the Jinkouhe Interchange and the Wushi Bridge Interchange at both ends of the tunnel.'

With horn-shaped peaks like a forest and deep-cut canyons, the harsh geological and topographical conditions have brought considerable difficulties and challenges to the project team. Various adverse geological problems such as rockbursts, high ground temperature, gas, and karst caves accompany the project.

Since construction started in early 2017, the Grand Canyon Tunnel has encountered geological and environmental disasters of varying degrees during excavation, including rockbursts, large karst caves, falling blocks from horizontal rock layers, and high ground temperature. Construction safety risks are high, and construction difficulty is extremely great.

'A large tunnel depth means the original crustal stress of the tunnel rock strata is relatively high. In addition, most of the surrounding rock of the Grand Canyon Tunnel is Grade 3 surrounding rock with high rock strength, making rockbursts of varying degrees highly likely,' Li Huaming explained. The high ground temperature of the surrounding rock in the tunnel, with a maximum temperature of 40°C, also seriously affects the construction working environment and efficiency.

To successfully break through this 'world's first', the frontline project team established a technical research group and a scientific research group, concentrating efforts on researching new processes and technologies in all aspects from design to construction. At the same time, they invited many well-known experts in the industry, including academicians of the Chinese Academy of Engineering, to guide on site, successfully removing many 'roadblocks' in the construction process.

Frequent rockbursts, rocks ejected like bullets

'Dodge, prevent, treat' to tackle world-class construction challenges

'Boom...' Not long ago, with a blast, the last few meters of rock wall on the left line of the Grand Canyon Tunnel of the Emei-Hanyuan Expressway were broken through, achieving double-tube breakthrough.

Since construction started in December 2017, the Grand Canyon Tunnel has undergone 1991 days and nights of arduous construction.

'This tunnel took 12 years from design to breakthrough, with a maximum depth of 1944 meters and a total length of 12.1 kilometers. The three numbers 12, 1944, and 12.1 have been written into my life and have become my pride,' said Ye Shaobo, project manager of Section 7, Bid 2 of the Emei-Hanyuan Expressway of Sichuan Road & Bridge, with emotion.

What was the biggest construction 'roadblock'? 'Rockburst!' Ye Shaobo blurted out.

Rockburst is a phenomenon commonly encountered in tunnel construction worldwide, and the rockburst phenomenon in the Grand Canyon Tunnel is extremely prominent.

'Rockburst is a world-class construction challenge that is difficult to completely eliminate. Theory is theory, but in practice there is still the risk of rockburst,' Ye Shaobo admitted. Although sufficient preparations for tunnel rockbursts had been made in the preliminary survey, due to the large depth, rockbursts still occurred frequently during tunnel excavation.

▲Image source: Jinguan News

Ye Shaobo clearly remembers that after nearly a year of excavation, weak rockbursts began to appear in the tunnel. 'Walking into the tunnel, you could hear muffled sounds, like distant thunder, not sharp thunder.'

Since March 2019, the rockburst problem at the exit end of the Grand Canyon Tunnel has become more obvious.

The deeper the excavation and the greater the depth, the stronger the rockburst. Rock ejection frequently occurred on the face of the tunnel face (the working face of tunnel excavation) and the surrounding rock of the arch waist. 'Rocks the size of a fist would pop out like bullets. Standing 40 meters away, being hit would still hurt badly.' Talking about his experience of being 'hit' on the calf by a rock, Ye Shaobo still remembers it vividly.

Prominent rockbursts were accompanied by collapses, with the maximum volume reaching 31 cubic meters. Several times, the construction trolleys for initial support and excavation were directly damaged, and construction faced great safety risks. At the most difficult time, the monthly excavation was only 48 meters, while under normal circumstances the monthly excavation footage should be about 120 meters.

With such great rockburst power, ejected rocks could injure people, and the safety of construction personnel became the primary issue. The project department adopted the method of dodging, preventing, and treating.

Dodging means leaving 1 to 7 hours after each excavation section to wait for rockbursts, and proceeding with the next construction step after the energy has been mostly released. Preventing includes measures such as sealing the rock mass with shotcrete, adding steel frames for initial support, and increasing mechanical equipment to reduce personnel, ensuring construction safety. Treating means using microseismic monitors to detect the rockburst level, and then helping the rock mass release energy through systematic drilling and repeated roof scaling.

48 meters, 80 meters, 120 meters... With a series of innovative measures combined, the excavation footage of the Grand Canyon Tunnel gradually increased. Some technologies applied in the Grand Canyon Tunnel became part of the scientific and technological achievement 'Key Technologies and Applications for Construction of Ultra-Long Depth High-Risk Highway Tunnels', which won the first prize of the 2022 Sichuan Provincial Science and Technology Progress Award. The 'anti-rockburst trolley' used in construction has obtained an invention patent.

Not only rockbursts, but during construction the project team also faced challenges from large karst caves, rainstorms, gas, and high ground temperature.

'During excavation, a karst cave was found on the face of the main line right tube of the Grand Canyon Tunnel (Hanyuan end). The cavity is 30 meters long, 35 meters high, and 28 meters wide at the bottom, and contains a large amount of harmful gas, posing high environmental danger,' Ye Shaobo said. To ensure smooth construction, the project team invited an expert group. After detailed surveys and demonstrations, they decided to adopt measures such as ventilation, gas detection, bottom backfilling, structural adjustment, and adding retaining walls, successfully passing through the karst cave.

Long tunnel 'divided' into multiple short tunnels by cross passages

Scientific operation design ensures bridge and tunnel traffic safety

The Emei-Hanyuan Expressway is built and opened in sections. Currently, only the Grand Canyon Tunnel and the 18-kilometer section at both ends remain unopened. The breakthrough of the Grand Canyon Tunnel marks that the Emei-Hanyuan Expressway has cleared the last 'bottleneck', and full-line opening is in 'countdown'.

For expressway tunnels over 10 kilometers, safety assurance after operation also attracts much attention.

'Although large depth is not directly related to tunnel operation difficulty, the Grand Canyon Tunnel is 12.1 kilometers long and is located in a dense section of the Emei-Hanyuan Expressway tunnel group. Emergency rescue support during operation is a major challenge,' Li Huaming said.

Among the pilot tasks for building a transportation powerhouse approved by the Ministry of Transport for Sichuan Province, the smart expressway construction pilot is one of the tasks. The Emei-Hanyuan Expressway is carrying out smart expressway construction focusing on tunnel scenarios, and has designed a complete process and plan for early warning and handling of emergencies during the operation stage.

Emergency rescue support after opening was carefully considered synchronously at the initial design of the Grand Canyon Tunnel.

The Grand Canyon Tunnel is designed as two-way four-lane, but at the exit and entrance ends of the single tube, traffic transfer zones are specially set up at 4-kilometer positions, each about 120 meters long, with a one-way width of four lanes.

'Moreover, between the left and right tubes of the tunnel, a 30-meter-long cross passage is opened every 700 meters,' Ye Shaobo said. These cross passages 'divide' the long tunnel into multiple short tunnels. Once an accident occurs inside the tunnel, vehicles can change lanes through the cross passages, facilitating the entry of rescue vehicles and avoiding long-time congestion inside the tunnel.

▲Image source: Sichuan Daily

Operational 'sharp tools' are being unveiled. 'About every 120 meters in the Grand Canyon Tunnel, a 'smart pole' will be installed,' Li Huaming said. The electronic equipment in the 'smart pole' is equivalent to 'clairvoyance' and 'clairaudience', enabling 24-hour monitoring of the entire section. Once an accident occurs on the road, the system can automatically identify it within seconds and automatically notify emergency personnel for handling.

The comfort of driving through the tunnel will also be improved. Ordinary tunnels do not need fan rooms, but the Grand Canyon Tunnel is an extra-long tunnel, and natural wind from the entrance and exit alone cannot ensure fresh air inside. The project team has set up an underground fan room at both the entrance and exit ends of the tunnel.

'There are 15 intersections in the underground fan room at the exit end of the left tube of the tunnel, and connecting air ducts are set between the intersections. Fresh air is blown in from an inclined shaft air inlet on the ground, driving the polluted air in the tunnel through the fan room and then discharged from the air outlet inclined shaft,' Ye Shaobo introduced. The underground fan room was poured in one go using a custom trolley. Compared with traditional fan rooms, it is smoother and more beautiful. After the tunnel is completed and operational, high-power fans will be installed at the main fan room position to extract exhaust gas and blow in fresh air at any time, keeping the air fresh inside the tunnel without a sense of pollution.

More far-reaching impacts are worth anticipating. 'The Emei-Hanyuan Expressway is a major transportation corridor and a major tourism artery,' Li Huaming said. The Emei-Hanyuan Expressway connects well-known tourist attractions such as Mount Emei, Heizhugou, the Grand Canyon, and Hanyuan Lake into a line and connects them to the expressway network, which will significantly improve traffic conditions along the route and drive tourism economic development. After the full line is completed and opened, it will take only more than an hour from Emei to Hanyuan. This will significantly improve the transportation infrastructure in the ethnic minority areas of Ebian Yi Autonomous County and Ganluo County, enhance traffic quality, and be of great significance for promoting economic and social development in ethnic minority areas, boosting ethnic unity and progress, and implementing the rural revitalization strategy.

Data shows that by the end of 2022, the total expressway mileage in Sichuan Province reached 9,179 kilometers, ranking third in the country, with 27 expressway corridors into and out of Sichuan completed. The province is making every effort to advance 39 expressway projects under construction with a total length of 3,500 kilometers. It is expected that by the end of this year, 12 projects (sections) will be completed and opened, and the total expressway mileage will reach 9,800 kilometers.