Inside the most complex geological tunnel on the North-South highway

Dien Chau - Bai Vot Expressway is part of the North-South Expressway, passing through Than Vu mountain range, constructed by Hoa Hiep Company Limited, Phuc Thanh Hung Investment Joint Stock Company and a consortium of investors as contractors.

According to the design, there are two tunnels, of which the left tunnel is 1,087 km long, the right tunnel is 1.13 km long, the north is in Dien Phu commune, Dien Chau district, the south is in Nghi Dong commune, Nghi Loc district. The width of each tunnel is 13.8 m, the distance between the two tunnel centers is 45 m.

The right tunnel will serve phase one of the project, the left tunnel is for the expressway extension in phase two.

On October 9, after excavating 748 m in the north and more than 350 m in the south, the contractor used explosives to close the tunnel, reinforce the arch, and then excavated the remaining rocks and soil to clear the route.

Mr. Vo Son Hai, Commander of the construction of Than Vu tunnel, Hoa Hiep Company Limited, said that the specialized agency assessed that the tunnel has the most complex terrain and geology among the tunnels through the mountains on the North-South expressway.

The tunnel is located halfway up the mountain, not at the foot of the mountain like the other tunnels. The tunnel on the left is next to a stream with many underground water sources. This area has weak geology, the rocks are strongly weathered and loose. The south side of the tunnel is all dirt, not rock, which is different from the original design documents.

"The construction process encountered many difficulties, especially when working from the two southern ends of the tunnel to the confluence point, we encountered a weak geological zone. If we are not careful, landslides can easily occur," said Mr. Hai.

To reinforce the tunnel arch, workers installed anchor pipes to create a landslide treatment cell. According to Mr. Hai, the original design only installed anchor pipes outside the tunnel entrance, but in reality, many landslides occurred during the excavation process, so they had to be installed at many locations inside to ensure the stability of the entire project.

The tunnel is designed to be permanent, with a reinforced concrete shell, constructed using Austrian technology. Inside the dark tunnel, the lighting system is constantly turned on for workers to work.

According to the Commander of the Than Vu Tunnel Construction, in early September, the inside of the tunnel had a landslide, causing hundreds of cubic meters of rock and soil to flow down. Throughout the entire construction process, similar incidents occurred many times, causing costs to increase, sometimes taking more than a month to handle.

Every time there is a landslide, engineers and workers must use concrete spraying machines to seal the entire landslide site, then close the pipe and pump in cement to turn the entire loose block into an artificial stone block, before continuing to dig.

Displacement monitoring equipment is installed at many locations in the tunnel to monitor whether the structure is shifting or experiencing any problems. If signs of subsidence or landslides are detected, the contractor must immediately take action.

After the tunnel opening ceremony on the afternoon of October 14, at 4:00 p.m. the same day, cars were able to run from the south of the tunnel in Nghi Loc district to the north of the tunnel in Dien Chau district.

Mr. Truong Duc Lien, Deputy General Director of Phuc Thanh Hung Investment Joint Stock Company, said that the tunnel opening is of great significance. After this point, the coordination of personnel, vehicles, supplies, and materials can go through the tunnel without having to travel on the official road, contributing to speeding up the construction progress of Than Vu tunnel as well as the entire project.

Steel frame and formwork system is used to construct concrete shell after tunneling.

Parallel to the right tunnel, the left Than Vu tunnel, 1,087 m long, is currently under construction. The tunnel is very airtight inside, and the project company has installed a large-capacity fan outside the door to provide clean air, helping staff and workers work safely.

From north to south, before Than Vu tunnel there are 2 viaducts, Xuan Duong 1 and Xuan Duong, each bridge has 20-22 spans, 800-880 m long.

After the tunnel, there are 3 bridges including Than Vu 1, Than Vu 2 and O O, of which Than Vu 2 bridge is designed with 32 spans, nearly 1,300 m long, some pillars are up to 50 m high. To build the bridge, the construction unit had to level the entire hillside in some locations. "The bridges have pillars 30-50 m high, all of which are of special grade," said Mr. Lien.

Located on the North-South Expressway, Dien Chau - Bai Vot road is about 50 km long, passing through two provinces of Nghe An (44.4 km) and Ha Tinh (4.9 km). The project started in May 2021 and is expected to be completed in May 2024. To meet transportation needs, phase one of the project has a scale of 4 lanes, a 17 m wide roadbed, and a design speed of 80 km/h.

The project has a total investment capital of over VND 11,150 billion, of which the investor mobilized capital is about VND 5,090 billion and the State capital participated in over VND 6,060 billion. The BOT contract was signed between the Ministry of Transport and the consortium of investors Hoa Hiep Company Limited - CIENCO4 - Nui Hong Investment Company Limited - Truong Son Construction Corporation - VINA2 Construction Investment Company and Phuc Thanh Hung Investment Joint Stock Company (project enterprise).