As night falls, another day of March draws to a close, with less than 50 days remaining until the road opens. Halfway up Mount Than Vu, the sounds of machinery intensify. Teams of workers, who have been on shift since 6 PM, are still working tirelessly, despite their weariness.
Returning from the northern tunnel entrance to his cramped, corrugated iron-walled room of less than 10 square meters, tucked away at the far end of the workers' barracks, engineer Bui Van Toan, the construction commander of the Than Vu tunnel under the Cienco4 Group, took off his dusty protective jacket and quietly walked to the outdoor water table so as not to disturb his colleague sleeping in the next bed.
"There were many difficulties. But what I thought was impossible is now almost within reach," Toan said.
Engineer Bui Van Toan briefs the workers on the work plan before they begin their shift.
While in charge of constructing the 567m left northern branch of the tunnel, he recounted that, unlike most other tunnel projects where the tunnel entrance is usually at the foot of the mountain, the starting point of the Than Vu tunnel is halfway up the mountain.
Geological conditions are constantly changing, and survey assessments are not always accurate; some sections assessed as hard rock are actually easily collapsible.
The most stressful period was when they reached the middle of the tunnel (around June 2023), as many of the most timid workers refused to go in to reinforce the tunnel arch after witnessing several instances of falling rocks, even though no serious incidents had occurred.
Colonel Le Minh Duc inspects the steel reinforcement construction before pouring concrete for the Than Vu bridge deck.
"They were right to demand that we ensure absolute safety before starting work. But what constitutes absolute safety? How should we explain that to them? As the highest-ranking official on the site, I took charge and signed a commitment stating that if any problems occurred, I would be fully responsible before the law," Toan recounted, adding that thanks to this, the workers gradually returned to their construction sites.
The excavation process was also more cautious and meticulous. According to the initial plan, if everything went smoothly, the contractor could excavate 3 meters per day, but due to difficult geological conditions, the average speed was only about 1 meter per day. On October 5, 2023, the last blast was safely detonated, and everyone felt relieved, ending the days of constant anxiety and uncertainty about when the risk might strike.
Mr. Vo Son Hai, the project manager of Hoa Hiep Co., Ltd., also admitted that working through areas with weak soil foundations is the most nerve-wracking period: "There was a case where the workers were building an arch, and after stepping back a few steps, a large rock fell down. Luckily, no one was injured, but about 10% of the workforce resigned afterward because they were too scared."
Weak geological conditions were also a bottleneck that caused construction of the Nui Vung tunnel on the Cam Lam - Vinh Hao expressway (the longest of the six mountain tunnels in the first phase of the North-South expressway project) to be repeatedly halted from December 2022 to June 2023.
The construction of the Nui Vung tunnel encountered weak geological conditions, and for four consecutive months, construction had to be temporarily halted several times to optimize construction solutions and ensure safety.
According to Mr. Dang Tien Thang, Deputy General Director of Cam Lam - Vinh Hao Joint Stock Company, the geological conditions in the southern tunnel were so complex that at the end of the excavation phase, water gushed out like a "water spout" in some places, and an 18-liter paint bucket filled with water in an instant.
"That difficulty forced us to choose: stop construction, delay progress, or continue digging, or accept the risks. While working and exploring solutions, the progress of tunnel excavation on the South side slowed down. If the North tunnel could excavate 3 meters per day, the South tunnel could only excavate 1 meter in 3 days," Mr. Thang recounted, adding that Deo Ca Group has increased resources and accelerated the progress of the North tunnel.
The excavation phase is calculated to be longer, and the execution cycle is optimized. Normally, the steps of drilling, blasting, excavation, reinforcement, and transportation take about 24 hours; now, all of these steps must be accelerated to reduce the time to 20 hours. Previously, a single cycle could only drill and blast 100 cubic meters; the accelerated process requires calculating an additional several tens of cubic meters.
When the tunnel sections were joined, approximately 350m were excavated on the southern side while about 1,800m was excavated on the northern side, instead of excavating half the tunnel length at each end as initially planned. Thanks to this solution, the Nui Vung tunnel was completed in August 2023.
"If I were to write a memoir, it would probably take me about six months to recount the construction journey on the Cam Lam - Vinh Hao section of the expressway," confided Mr. Nguyen Van Ngoi, Deputy General Director of Construction Investment Corporation 194 (Company 194), as he slowly walked along the road leading to the headquarters of the management board.
Having been involved in the project from its early stages, Mr. Ngoi was impressed by the construction period of the two largest overpasses in the project. "This is the bridge at Km55+982," pointing to the central pillar of the bridge, Mr. Ngoi said, explaining that the construction of bored piles was an extremely stressful task.
The area was all hard rock, and in the early stages of construction, conventional drilling methods only lasted a short time before the drill bit broke. At least five companies attempted to drill one pile before giving up; even with additional payment from the investor, they wouldn't continue. It took three months of trial and error, trying various solutions, before success finally came with using a larger drilling machine, a higher rotational speed, and a more specialized drill bit.
For Mr. Tran Nam Trung, General Director of Company 194, the most memorable experience was the Km60 bridge. The bridge is situated in a valley between two high mountains. According to the approved construction method (longitudinal girder launching), completing one span would take one month, meaning it would take 22 months to construct all 22 spans, and the bridge was expected to be completed around June 2024.
To optimize the project's progress, Company 194 switched from a longitudinal to a transverse launching method. The contractor expanded the service road, lowered 60-70 ton girder sections into the bridge pier area, and used two cranes of 350 and 450 tons to lift them, at an average speed of one girder section every three days. As a result, the bridge was completed in less than 18 months, meeting the deadline for opening to traffic on April 30, 2024.
The overpass at Km 60+552, with the tallest pillars (over 47m) on the Cam Lam - Vinh Hao expressway, was designed with a length of 883m, 21 pillars, and 22 spans, and has now been completed.
Boldly changing construction solutions is also the approach that helped Truong Son Construction Corporation optimize the construction time for 2 out of 4 of the largest overpasses in the Dien Chau - Bai Vot section of the North-South Expressway project.
Standing on the Than Vu 2 bridge - the longest overpass bridge in the project (1,300m), Colonel Le Minh Duc, Director of Package XL2, said that according to the initial plan, the construction of the bridge girders and deck would be carried out from the middle outwards to both ends.
During construction, realizing that the section of the Than Vu tunnel required a longer construction period, the contractor improved the method, doubling the number of construction lines instead of using just one.
Than Vu 2 Bridge is the longest overpass bridge (nearly 13,000m) on the Dien Chau - Bai Vot section of the expressway, consisting of 31 piers, 2 abutments with 32 spans, and 224 girders.
The 70-80 ton girder sections are not transported on the road but are lifted from the casting yard below and moved across the bridge surface. Work capacity has increased to 5 spans per month compared to the initial 2.5 spans per month.
"If we don't plan ahead, the construction time could be extended by at least another 1.5 months, meaning it won't be completed until May or June 2024. Given its characteristics as a high, long bridge with a large volume of work, the Than Vu 2 bridge was initially a project that many units were hesitant to undertake," Colonel Duc stated.
At Than Vu 2 Bridge, constructing 346 bored piles with a total length of approximately 6,000m on entirely rocky soil presented a huge challenge. Some piles, as long as 7m, took 7 days to complete. That's why the goal of completing the bored piles within 6 months was something no one dared to believe.
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