The challenges in building the Bangabandhu Tunnel

While underwater and subterranean tunnelling to reduce travel distances has long been a tradition in China, the US, and Europe, the Bangabandhu Sheikh Mujibur Rahman Tunnel under the Karnaphuli River in Chattogram marks a new era for South Asia.

Typically, in construction projects like this, a Tunnel Boring Machine (TBM) is employed to excavate soil and water, creating tunnels under rivers. The TBM used beneath the Karnaphuli River is an impressive 94 metres in length and has a diameter of 12.12 metres. 

This massive machine weighs approximately 2,200 tonnes and with a colossal cutterhead, which itself weighs 220 tonnes. The specialised cutterhead is complemented by seven different types of 455 cutters and blades, all working together to make this engineering feat possible.

Excavation is primarily carried out using the cutterhead, and changing the cutterhead requires access to the chamber twice. The pivotal stage in tunnel construction is tunnelling through excavation. The most daunting aspect of this undertaking was the replacement of cutting tools within the chamber. Several individuals involved in the project, including two of its engineers, shared their tales of adventure with The Business Standard.

Usually, the pressure at the bottom of a river is several times higher than that in normal conditions. This pressure is primarily generated by water and the earth. Even with the necessary equipment, including oxygen, it is nearly impossible to remain in such an environment due to the immense pressure. The pressure level increases with the depth underwater. According to the engineers involved in the tunnel project, the maximum pressure encountered at the construction site beneath the Karnaphuli River was five times that of normal atmospheric pressure.

The China Communications and Construction Company Ltd, contracted by the Bridges Authority, commenced excavation of the first tube, the North one, on 24 February 2019. The tube was completed on 2 August 2020. During this period, the cutter tools on the boring machine's cutter head had to be changed twice. For this task, a team of 42 experts was brought in from China. These experts entered the cutterhead chamber and replaced the tools.

Engineers noted that the cutting tools had to be changed when the cutterhead was flush with the ground on the riverbed. At this depth, the pressure was immense. The experts from China could not remain in the chamber for more than half an hour at a time. As a result, they worked in shifts. Additionally, they would first enter the chamber for brief periods to acclimatise to the excessive pressure. This acclimatisation process is essential, as the pressure is unbearable for those who are not accustomed to it.

Of the 42 experts, some had prior experience in the construction of 52 tunnels. Their expertise and knowledge were invaluable in ensuring the successful completion of this arduous task.

The excavation of the first tube took approximately 18 months. However, the experience gained during this process enabled the excavation of the second tube to be completed in just 10 months. Excavation commenced on 12 December 2020, and was concluded on 7 October 2021. The reduced excavation time was attributed to a better understanding of the soil type and the experience gained from the previous excavation. Unlike the first tube, the cutting tools did not need to be replaced during the excavation of the second tube.

A total of 560,000 cubic metres of soil were removed from each tube, with approximately 280,000 cubic metres being removed from each side.

The TBM utilised in the construction of the Bangabandhu Tunnel was manufactured at the contractor's factory in Changshu, China, which is located between Shanghai and Nanjing. The construction of the device took approximately one year and cost Tk500 crore. Once the necessary components of the machine were manufactured, they were assembled at the factory. Following successful trials, the machine was disassembled and transported to Bangladesh in 120 containers. Upon arrival, it was reassembled and put into operation. The machine had a boring capacity of 12.5 kilometres, of which 5 kilometres were utilised for this project.

The initial discussion regarding the construction of a tunnel under the Buriganga River through the unused section was discussed by the Bridges Division. However, there is uncertainty as to whether the current contractor company will be tasked with the job. As per global practice, a TBM is typically used for tunnel construction. However, no further discussions have taken place on this matter. In accordance with the contractor agreement, the TBM was returned to China by the contractor company upon completion of the excavation.

There are two primary methods used for soil removal during the construction of the Karnaphuli River Tunnel. One is the up-pressure method, employed for excavating hard soil, while the other is the slurry-pressure method, utilised for excavating soft soil. Due to the presence of soft soil beneath the Karnaphuli River, the slurry-pressure method was chosen for this project.

The two tubes of the Karnaphuli River Tunnel are constructed beneath the mud soil on the riverbed. Segments are excavated and placed by a TBM. Eight segments, each weighing 11.5 tonnes, are placed around the circumference of the tunnel to form a ring with a diameter of two metres. Each ring weighs approximately 88 tonnes. During tunnelling, 5-8 rings are installed daily. A total of 19,616 segments were used in the construction of the two tubes.

'It took one year to connect the two tubes'

A total of three cross passages have been constructed in the tunnel to connect the two tubes. These passages allow vehicles to make a U-turn in the event of an accident inside the tunnel. The main tunnel, which is 2,450 metres in length, was divided into four parts, and the three cross passages were constructed approximately 600 metres apart. The cross passages are two metres in length and 2.5 metres in width.

The distance between the two tubes built under the river is 10–12 metres. This means that there is mud and soil surrounding both tubes. The question arises as to how the surrounding mud is blocked when the cross passages are made.

Engineers explained that the soil is removed using a freezing method. Before removing the rings containing the tunnelled segments, calcium chloride brine was placed at a temperature of minus 33 degrees Celsius. At minus 28 degrees Celsius, the soil would turn into ice. As a result, the soil was frozen for 100 days. The segment was then removed from the frozen soil.

In order to move each segment, it was necessary to freeze two to three times the surrounding ground. The frozen soil was then cut into small pieces and removed. In this way, 10 to 16 metres of soil had to be cut for each cross passage. It took about a year to complete the three cross passages using this method.

Engineers commented that this was a time-consuming and delicate process. The results were also challenging to achieve. Because the pressure under the river is high, the risk is also high. Small pieces of soil had to be slowly removed by freezing. There was no opportunity to use large equipment.

Project director Md Harunur Rashid Chowdhury told TBS, "We have faced a number of technical issues throughout the project. With the help of Chinese specialists, we have resolved them and achieved success."