Bangladesh's first tunnel: Bangabandhu Tunnel a.k.a Karnaphuli Tunnel aiming on 'One City, Two town Concept' of Shanghai.

The Bangabandhu Tunnel, also known as the Karnaphuli Tunnel, is a significant infrastructure project in Bangladesh. The construction of a tunnel under the Karnaphuli River is a first for South Asia. The purpose of the tunnel is to improve transport connectivity in the region, reduce traffic congestion & travel time.

Under the project, two tubes were excavated under the river and subsequently converted into a road. The contractor was China Communications Construction Company Lim

ited (CCCC) & six consulting firms collectively participated in the project under the Bangladesh Bridges Authority.

The main challenges in building the tunnels are:-

1. The water & earth Pressure under the bottom of the river which was five times higher than the normal atmospheric pressure.

2.The tunneling excavation was carried out using cutterhead & the replacement of the cutting tools was a challenging task. Though cutting tools did not need to be replaced during the excavation of the second tube

3. Blocking the surrounding mud while constructing cross passage.

The implementation of first tube was the most challenging one as the experience gained in the first tube helped a lot in the 2nd tube.

A Tunnel Boring Machine (TBM) is employed to excavate soil and water. The TBM used beneath the Karnaphuli River is an impressive 94 metres in length and has a diameter of 12.12 metres. The machine had a boring capacity of 12.5 kilometers, of which 5 kilometers were utilized for this project. 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 tunnel were constructed beneath the mud soil on the riverbed. Eight segments, each weighing 11.5 tones, are placed around the circumference of the tunnel to form a ring with a diameter of two metres. A total of 19,616 segments were used in the construction of the two tubes.

A total of three cross passages have been constructed in the tunnel to connect the two tubes. 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. While constructing the cross passages, the surrounding mud was frozen out using calcium chloride brine. Then the frozen soil was cut into small pieces and removed. After that, the rings containing the tunneled segments were removed on the side of cross passage. It took about a year to complete the three cross passages using this method as the pressure under the river was high, the risk was also high. Small pieces of soil had to be slowly removed by freezing.

However, other important issues such as fire safety, emergency exit, and ventilation systems etc. were also considered.

The Bangabandhu Tunnel stands as a testament to Bangladesh's commitment to infrastructure development and improving the lives of its citizens. Its construction represents a significant milestone in the country's efforts to enhance connectivity, promote economic growth, and ensure sustainable development.

Article Courtesy: Nazmul Huda Abdullah (CE'20), Jannatul Akmum (CE'19)

Design Courtesy: Sourov Saha (CE'19)

Photo Courtesy: Dhaka Tribune