India recently welcomed one of its biggest landmarks! The country’s longest rail-cum-road bridge—Bogibeel Bridge over river Brahmaputra in Assam—was recently inaugurated, after having encountered several engineering and other challenges, almost 22 years after its foundation stone was laid in January 1997.
For the construction of this mammoth bridge, HCC set-up huge facilities on the left bank of the river for the three sequences of fabrication, assembly and launching.
Fabrication: Fabrication shops of 2,000 mt per month capacity were set up with two parallel bays. The team had assembled customised platforms in house to fabricate and fit various joints employing gas metal arc welding (GMAW). To ensure error-free welding, magnetic particle testing, dry penetration testing and ultrasonic testing were deployed. A specialised beam-making CMM machine from Italy has been used for the first time in India for the fabrication of box and I-sections using the submerged arc welding (SAW) procedure. A blasting gun was used to achieve the surface roughness of SA 2½ before applying paint.
Assembly: Thereafter, these fabricated sections were moved to the assembly shop where they were installed on their designated beds. They were correctly positioned through jacking and welded by GMAW process. These segments were arranged in a sequence and sent for the vertical assembly using horizontal lifters. After installation of the top and bottom girders, the final truss bridge dimension design chambers were examined thoroughly and approved to complete the fitout. A nose was fabricated and fitted on the first truss before launching it on the piers.
Launching: While determining the methodology of erecting the steel trusses on pillars, the engineers had two choices: lift the spans with floating cranes or erect them with a launching truss. They had to choose the more practical and economical option. After weighing the pros and cons, the HCC team came up with the solution of pulling the steel trusses with a set of jacks and winches on the pillars. This eliminated the need to enter the river, which was often turbulent during monsoons. Besides, it also ensured safer working conditions, precluded the mobilisation of a giant set-up on either side of the river having a width of 4.8 km, and accelerated the pace of the project. A 1,000-tonne hydraulic jack and strand jacks linked with the substructures have been used for moving the steel truss over the pillars. Two sets of steel cable strands were anchored to the end cross beams of the truss and hauled by hydraulic jacks. The truss slides over the launching bearing with the help of sliding plates, which were inserted at one end and taken out at the other, thereby moving the truss towards its desired position. To limit the required launching forces, the superstructure was pulled in four launching segments of 10 spans each. Thus, the superstructure was pulled over the pillars just like a train of 10 spans, with each span weighing 1,700 mt. Finally, the launching bearings were replaced by final bearings. The tracks were laid and the road was constructed adapting RCC construction. After fulfilling the electrical and other ancillary requirements, the assignment was completed.