Two bridges coming up at two ends of Calcutta are set to ease traffic movement once they are commissioned.
The bridges — both 58 years old — one in the north and one in the south had to be pulled down: the Majherhat bridge, after a portion of it collapsed in September 2018; and the Tallah bridge after cracks surfaced.
The 750m-long bridge will connect BT Road with Shyambazar in the north over the rail tracks at Tallah.
The foundation work for building the bridge has begun. The old structure was demolished between March and April, ahead of the 75-day target set for the demolition.
L&T engineers have started fixing piles nearly 80-90m into the ground to build the foundation of the structure. The depth of the foundation for this bridge is almost four times that of a house, according to civil engineers.
Once the piles are concreted, each of the piles will be covered with a pile cap. This will complete the first phase of the construction.
In the second phase, pillars will be built over the pile caps as part of a “sub-structure” of the bridge, which will be a bowstring arch steel structure.
Eight pipes of various diametres, ranging between 30ft and 72ft, are located under the ground. These pipes bring water from Palta to the Tallah tank and from the tank carry potable water to the entire city. Any damage to any one of these pipes will spell disaster — either Tallah and adjoining Paikpara will be flooded or the city will go without water for days.
“Prior to the demolition we had to fix the exact location of each of these underground pipes and create a tunnel to physically run a check on them. We repeated the same exercise all over again before the start of the piling work,” a PWD official said.
There is a design challenge, too. The width of the main deck of the bridge will be a little more than double that of the old bridge, around 24m. But the width of the approach roads will be almost the same (11m) as it was earlier for want of space.
“The gradients have to be carefully calculated so that there is no jerk when vehicles ascend or descend the bridge. It will be a gradual tapering on the sides,” the PWD official said.
The new bridge will be a four-lane structure with a load-bearing capacity almost four times that of the old one (around 400 tonnes). There will be footpaths on either side, each measuring 1.5m.
Expected date of completion: January 2022
Project cost: Rs 348 crore
The 650m-long bridge over the tracks at Majherhat will connect New Alipore with Mominpore in south Calcutta.
The cable-stayed bridge will have the main deck held by 84 cables, which have been brought from Switzerland. The work of erecting these cables began on September 19 and is likely to be completed by October 10, about 12 days before Durga Puja.
“Once the cables have been erected we will begin casting the concrete crash barriers of the bridge. The barriers will be allowed to rest for some time for strengthening. Then bitumen will be laid on the bridge,” a PWD official said.
This is the most crucial part with a lot of calculations involved, according to engineers.
“We will have to study how the cables function in bearing stress while holding the bare deck slab and compare it when bitumen has been laid on the deck,” the PWD official said. “The design has a theoretical aspect and a practical aspect… the two will have to match.”
A team of experts from the railways and IIT Bombay will ascertain whether the two aspects are matching or there are discrepancies.
Based on their observation, the stress-bearing capacity of the cables will be checked and fine-tuned, if needed.
In between, the four service roads around the bridge will be readied and the entire structure will get a coat of paint after lights have been installed.
The four-lane bridge will have a load-bearing capacity of 350 tonnes against the previous 150 tonnes.
The bridge will be the first cable-stayed bridge built over rail tracks by the PWD in Bengal.
Expected date of completion: Just ahead of Diwali
Project cost: Rs 246 crore