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Silt & sloth in cave-in city

Here a cave-in, there a cave-in — as many as two cave-ins occur every three days in the city with dredging robbing its decaying British-era brick sewers of the last line of defence, says a report commissioned by the civic body.

The silt that chokes the century-old sewers is apparently also their only saviour, forming a thick layer that acts as a casing for the wobbly walls. So every time an underground sewer is cleared of silt, it makes the walls weaker and more susceptible to caving in under the weight of wheels on the road above.

“The only way out is glass-reinforced polymer jacketing, which is currently being used in parts of the city. But it would cost the Calcutta Municipal Corporation Rs 5,000 crore to reinforce the entire network,” said a senior official of the sewerage and drainage department.

Around 249 cave-ins were reported during the five-year period between 1980 and 1985. The number shot up to 1,266 between 2001 and 2006.

The pathetic state of the city’s sewer network was documented in a study conducted by the Bengal Integrated Infrastructure Development Ltd in 2006 but the report has been gathering silt.

Tuesday’s cave-in on the road to Writers’ — in front of the west gate of Raj Bhavan — did send the civic bosses scurrying to the spot but not many, including the city’s new first citizen, seem to be aware of the existence of any such report.

“I have asked for a report on the present condition of brick sewers. It is essential to chalk out a strategy for the future,” said mayor Sovan Chatterjee even as another cave-in occurred in the city’s business district on Wednesday.

A 3ft wide and 4ft deep crater was detected on Raja Woodmunt Street, a few metres ahead of Nandram Market, after a water pipe running underneath the road sprung a leak. The subsidence forced police to divert traffic headed for Brabourne Road to Strand Road.

Anik Kumar Roychowdhury, a member of the Storm Drainage Manual Board under the urban development ministry, warned of more frequent cave-ins across the city over the next few months.

“The trunk sewers running under APC Road, AJC Bose Road and Lenin Sarani have all being cleared of silt and jacketed but what about the branch sewers that can’t be manually accessed? An imbalance between pressure inside and outside the sewer is bound to make the walls of these old branch sewers more unstable,” he explained.

“Non-man-entry” sewers need to be jacketed with laser-guided machinery, which is an expensive proposition.

Sewers on Free School Street have been fully jacketed with machines, but officials say the civic body does not have the resources to cover the entire network.

So will the city always remain on shaky ground?

“Former municipal commissioner Alapan Bandyopadhyay was the first to give this serious thought. He commissioned the 2006 report to apply for funding under the Jawaharlal Nehru National Urban Renewal Mission to bolster the brick-sewer network,” said a CMC engineer.

The city has 90km of manually inaccessible sewers and another 88km of “man-entry” lines. The weakest points in the network are the sewer junctions under APC Road, Bidhan Sarani, BB Ganguly Street, Grey Street, Nirode Behari Mullick Street, Vivekananda Road, Canal West Road, Raja Dinendra Street and Rashbehari Avenue.

According to the 2006 study, the number of sewer collapses has increased five times in the past two decades. “The vicious cycle started with acidic waste eroding the brick walls. Over time, the accumulated silt also became a buffer for the walls. Now removing silt without jacketing is the biggest danger,” said Roychowdhury.

The cost of repairs has increased nearly 15 fold. The civic body spent Rs 36.43 crore on repairing damaged sewers and roads in 2001-2006, against Rs 2.79 crore in 1980-85.

Former mayor Bikash Ranjan Bhattacharyya said the Trinamul-run civic body should focus on refurbishing the brick sewers rather than wasting money and resources on beautification of the city. “How can you turn Calcutta into London when the sewer network is on the verge of collapse? I laid emphasis on strengthening the old sewers and I hope the new board would do the same,” he added.

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