The underground sewage network spreading under Calcutta, built back in 1875 by the British settlers, was equivalent to the systems of European cities like London and Hamburg, Germany. Being in operation for over than 130 years, but with no significant maintenance work performed, the 2006 cave-ins urged for de-silting operations and structural rehabilitation. The maintenance works were undertaken in 2007 by the Kolkata Municipal Corporation, which today are almost complete. Upon completion, it is estimated that the flooding affecting the city every monsoon will be largely controlled.
Being built 130 year ago, Calcutta's sewage tunnel system is perhaps the most complicated system than any other city's system, receiving both storm and waste water. It is still amazing that it was built so large than it can nowadays serve the needs of its 22 million people, daily visitors included. Given the above and the lack of any serious maintenance work performed since its first operation, engineers and contractors faced a major challenge during the restoration works.
The first phase involved the refurbishing of about 26 kilometers of the largest brick tunnels, performed under difficult conditions, given that manholes constructed every 30 meters by the British had been covered by the uncontrolled urbanization. Also, given this densely populated city of 25,000 residents per square kilometer, and the lack of an adequate road network (only six percent of Calcutta's land is assigned to roads) the construction of new tunnels and the increase of the system's capacity became impossible. Hence, engineers selected the lining of the existing tunnels with glass reinforced polymer (GRP), sacrificing some capacity, but accelerating water flow on the much smoother surface of this material. According to one of the engineers working in the tunnels, the successful performance of the sewage system was proved last August, when streets did not flood for long after the city received about 430 millimeters of rainfall in just four days.
Source: nextcity.org
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