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Since 1st March, 1999
 
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Warning on unstable soil

Calcutta may have managed to go unscathed, but if geophysicists are to be believed, the city is vulnerable to devastation caused by an earthquake, as is evident from cracks in new houses or flyovers. The ground beneath the overpopulated city is not stable.

'Studies show that the soil cover in the city is thick alluvium, which is unstable by nature. Surface waves result in 'liquefication' of the soil. Cracks occur in new structures because the soil underneath tends to spread and move away,' explains Sankar Nath, head of the department of geology, IIT, Kharagpur.

As it is, Calcutta falls within the seismic scale of 3 and 4 and studies reveal that the city is headed for a 'construction doom'. Nath and other experts believe that unless immediate steps are taken to ensure examination of the soil in consultation with seismologists and geologists, even the most sound of built structures are bound to take a beating a few years hence.

According to seismologists, when surface waves hit alluvium topping, the soil is prone to turn into clay and lose its tensile strength. The soil begins to slither and this affects the foundation of a structure.

Experts from seven IITs and the Indian Institute of Science, Bangalore, discussed the issue recently at IIT Kharagpur. It was decided that micro-zonation ' a zone-wise break-up ' of the city was imperative to identify the particular areas prone to intensive seismic vibrations before the threat could be quantified in exact terms.

Most experts were more concerned on Sunday with the immediate threat in store. They believe that Bengal is still vulnerable to a tsunami in spite of being spared this time.

Moreover, with thousands of litres of groundwater being drawn out every day, of which little is recharged, the soil cover is turning into a crust with little support beneath.

The Central Groundwater Board (CGB) has already sounded a red alert as the water level from city aquifers (water-bearing level) has dropped several metres owing to indiscriminate drawing of ground water and the absence of any effort to replenish it.

'This is a dangerous situation and unless stopped immediately, the impact of any disaster due to breakage of the earth layer will increase manifold,' said a CGB official.

The most vulnerable areas are Rajarhat, Dum Dum, Kestopur in north Calcutta; Jadavpur, Garia, Santoshpur and Prince Anwar Shah Road in south Calcutta; and places around Chittaranjan Avenue in central Calcutta.

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