Already, as the tunnelling work of Chennai Metro Rail paces ahead, several buildings have suffered cracks as result of this work. The giant tunnel boring machine (TBM), as it drilled underneath at least 700 buildings in the Broadway area, caused cracks in about 20 of them including heritage structures.

 

Samira Khan (name changed), a resident of STS Complex in Broadway, a building that recently developed cracks, said that her daily schedule was disrupted as the repair work was done. “The work is almost over now and I am completely exhausted dealing with this. For the last two weeks, I have sacrificed all my time for the repair work. Not just that, my family members have developed dust allergy and fallen sick due to the cement dust,” she said.

 

Chennai Metro Rail Limited (CMRL) may have taken care of the repair work on the affected buildings, but the owners say that they were unaware that a tunnel boring machine (TBM) would go underneath their building. “We only woke up to find cracks all over our house. Now, we are struggling to get compensation from them,” she added.

 

The CMRL is required to inform the residents of areas in which boring is to take place of the date on which the activity will begin, according to experts in IIT-Madras. “It is the duty of CMRL to inform the residents before TBM drills below their building. The only thing residents can do is to ensure that the cracks don’t widen; if they widen, then it is cause for concern,” said a structural engineering expert from IIT Madras.

 

Cracks usually occur when the soil is weak and overburdened in the tunnelling location. Plus, the strength of the foundation of the building also plays an important role, he added.

 

Before tunnelling, CMRL has been surveying the area to see if any old and vulnerable buildings need protection. But, incidents of crack are largely unavoidable in such large scale constructions particularly in congested area where known methods of strengthening cannot be carried out, said S. R. Gandhi, faculty, and the Head of the Department of Civil engineering in IIT- Madras.

 

“The designers do have a fair idea of the type of soil that the machine would bore through and a detailed investigation is carried out by drilling of bore holes at regular intervals, but small pockets of different quality of soil in between these boreholes cannot be ruled out. This might cause such damage,” he said.

 

It is certainly challenging to carry out tunnelling in Chennai's soil conditions which is a coastal region with large variations and occasional soft marine clay layers, Mr. Gandhi added.

 

CMRL looks into the nature of the structures before beginning to tunnel in a stretch, said officials. “If we suspect that the building may collapse, we evacuate the building. I’m told that few structures had to be dismantled because they were too weak to stand the tunnelling process,” said the official of CMRL.

 

The piers of Delhi Metro Rail too developed many cracks intermittently in some stretches after the operations commenced.

 

When asked if such cracks would continue to occur when the Chennai Metro Rail trains are up and running, the official denied any such possibility. “The vibrations of trains certainly will not have such an effect on our buildings,” he added.