Of the 740 buildings under which the tunnel-boring machine ploughed through in the Washermenpet-Mannady stretch in north Chennai, 18 have developed fresh cracks, according to an official of the Chennai Metro Rail Limited (CMRL).
While 14 buildings suffered minor cracks, four developed bigger cracks due to tunnelling work carried out as part of the construction of the Chennai Metro Rail network.
“Most of the affected ones — comprising both residential and commercial buildings — were structurally weak, aged over 30-40 years. Both minor and visible cracks were attended to and repaired immediately. So far, most of them were minor settlement cracks and less than 5 mm in width. The heavy congestion in the area acted as a contributing factor,” said the official.
While a minor crack has a width of 3-5mm, a settlement crack is one that may occur owing to the decrease in the load-bearing capacity of the soil on which the building stands. This may happen in any area, not just a congested area like Washermenpet, but it becomes increasingly difficult to bore through in the congested areas as the soil is already under much pressure, he added.
On Saturday, two commercial buildings on Prakasam Salai in George Town had developed cracks and were evacuated on precautionary grounds. The officials said that one building was repaired subsequently and work on the other is in progress.
The official said, however meticulous the planning may be, cracks cannot be completely avoided in every building.
“With very old buildings, we can’t escape cracks. We foresee certain eventualities and keep it to a minimum limit,” said another CMRL official.
Before tunnelling operations begin, an existing building survey was carried out which indicated the old cracks and vulnerability of every building. The building is also checked with instruments including a crack meter, tilt meter and ground settlement meter.
If these readings show the structure may be weak and susceptible to cracks, some preventive measures are taken which include giving support systems to the building, TBMs drilling at a slower pace, reduction in the volume of construction material ‘grout’ and people in the building being evacuated, he added.
“Also, we give good shell and lining between the tunnel and the base of the building which may reduce the chances of cracks when the tunnelling operation occurs. Despite such measures if the cracks occur it may be only because the building is fragile,” the official said.
An engineering expert from IIT-Madras said that the cracks may not pose a threat to the survival of the building; but constant monitoring may be required to stop the growth of the crack. “When a TBM drills underground, there may be stress on the building which can be nullified by providing good cushioning system like lining and shelling between the tunnel and the base of the building to avoid crack formation. If the building develops a crack in spite of this then repair and monitor mechanism may be the only resort,” he said.