A survey to assess seismic vulnerability of the built environment in Chennai has classified 29 per cent of the buildings in the city as highly vulnerable in an earthquake event.
The survey undertaken by the Centre for Disaster Mitigation and Management (CDMM) at Anna University aerially mapped 22,758 medium-to-tall buildings in the city deploying the Rapid Visual Screening technique.
The surveyed buildings included 11,224 three-storeyed structures, 7,500 four-storeyed buildings and 2,053 buildings with five floors or more.
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Launched as part of a Government of India-funded project to evolve a GIS information based e-governance model for vulnerability assessment of Chennai, the study also recommended “ground truth” (physical) verification of select buildings to confirm the extent of vulnerability.
The study that also analysed soil types in the city found that the clay and unconsolidated sediments were likely to cause higher local site amplification of earthquake energy effects.
Such amplification areas were identified at Vyasarpadi, Siruvallur, Otteri, Purasawalkam , Vepery, Egmore, Kolathur, Villivakkam, Anna Nagar, Aynavaram, Koyambedu, Arumbakkam, Kodambakkam, parts of T. Nagar, Taramani and parts of Velachery.
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Recommendation
The soil liquefiable areas were mapped in Vyasarpadi, Purusawakkam, Koyambedu, Villivakkam, Saligramam, T. Nagar and West Mambalam. The study recommended soil improvement in these areas for mitigation of seismic damage.
Buildings coming up in future could also incorporate the amplification factor to realistically design appropriate earthquake resistant features relevant to the varying levels of seismic hazards across the city instead of uniformly following the coefficient for Seismic Zone III, the study has recommended.
It is pointed out that while Chennai was re-designated from Hazard II zone to Hazard III zone in 2001, several buildings that came up prior to that year remain ill-designed to withstand earthquake events. “As a result, even relatively moderate earthquake events can be the source of huge socio-economic impact,” said S. Rajarathnam, Director, CDMM, Anna University.
While the draft seismic hazard map for the city is ready from the perspective of borehole data analysis and engineering seismograph survey, the GIS-based e-governance model will be put together after gathering inputs from various user departments, said A. R. Santhakumar, CDMM consultant.