Consider this: an earthquake of even moderate intensity can liquefy the loose soil in some parts of the city and bring buildings crashing down. A new study by the Indian Institute of Technology – Bombay (IIT-B) states that foundations of buildings need to take soil into account if the structures have to withstand strong ground accelerations during an earthquake.
Soil liquefaction is a phenomenon in which cohesionless soil — usually sandy or clay — flows like muddy water (see box). The study observed that reclaimed areas of the city will be more prone to liquefaction.
A seven-member team simulated pile design or foundation, which can withstand different earthquake magnitudes depending on different soil conditions.
“We are trying to find out the simplest formula to design foundations of buildings in terms of piles under
earthquake circumstances,” said professor Deepankar Choudhury, civil engineering department. “Earthquake codes were last changed in 2002. Since then, many new earthquakes have taken place. Therefore, we need revised codes.”
The Mumbai region, which includes Thane and Navi Mumbai, falls under moderately seismic zone 3 with a possibility of earthquake between 6 and 6.5 magnitude. There are 23 fault lines that can cause ground motion in and around the city.
According to the study, Mumbai has the most high-rises in India because of scarcity of land. “All these structures are mostly founded on pile foundation because of typical soft nature of soil strata in Mumbai, which is a typical coastal city,” read the paper Seismic Liquefaction Hazard and Site Response for Design of Piles in Mumbai City.
The team collected more than 500 borehole data and soil testing reports for Mumbai from different government and private institutions and consultants.
Having developed GIS-based seismic liquefaction hazard maps for the city — critical, moderate and non-liquefiable magnitudes of 5.5 and 7.5 — the team observed that reclaimed areas will be more prone to liquefaction during an earthquake of magnitude 6 and above.
“As the city originated from seven different islands, the reclaimed land can easily liquefy in the event of a major earthquake. In the absence of a proper soil exploration, the buildings will get damaged,” said Choudhury.
A similar study in 2010 revealed that the level of safety of structures in case of an earthquake will not be the same, and depends on the soil quality. For instance, between magnitudes of 6.5 to 7.5, soil in Bhandup and Malvani in Malad will liquefy faster compared to Dahisar or Borivli, where liquefaction will occur between magnitudes of 7 to 7.5 on the Richter scale.
“Although Mumbai looks safe, it is not. The manner in which high-rises are constructed in the city, I hope precautions are being taken to make earthquake-resistant buildings,” said VK Joshi, former director, Geological Survey of India.
“A micro-seismic zonation of Mumbai will bring out area specific vulnerabilities based on which buildings must be constructed.”