The use of emulsion explosives and its reduced usage at the blast sites helped in the effective demolition of the concrete buildings while containing its impacts, said R. Venugopal, Deputy Chief Controller of Explosives.
Speaking to The Hindu in the post-demolition scenario, Dr. Venugopal said the controlled implosions carried out at the sites had brought down Golden Kayaloram, Jain Coral Cove, H2O Holy Faith and Alfa Serene as planned. The impact was contained mostly within the compound walls of the structures. All the buildings could be brought down in the exact angle as planned, he said.
For Holy Faith, only 212 kg of explosives were used against the 215 kg ordered, 14.5 kg in Golden Kayaloram against 125kg, and 343 kg at Alfa Serene against 600 kg. It was only at Jain Coral Cove that the explosives were used above the earlier estimation.
Here 372 kg of explosives had to be used against the 312 kg considering the size of the building, he said.
While reducing the quantity of the explosives, the length of detonating fuse was increased, which improved the efficiency of the blasts while reducing its impact, he said.
Vibrations’ duration
According to A. Bhoominathan, Head of the Civil Engineering Department of the Indian Institute of Technology, Madras, who led a team of experts that assessed the vibrations created by the explosions, the short duration of the vibrations triggered by the explosions, and the distance between the buildings and the point of origin of the vibrations saved the nearby structures from any significant damage.
The vibrations generated from Jain Coral Cove lasted 12 seconds, while it was 8 seconds for Golden Kayaloram. The average duration of the vibrations in Maradu demolition was less than 10 seconds, he said.
The vibrations created by earthquakes would last longer. In one of the recent earthquakes which rocked Japan, it lasted two minutes. Any vibration that exceeds 30 seconds will leave a high impact.
While the frequency of common earthquakes range from 0.1 Hz to 5 Hz, the vibration from the demolition had frequencies starting from 4 Hz and continued to high frequency ranges. This is another reason for the minimal damage, he said.
No cracks
The control room set up 120 metres away from Jain Coral Cove was shaken by the ground vibration generated by the blasts. However, no major cracks were found in a building located nearly 20 metres from the building. The department will submit its report to Maradu municipality shortly, said Dr. Bhoominathan.
