The City of Johannesburg's preliminary investigation has identified three possible scenarios which could have led to Wednesday's explosion in the CBD — but only one scenario seems likely.
City manager Floyd Brink said the ignition of methane gas in underground storm water systems due to sewage ingress could have led to the blast.
He said the second possibility was the ignition of natural gas, mixed with oxygen in underground storm water drainage systems or service ducts and the ignition of gas from a gas pipe burst could have led to an explosion.
“The third and last possibility is the ignition of gas from a gas pipe burst,” Brink added.
He addressed journalists on Thursday evening at Mary Fitzgerald Square where the city has set up the command unit after Wednesday's blast that left almost 50 people injured and one person dead.
“At this stage only one of the above potential causes appears to be warranted, accidental leakage of natural gas into the service duct reaching explosion concentration levels of up to 15% and was ignited by a source unknown at this stage,” he said.
He said the source of gas in the service duct is unknown, with investigations to continue to try to locate the source in the next few days.
Brink said the buildings adjacent to the collapsed road were inspected and on Thursday evening there was no sign of failure detected, so the buildings were regarded structurally sound for occupation.
“We have also assessed the gas leaking into the atmosphere and have been assured by the environmental services and air quality controllers of the city that the gas leaking into the atmosphere poses no risk to residents as its concentration reduces rapidly in the open air,” he said.
He said, however, gas leaks into subsoil cavities such as basements can be fatal.
“From yesterday evening, the city engineers and specialists from Johannesburg Roads Agency, Johannesburg Water, City Power and representatives of Egoli Gas have been on the site to ascertain facts on the source of the explosion and the triggers,” he said.
He said Egoli Gas was shutting down the gas pipelines that run through the site of the explosion. The gradual shutdown was due to be completed by 10pm Thursday.
“The process to purge the gas then will commence under the supervision of the city teams on site,” he said.
Johannesburg Water has already isolated water supply to the area and deployed water tankers. This will be augmented by the deployment of mobile ablution facilities.
City Power will be deploying technicians to assess the power lines for damage along the 400m site and will only be able to re-energise the area once it has been declared safe.
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