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Investigators probe three likely causes of Johannesburg explosion, Lilian Ngoyi Street closed over safety fears

Investigators probe three likely causes of Johannesburg explosion, Lilian Ngoyi Street closed over safety fears
The collapsed road on Lilian Ngoyi Street in the Johannesburg central business district on 20 July, 2023. (Photo: Gallo Images / OJ Koloti)

City of Johannesburg has stated that ignition of methane gas, natural gas, and a possible leak in a gas pipe were three possible causes of the blast.

More than 60 hours following a deadly explosion that ripped through the city of Johannesburg’s Lilian Ngoyi Street (previously Bree Street) causing mayhem, the exact cause of the blast remains unknown, with officials speculating over three possibilities.    

Despite not knowing the exact cause of the blast — which led to one fatality, multiple injuries and damage to infrastructure — authorities believe there is no need for mass evacuations, as buildings near the cracked road are “structurally sound for occupation”. 

The three possible causes of the explosion were identified, according to the city manager of Johannesburg Floyd Brink, who led a technical media briefing on Thursday evening: 

  • the ignition of methane gas in underground stormwater systems due to sewerage ingress;
  • the ignition of natural gas, mixed with air (oxygen) in underground stormwater drainage systems; or
  • the ignition of gas from a burst gas pipe.

“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 5-15% [that] was ignited by a source unknown at this stage,” he said.    

While a gas line explosion was the suspected cause — stemming from gas lines running beneath the pavement, not under the roadway itself — such lines operate at low pressure and are unlikely to cause an explosion, according to Egoli Gas which manages the municipal lines in the city.  

On Thursday, however, the company indicated the detection of a small leak but said it was unlikely the cause of the explosion. 

“A small leak has been detected on the servitude pipeline at the corner of Bree and Eloff on a 100mm pipe. We believe the crack in the pipe has been caused by the collapse of the road. Our team is busy repairing the leak.”     

Despite this statement, a civil engineer contracted by the city, Johan La Grange however cautioned against pointing fingers at Egoli Gas.  

“This is not uncommon, it is a very sad event that happened, so we won’t point fingers. I think as we just know it is gas, and their detection system is in good shape, it actually withstands the explosion. What we indicate is that leakage could either be from a mainline or it can be from a distributor.”  

Read more in Daily Maverick: Emergency teams race to find source of Johannesburg explosion as city centre remains volatile 

Dr Phathutshedzo Khangale, Senior Lecturer in the Department of Chemical Engineering Technology at the University of Johannesburg ruled out the possibility of the explosion being the result of a gas leak.  

Johannesburg explosion

A collapsed road in the central business district along Lilian Ngoyi Street (formerly Bree Street) on 20 July, 2023 in Johannesburg, South Africa. It is reported that at least 24 vehicles, mostly taxis, were damaged when the explosion occurred. (Photo: Gallo Images / OJ Koloti)

“If that was the case, I would have expected to see some flames, in parts of the area, it is also possible that the temperature in that area of the pipe could have risen, possible induced by human beings, but again if that was the gas because they are transporting flammable gas like natural gas, you again would expect some flames…”

“There would also be a sudden cut in the supply of gas to the end users, if that did not happen, you can most probably rule out the possibility of the pipe actually bursting.”    

Despite ruling out the possibility of gas being the cause of the blast, Khangale conceded that the water pipes running beneath the ground in the area could have played a role in the explosion.   

“There is a possibility that some of the water pipes run on top of a gas pipe, in an event something happens, it would most likely create an impact that might cause an opening in the gas pipe and that can lead to an explosion.” 

Illegal mining?

Another expert, Prof Felix Okonta, the Head of Geotechnical and Pavement Engineering Research at UJ did not rule out the possibility of a gas leak, further suggesting that “it could also be the work of informal miners who are actually trying to extract minerals. It is possible because Johannesburg is underlined by old mine shafts, they run across the ground everywhere.”   

Chief Executive of mining consultancy AmaranthCX, Paul Miller suggested this was unlikely. 

“The gold reef outcropped near the Standard Bank precinct — about a kilometre away to the south. And the reef dips to the south away from the CBD so there would not be old gold mine workings directly associated with the area of Lilian Ngoyi Street. It is too far north.”

Meanwhile, Department of Mineral Resources and Energy spokesperson Ernest Mulibana, poured cold water over the ascertains.   

“We can confirm that there are no old mines in the area at which the explosion occurred. The DMRE is confident that investigations led by the relevant authorities which include the Gauteng provincial government and the city of Johannesburg will be able to ascertain the actual cause of the tragedy,” said Mulibana.   

Read more in Daily Maverick: Johannesburg’s emergency call for engineers, gas-detection experts after CBD explosion exposes dire skills gap 

Johannesburg explosion

Several people were injured following an explosion that rocked Johannesburg on Wednesday, 19 July 2023. (Photo: Felix Dlangamandla)

Public dangers

Okonta said although there was no physical structural damage that could be seen on the buildings near the scene, there was no doubt that the surrounding buildings were affected by the vibrations of the blast, adding there was a need for structural integrity tests to be carried out. The outcome would then determine whether they would remain stable or not.  

“For me what we should be worried about is the stability or structural integrity of the buildings more than  the explosion…the cracks we see today will not be the only cracks we see in the years to come.” 

Brink however said buildings adjacent to the collapsed road had been inspected and so far there was no cause for concern, this amid conflicting information on whether or not residents needed to be evacuated as the smell of gas lingered. 

“At this stage, no signs of failure have been detected and the buildings are regarded as structurally sound for occupation,” said Brink.

“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 and as its concentration reduces rapidly in open air. However, gas leaks into subsoil cavities such as basements can be fatal.”

In addition to the inspections done so far, Brink said advanced technologies to assess and review underground damage to infrastructure had also been deployed.  

“The technologies deployed are world-class and first of its kind on the continent and have assisted us to identify underground defects underneath the surface of the roads that are still intact and unaffected at this stage.” 

Johannesburg explosion

Several people were injured following an explosion that rocked Johannesburg on Wednesday, 19 July, 2023. (Photo: Felix Dlangamandla)


Residents in several parts of the city have been without water and power since the explosion, with this likely to be the case indefinitely. Joburg City officials said Joburg Water would supply residents with water and ablution facilities. Meanwhile, City Power’s general manager Charles Mohlaka could not say for sure when power was likely to be restored. 

“The Bree Street substation remains affected by power outages. The city is assessing whether it’s safe to restore it after the explosion,” said Mohlaka, expressing concern over electricity possibly clashing with gas which would lead to yet another “serious explosion”. 

Meanwhile, businesses in and around the area remain closed following the city’s decision to close down and cordon off Bree Street.   

A retail worker in the area, Sibongile Dlamini was concerned about being unable to make any income. “Some of us in retail get paid hourly, so this is affecting us really badly because we will not earn an income for a long time, it seems.”  

Read more in Daily Maverick: Residents describe Joburg blast — ‘I felt my intestines go cold from fear’ 

Unable to make an income 

If the street remains closed for the next week, for Dlamini — who gets paid weekly — it means “I will not be able to buy food, and if it goes on for another week, then I may not be able to pay rent”.   

Not only are small businesses affected but also offices of the Commission for Conciliation, Mediation and Arbitration (CCMA) which announced that it would remain closed in order to safeguard the health and safety of the staffers and clients.  

The city officials said the next 72 hours would be critical, having established a multi-disciplinary team that is working around the clock to secure the site, conclude investigations and restore services for residents and businesses in the area. 

Immediate action plan

Brink outlined immediate Actions being undertaken as follows: “

  1. “E-Goli Gas is currently shutting down the gas pipelines that run through the site of the explosion and this gradual shutdown will be completed in the next two hours by 10pm [Thursday] evening. The process to perch the gas then will commence under supervision from the City teams on site.
  2. “Johannesburg EMS has deployed a Fire Engine on stand-by duty at the site to monitor the site and be available to react to any incidents that may occur.
  3. “Johannesburg Water has already isolated water supply to the area and has deployed water tankers to support residents in the area. This will also be augmented by the deployment of mobile ablution facilities to provide for residents in the affected area.
  4. “Similarly, once the site is declared safe, Johannesburg Water will require three days to repair the sewer lines in the area to re-activate services.
  5. “City Power will also be deploying technicians to assess the power lines for damages along the 400 meters of the site and will only be able to re-energize the area once it has been declared safe to do so.
  6. “JMPD and SAPS will barricade the area to limit access and movement to allow for the efficient management of the site and the creation of a safe environment that minimises the risk to residents.
  7. “The barricades will also be supported by 250 members per shift and this will be supported by 50 private security personnel from The Firm.
  8. “The City will also commence with the work of sampling all bulk service manholes in the area for gas leaks and ruminants.

“This will include the opening of bulk manholes belonging to City Power, Telkom and other service departments and entities. Some of the manholes are welded, due to theft and vandalism and at this stage we are unable to immediately open them for fear of igniting a further blast.

“Where possible our teams, led by EMS, will be placing Positive Pressure Ventilation Fans down the manholes to blow out any gas that may still exist below. This is intrinsically safe equipment and will assist in efforts to disperse trapped gas.” DM


Comments - Please in order to comment.

  • Rod McLeman says:

    I’m glad I live in the Cape!

  • Lesley Young says:

    “…perch …” the gas pipes? Do they mean ‘purge’?
    “… for gas leaks or ruminants”. ??? Cows? Goats? Or camels?

  • Lesley Young says:

    I have not duplicated my comment!

  • Samantha Vandersteen says:

    So the same cadres who don’t have the qualifications to ascertain the cause of the blast are also the ones inspecting the nearby buildings & saying they are safe for occupation? That’s a bit scary

    • johanw773 says:

      My thoughts exactly!

    • Ryckard Blake says:

      * Not chaps provided by Sodi’s Blackhead Consulting, but SQEP structural engineers working many years for major insurers on this type of investigation, and probably having to be flown in from around the globe.

      (This comment continues my previous one, excuse me for having tagged it on to yours, SvdSteen)

  • Peter Hartley says:

    The cause of such an explosion can only be caused by a gas igniting. The source of the gas leak or origin obviously has to determined but from the photos you have shown, it would appear that a large pipe – a pipe much bigger than a gas supply line – has ruptured as a result of the explosion. I say this because very little material is lying outside of the blast area but the road has caved in. This material had to go somewhere and my guess is it has fallen into a large – maybe 2m diameter – sewer pipe. Gas from a leaking gas pipe might have made it’s way into the sewer but it is more likely to be a build up of methane. Let’s wait and see.

  • Ryckard Blake says:

    Dr Phathutshedzo Khangale of UJ should familiarise himself with studies in explosion science, before making a bigger fool of himself. If all the texts on the subject in UJ’s engineering library have been stolen, I can lend him my copy of the classic “Loss Prevention” (Frank P. Lees), wherein Ch. 17 will introduce him to the differences between UVCEs, BLEVEs, and Deflagrations or Detonations in confined spaces.
    The early evidence must be that this event was a DETONATION in a confined space; not in the pipeline, obviously, but of leaked gas in the duct / tunnel surrounding it. Doctor PK will find that flames after a gas detonation in a confined space would be quite unusual.
    The visual record shows that the shock wave from the explosion lifted heavy slabs of road plus the taxis on top of them a good metre into the air. That was SOME destructive shock, and I query how any competent structural inspector can within 2 days declare all the surrounding multi-storey buildings safe for habitation.
    Obviously, Lesufi and Kabelo “Not MY Mayor” Gwamanda lack the political will to evacuate the thousands of residents whose buildings felt that shock. The buildings may remain safer than the shacks which many of the occupants previously called home, before invading office blocks in Bree Street. But I would bet that a thorough, detailed inspection of all those structures by competent* Assessor-Inspectors will take many weeks to complete, and may result in more than a few demolition orders.

  • valerie.scofield says:

    I think it is more likely that sewerage pipes have built up the gas over a long period of time.

Please peer review 3 community comments before your comment can be posted


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