The South African National Roads Agency (Sanral) has confirmed that the massive N1 sinkhole that formed in January 2022 near the Flying Saucer interchange in Centurion, Gauteng will cost an estimated R280 million to repair.
The rehabilitation work kicked off on 11 July 2023 and is expected to take around nine months.
Sanral Northern Region project manager Oakley van Eyk told MyBroadband that the project is “on schedule” – thus putting the estimated date of completion in April 2024.
The reason for the year-long delay in implementing repairs, according to Van Eyk, is that the remedial work on the sinkhole is highly complex and required extensive research before any construction could be started.
“Geophysical testing and extensive percussion drillings had to be conducted to establish the extent of the poor dolomitic conditions and provide input into the design of remedial measures,” he said.
Further complicating matters is that the N1 must stay open during the entire construction period, calling for the contractor to build an anchored-pile retaining wall to allow traffic to continue flowing whilst work is being done.
The tender application was therefore opened and closed several times to give candidates sufficient time to study the requirements, Sanral previously told TopAuto.
The final contract was awarded to Jodan Construction (Pty) Ltd on 26 June 2023.
What caused the N1 sinkhole
Sanral previously confirmed that the N1 sinkhole formed in the early weeks of 2022 after the Centurion area experienced heavy rains in December 2021 which subsequently dissolved a sizeable portion of dolomite rock underlying the freeway.
Around 25% of Gauteng is covered by dolomite rock in one way or another, making the province, and especially the Centurion/Midrand area, more prone to sinkholes than most other regions in the country.
It is nearly impossible to predict the formation of sinkholes, leading to Sanral having to implement proactive measures to protect a road from these natural disasters.
Underground concrete pillars can be installed at the onset of a building project in a dolomite-rich area to mitigate the risk of sinkholes damaging the finished structure, which can be seen in the Gautrain bridge crossing the N1 a few kilometres away from the sinkhole.
However, should a cavity form, Sanral said the size thereof will inform the type repair solution.
If it’s a small sinkhole, the agency can fill it with rocks and compact the area, but larger ones need other options, which could entail building a reinforced concrete slab above the cavity that would allow a road to be laid on top.
Sanral said it is consistently monitoring at-risk areas and that if a sinkhole occurs, it will not only survey the sinkhole but also a large part of the surrounding area to detect any other cavities that might have formed.