Good news for massive N1 sinkhole repair

The R280-million rehabilitation of a huge sinkhole that formed next to the N1 in Centurion nearly three years ago has been completed, and all vehicle lanes have been reopened for traffic.

MyBroadband contacted the South African National Roads Agency (Sanral) after several readers who regularly use the highway as part of their commute noticed that the roadworks appeared to be complete.

Sanral Northern Region project manager Thabo Mahlakwana confirmed with contractor Jodan Construction that the construction work was completed by 28 November 2024.

Mahlakwana added that “minor roadwork snags” were planned for January 2025 but did not make mention of signficant lane closures being required for this.

The contractor appears to have made better progress than anticipated, as Sanral’s last communication on the project estimated completion by the second quarter of 2025.

However, that date was already significantly later than when Sanral initially believed the hole could be filled up and the road safe for a full traffic load.

The sinkhole formed on the eastern side of the N1, just south of the highway’s interchange with the R21 Flying Saucer, after heavy rains in January 2022.

The area is underlain with dolomite, a type of limestone susceptible to collapse due to water ingress.

The sinkhole’s proximity to the southbound carriageway forced Sanral to close several lanes, resulting in significant traffic pile-ups for nearly three years.

Although Sanral reconfigured the layouts to accommodate the large volumes of traffic using the road daily, the closures continuously impacted at least one of four lanes on the main road and another coming off the R21 offramps.

The narrowing to two and later three lanes affected drivers going south from Pretoria on the N1 and from both directions on the R21.

Motorists attempting to avoid this had also caused additional strain on arterial routes in the adjacent area.

Sanral originally estimated that the hole could be fixed within four to six months, putting a return to normal traffic flow as early as July 2022.

While cost estimates were challenging in the early stages, it said the project would likely cost “tens of millions” of rand.

Former Sanral Northern Region project manager Oakley van Eyk subsequently explained that the rehabilitation needed prior to construction turned out to be highly complex.

“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.

The satellite images below from Google Earth show how the area changed from before the sinkhole’s formation to the middle of construction work at the site.

From four to 36 months

The contractor was only appointed in late June 2023 — well over a year after the sinkhole formed — and began its work on 11 July 2023.

The original timeline was nine months, and work appeared to be on schedule, suggesting it could be done with lanes repainted by April 2024. That did not happen as planned.

In May 2024, Sanral explained that during project execution, the complex work required on the piling and soil anchoring proved difficult due to the area’s potentially instability, worsened by additional rainfall during the 2023/2024 summer.

“As a result, the engineering team focused on ensuring that workers remained safe during the execution of this work,” the agency said.

“They paid special regard to ensuring that the sinkhole area remained stable while heavy machinery was in use.”

Sanral listed several steps taken by the contractor to ensure long-term stability after the repairs, including:

• Drilling 278 holes and pumping grout into them
• Installing 55 no. 0.610m diameter piles, which were, on average, 25m deep
• Installing 161 no. 52/26 self-drilling anchors and a retaining wall

The construction project itself took 17 months, while the time from collapse to full restoration of traffic flow was about 34 months.

Sanral has also been working on two large sinkholes that formed next to the R21 near the Olifantsfontein

The cost of rehabilitating these holes was estimated to be around R144 million, with completion set for August 2024.