While Gauteng province Premier Mbhazima Shilowa attended the unveiling of the first of the Gautrain coaches in England on 8th July, a section of Killarney’s Oxford road collapsed in Rosebank, north of Johannesburg. The collapse created a hole measures about 12 by 8 metres that is filled with water, making it difficult to determine the depth. A precast private residential wall adjacent to the affected surface collapsed but no other buildings were affected. View a video at http://multimedia.thetimes.co.za/videos/2008/07/road-above-gautrain-collapses
The Bombela consortium, the Gautrain construction company, has been instructed to cease excavations in the area where the sinkhole appeared. The order was issued in terms of mine, health and safety legislation. Safety officials were trying to determine if there were any other cracks along the road which could be dangerous. The area was cordoned off and motorists were being diverted off Oxford road. The road is expected to be closed for two weeks. The Johannesburg Roads Agency and the Bombela consortium will be repairing the road over the next two weeks. Bombela and the provincial support team are working together with the city of Johannesburg to provide temporary alternative routes for motorists while the road is being repaired.
The Johannesburg metro police department said the reason for the collapse would be investigated, but that it was very likely that Gautrain construction work going on in that area was the major cause. Construction of the Gautrain tunnel is currently underway beneath the road. Click here.
During routine monitoring process, the TBM technical team noticed surplus material coming through the machine, which is symptomatic of a void being formed above the TBM. Upon investigation they identified water seepage in the TBM chamber and a void above. To give more support to the ground, the TBM’s operational mode was changed to earth pressure balanced mode (EPBM).
At 09:00 a.m., Bombela’s construction team cordoned off the northbound lanes of Oxford road as a precaution and established additional survey monitoring points which were regularly checked. Notwithstanding the further measures put in place a ground loss occurred between the TBM and the utility services above which fractured, causing significant water flow exacerbating the situation and causing the road surface to subside.
At approximately 13:30, an area of Oxford road between North Road and 8th Avenue subsided. An initial void, approximately 6 m x 4 m and 2 m to 3 m in depth, formed at the road surface. This initial subsidence was a consequence of ground loss above the Gautrain TBM, which is currently 12 metres below ground level. No injuries were sustained and no damage occurred to either the TBM or the completed tunnel. The initial hole of 6 m x 4 m increased to 12 m x 7 m as it took approximately two hours for the water mains to be shut down.
Preliminary investigation carried out by Bombela after the event “suggests that water seepage from the utilities (water and effluent pipelines) above the tunnel alignment led to a weakening of the soil being excavated”, causing ground loss directly above the TBM. All steps that could have been, and should have been taken by the construction team were implemented timeously. Since the event, all structures in the vicinity of the subsidence have been continuously monitored and remain stable i.e. surrounding buildings are not at risk. Gautrain representatives said the tunnelling would resume after geotechnical and geological investigations into the collapse are completed, which could take "several weeks". It is not clear how long tunnelling will be delayed by the collapse, which caused water to seep into the tunnel.
Johannesburg Water said it was concerned that “fingers were being pointed” about its water pipes not being sound and that it wanted a copy of Bombela’s preliminary report into the cause of the sinkhole. “When we repaired the broken water main, technicians checked about 100 metres of the pipe for leaks using cameras. They couldn’t find anything” said Johannesburg Water. Johannesburg Water added that if Bombela’s engineers had suspected “seepage from utilities” when they realised a cavity had formed above their tunnel, 12 m below the surface, they should have reported the leak to them. Instead, Bombela changed the setting of the TBM so that it would support the softer soil above it and continued drilling.
Gautrain management said on 9th July that the tunnel collapse on Oxford road would not have any significant impact on the project’s schedule. A Herrenknecht tunnel boring machine is used to complete the 3 km tunnel from Rosebank to Park station because soft rock and waterlogged soil have precluded the use of conventional drilling and blasting methods.
The Gautrain line between Johannesburg, Pretoria and OR Tambo international airport is currently under construction, in one of the biggest infrastructure projects ever seen in South Africa. Construction of the first link in the Gautrain network, between the airport and Sandton in Johannesburg, began in September 2006 and is due to be complete in 2010, with the rest of the network due for completion in 2011. Visit www.gautrain.co.za 29/08.
