A leaking pipeline has been blamed for softening ground so much that a large hole opened in the road over the TBM excavating part of the Gautrain rail project in Johannesburg, forcing the shield to stop work for almost a week in early July.
Tunnelling work has resumed but the local authority’s concerns over the condition of utilities has led to the introduction of a precautionary, three-phase programme of road closures ahead of the machine. Surrounding buildings are being monitored and have been stable, said contractor Bombela Civils JV.
The 6.68m diameter TBM was driving under Oxford Road at a depth of 12m, and had reached the section between North Road and 8th Avenue, when the road subsided in the early afternoon of 8 July. A hole up to 3m deep and 6m long by 4m wide first appeared, then it increased 12m long by 7m wide due to a fractured water supply pipeline.
Bombela said its preliminary investigation suggested that water seepage from the utilities above the tunnel alignment had weakened the soil, and led directly to the ground loss above the TBM, which was capable of, but not operating in EPB mode.
The contractor became aware of the problem underground when surplus material began to come in. The machine was switched to EPB mode.
Bombela, which is led by Bouygues, said repairs to the road were quickly completed but as a precautionary measure a series of phased road closures would be introduced as the TBM advanced.
The TBM (S-386) was manufactured by Herrenknecht to drive just over 2,820m from Rosebank towards Park station (T&TI, December 2007, p7).
In total, almost 16km of tunnel (excluding stations) is being excavated for the 77km long rail project.
Geology comprises mainly weathered granite with some silt, clay and boulders. There are several faults. Cover is 15m-80m and groundwater levels are at depths of at least 25m, Bombela said previously.
