The Gautrain Rapid Rail Link will comprise about 80km of double track from Johannesburg International Airport in the south to Pretoria and Hatfield in the north. The trains are intended to operate at speeds of up to about 160km/hr. At peak times, trains are expected to operate at 10 minute intervals with up to 2000 people per train.
Between Centurion and Pretoria, in the northern section of the route, the alignment traverses about 10km of dolomitic limestone geology. The presence of large cavities in the limestones and overlying metastable sediments has in the past led to rapid collapse settlements (sinkholes).
At tender stage, the Gautrain Civil Works Joint Venture (Bombela) accepted the full ground risk for both construction and operation of the railway. Later assessments suggested greater than expected risks associated with the potential formation of sinkholes and the effects of solution cavities and rock pinnacles on pile design.
GCG were requested by Bombela to review the risks and comment on the adequacy of the original investigation and its interpretation, current understanding of the geology, and design solutions proposed by Bombela relating to cuttings, embankments and structures. A team from GCG visited the site of the project and provided a comprehensive report, with specialist input on karst geology from Dr Tony Waltham and on geophysics from Mr John Arthur.
The report identified the worst reasonably likely geological conditions and also extreme conditions that would be highly unlikely to occur but could have disastrous consequences. Recommendations were made for the design and safe operation of the railway taking all conditions into account.
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