A modified compaction grouting procedure has been implemented on the Gautrain Rapid Rail in South Africa for viaduct foundations to mitigate against the formation of sinkholes by filling voids in dolomitic soils and karst bedrock. Two full scale trials were carried out at the start of the construction and the results were used to develop a methodology for the grouting works based on an attri-tive approach with stages of grouting and depth related pressure and volume criteria. Drilling and grout injection by upstage grouting comprise the two main components of the work. For the works all drill rigs were equipped with drill parameter recorders and grout injection was monitored by instrumentation developed specifically for the project. Fifty five pier locations and a railway station site were grouted in 24 months. The works are presented to show how grouting in karst bedrock and overlying thick residual soils has been achieved.

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