Mountain Pass Slope Failure Retrofitted with a Half Viaduct Bridge Structure, South Africa

The Garden Route, one of the main tourist attractions and also a major national road on the south coast of South Africa, experienced abnormally high rainfall in August 2006. A major slip failure occurred in one of the road cuttings, known as Kaaimans River Pass, which severely damaged the roadway. The owner (South African National Road Agency Ltd.), immediately took measures to restore this vital link in the national road network. Several preliminary concepts were developed by the project team which included bridge and geotechnical specialists from the owner and the project-consulting engineers. These included a tied pile anchor wall, a tied mechanically stabilized embankment and a half-width pre-cast beam bridge structure. The selection of the most suitable option relied heavily on the geotechnical conclusions by specialists from the project-consulting engineers, after an extensive drilling programme was undertaken to investigate the competency of the underlying rock substrata. A modified version of the first concept was finally decided upon, as this was considered to provide the optimum solution, considering interaction with the adjacent road formation as well as the bedrock profile. The new viaduct is supported on eight 1200 mm diameter reinforced concrete oscillator bored piles approximately 14 m long, spaced at 8 m centres and socketed 4 m into competent rock. The superstructure comprises a 7 m wide, 60 m long cantilevered deck that is counterbalanced by an integrated buried jockey slab and supported on the piled substructure. This configuration eliminated a problematic longitudinal joint. The structure is anchored with re-stressable rock anchors, ensuring the stability of the structure and rock formation. In order to minimize the movement between the roadway and structure, the jockey slab is also tied down with anchors.