Concrete Retaining Blocks - Architect & Builder , October/November 200
This retaining wall project began when two engineering firms, Jeffares & Green and Mike van Wieringen and Associates were requested to investigate a cut slope failure at Suikerbossie Nek, situated above Victoria Road between Camps Bay and Hout Bay.
The engineers found that erosion and collapsing were caused by the angle of the slope. Although initially stable in terms of wedge or slip circle failure, over time the slope had been rendered unstable through weathering which reduced its surficial strength parameters. In addition, non-functional surface cut-off drains, situated above the cut face, had accelerated erosion and slip failure. Moreover, a lack of regular maintenance had led to silting and vegetation growth in the drain, resulting in choking with consequential over-topping.
Duncan Murphy, a civil engineer at Jeffares & Green, outlined the nature of the failure: “At some point erosion activity would have intersected the cut-off drain, and the formation of a large eroded gully in the cut face would have been the likely outcome. This could have resulted in the formation of a deep and irreparable gully extending a considerable way up the slope. Moreover, judging by the crack pattern in the head-scarp, a significant collapse onto the road during a period of high rainfall, was imminent.”
Remedial options such as gunite and soil nails or an 8,5m vertical concrete retaining wall were discarded for aesthetic reasons. Instead a combination of a near vertical gabion toe wall up to 2,5m high, a 35° Terrafix covered slope topped with a concrete-lined cut-off drain, and Terraforce L12 side walls, was the option selected. Terrafix 150 blocks were laid to trap the maximum amount of topsoil on the face, a solution which has prevented erosion and weathering, and encouraged plant growth.
Because the slope was large and curved, near-horizontal concrete infill walers were used at five metre intervals to fill the uneven spaces between adjacent panels. These were secured with anchor bars so that any possible pressure on the gabion wall would be reduced, and Terrafixes' maximum laying density of 10 blocks per square metre was used to prevent lateral sliding.
Thousands of water-wise indigenous plants, ground cover and bulbous species, were planted and these have started to relieve the nondescript appearance of the slope.
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