“Also, the high probability of a significantly sized collapse onto the roadway during a period of high rainfall and low visibility could prove fatal to passing motorists. The size of such collapses could be expected to increase for a time before they gradually reduce to sheet erosion in 20 to 30 years time. Judging by the crack pattern that was present in the head-scarp, such a collapse appeared imminent.”
In developing possible designs for the remedial works at the site, a number of considerations were taken into account in terms of guiding the selection process and included:
a) Cost – the less expensive the better.
b) Aesthetics – it was felt that given the environment in which the site resides the more natural approach with softer irregular lines and allowing concealment with vegetation is more preferable than the hard engineering approach. It was also felt that it would be beneficial, as much as possible, to use methods already used along Victoria Drive in order to develop a degree of visual compatibility along the entire route.
c) Disruption to traffic – this should be kept to a minimum and, if possible, without any closure of the road. Longer contract periods can be entertained if they imply less disruption to traffic.
d) Intrusion onto neighbouring property – this to be kept to a minimum in terms of both access and temporary or permanent disturbance to land or vegetation.
e) Adaptability and extendibility – remedial work needed to be such that it can be easily adapted and extended without having to demolish existing initial measures implemented. This allows for the minimum intervention now, with the option to extend and easily repair if and when further collapse or erosion occurs.
Remedial measures considered
The “do nothing” option was considered and rejected for a number of reasons as was the possibility of using light weight erosion control measures.
Options such as gunite and soil nails over portions of or over the whole slope, or a vertical reinforced concrete retaining wall up to 8.5 m high, were discarded for aesthetic reasons; the cutting being visible from a large portion of Hout Bay and adjacent to the Cape Peninsula National Park. A combination of a near vertical gabion toe wall up to 2,5 m high at the base of the cut, with a 35° Terrafix covered slope above, topped by a concrete-lined cut-off drain, was ultimately selected.
The gabions match numerous other gabion structures along Victoria Road and a feature of these particular ones, as developed by Gabion Construction, is the smooth unstepped nature of the face and the top of the wall as it follows the contour of the cut face.
Van Wieringen explains that Terrafix 150 blocks were selected in order to trap the maximum amount of topsoil on the face thus enabling plant growth, to prevent erosion by surface run-off and to retard the weathering process. “Because of the very large area covered on a curved surface, near-horizontal concrete infill walers were introduced at 5 m intervals to take up the uneven spaces developed between adjacent panels.”
“These were anchored to the slope with anchor bars in order to reduce any possible downward sliding force coming to bear on the gabion wall. The maximum density of Terrafix laying pattern (10 blocks per m²) was used so as to maintain an interlock between blocks against lateral sliding should support from below be disturbed in any way.”
To soften the appearance of the Terrafix blocks, thousands of water-wise indigenous plants as specified by the landscape architect were established early in spring and it is hoped that late winter rains will provide them with a good start in life. Ground covers and bulbous species such as Tatragona Fructicosa, Helichrysum Cymosum, Jordancella Louisa, Sparaxis Grandiflora, Metalasia and Aristea Majour should soon relieve the current fairly nondescript appearance of this slope.