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Engineering Chapman's Peak Drive

Sir Frederic De Waal, first administrator of the Cape, ordered the construction of the Hout Bay- Noordehoek Road in the early 1900’s. Despite the steep unstable cliffs and ravines the road was cleverly planned with the road surface based on the solid and conveniently located 630 million year old Cape Granite contour, while the many roadside cuttings were carved out of the more workable Malmesbury series sediments. The route took 7 years to complete and was opened in 1922. See more on the history of Chapman’s Peak Drive.

In 1962 the road was widened and in 1977 a portion of the road was washed away resulting in a bridge to replace the damaged section.

Despite its spectacular views and extraordinary beauty, the drive is not without danger, as rock falls and mudslides have always been a hazard for motorists-particular during storms in winter. Four deaths and several serious injuries resulted from rock falls incidents between 1998 and its closure as a public road in January 2000 after the last fatal occurrence, until suitable rock-fall protections measures had been implemented.

In order to re-open the road to the public, the hazard from and debris landing on the road had to be substantially reduced and the associated risk and liability of injury or death to road users minimized. At the same time, the touristic, historical, aesthetic, environment and traffic implications of road widening and other substantial construction works had to be considered. All of these had to be carried out in a very short time and within a limited budget. This was the task that faced Entilini Concessions in 2002 when it was awarded a 30 year concessions for the rehabilitation and operation of Chapman’s Peak Drive as a toll road.

The rock-fall protection measures implemented at Chapman’s Peak Drive were selected on the basis of what is considered to be best international practice and consist of a half tunnel, catch fences, slope stabilisation and canopy structures. In 2004, Chapman’s Peak Drive received an excellence award for rock fall protection.

The Project Engineering Solutions

Rock Fall Protections Measures: The scale of the design and construction of the Works was formidable, with many of the activities required being ‘first-offs’ in South Africa. The design of rockfall protection measures involved sophisticated three dimensional computer modeling of the topography assessment of mountain and location of boulders on the slopes and cliffs above the road to predict the associated rockfall patterns. This modeling predicted, on a statistical basis, the location, trajectory, frequency, bounce high and energy of falling rocks along route. These predictions formed the basis for determining the optimum size and shape positions of the rockfall protection structures and for their design.

Working conditions were challenging and up to 11 different mobile cranes were deployed, as the road was too narrow to constantly move the cranes up and down. Helicopters were also used to lift drilling and grouting equipment into position at heights above 65 m, while abseilers completed the work in those inaccessible locations.

Terrain Modelling: The engineers, in associated with Swiss specialists utilized a digital terrain model to generate three-dimensional simulations of rockfalls. This was supplemented by the interpretation of high-resolution aerial photographs to determine the distribution of relative frequencies, bounce heights and energies of simulated rocks of various sizes.

Half Tunnel: This is the first time that half tunnelling has been used in South Africa and entails cutting into the mountain at road level, then moving the road in under the protection of the resulting overhang. The 155m half tunnel was built at the base of Chapman’s Peak and the overhanging rock-mass is supported by 95t rock anchors. The soffit and side walls are supported by 150mm thick steel reinforced shotcrete lining.

Rock Shelters: In other places the solution to rock-fall protection measures was to build a concrete canopy, similar to those at Lake Garda, Italy. These measures ensure that any rocks that do fall off the mountain will at worst land close to the old rock faced guard wall, while traffic will travel safely under the canopy.

At two locations where the sandstone cliffs extend to more than 400 m above the road, concrete impact protection canopies were constructed. These structures arch over the road and protect it by intercepting rockfall and debris or deflecting the material over the road and into the sea below.

A 40m long curved cantilever canopy, arching over both lanes was built on a tight bend at the confluence of three gullies. It is tied back into the cliff face at each end by 100t rock anchors and supported in the middle by 11 large pre-stressed counterfort ribs, as the fault resulted in stability concerns due to hard granite on the northside and soft sandstone on the southside.

A portal canopy coincides with the highest predicted rockfall energies and the structure is free of the cliff face. It is supported from behind by tapered columns with a row of circular sloping columns to support the front edge.

Catch Fences: A new feature is the catch fences, a sophisticated Swiss design not used in South Africa before. They consist of interlocking rings of high tensile wire designed to trap rocks as they fall, and are anchored into the rock with steel wire ropes. There are 1,6km of catch fences in different sections that are monitored and fallen rocks removed. These fences have capacities ranging from 500 to 3500kj and vary in height from 4-6metres. They are positioned at various locations and elevations above the road to suit the local topography and rockfall trajectories.

Advanced Traffic Management Systems (ATMS)

CCTV cameras, variable message signs, radar traffic detectors and a weather station linked via optic- fibre cables to operator consoles all enable the concessionaire in monitoring risk and ensuring ongoing adequate maintenance.

According to the Chapman’s Peak Engineering Group the rockfall protection measures have reduced the risk of rockfalls reaching the road by 90%. However in cases of severe weather conditions the risk will be further reduced by closing the road temporarily.

In terms of adhering to environment criteria, an independent environment scoping report was completed prior to the design and construction work commencing. An environment monitoring committee was appointed and an independent environmentalist constantly monitored the work. A environmental consultant supervised the removal of alien vegetation and harvested indigenous plant seeds. These were propagated in a nursery and planted in disturbed areas.

A landscape architect ensured that the structures conformed to the ‘sense of place’. This included ensuring that rock-packed retaining walls were reinforced with pigmented shotcrete and rocks packed into pigmented gabion baskets.

According to the Chapman’s Peak Engineering Group all targets for black economic empowerment and the development of local small businesses were achieved. A portion of the project’s equity was invested in a community trust to benefit local disadvantaged communities. During construction 40 members of the local community were trained as abseilers, while others were trained in the art of constructing stonewalling.

A Triumph for South African Engineering

The re-opening of Chapman’s Peak Drive in 2003 was heralded as a great success, enabling tourists and locals to once again experience the exquisite beauty and engineering magnificence of the drive. The coupling of 21st-century cutting-edge and construction processes with the courage and determination of the early 1920s who built the original drive has resulted in this project showcasing the outstanding talents of South African engineering.

Awards

  • Winner of the SAACE National Award for Engineering Excellence (2004)

  • Winner of the SAFCEC National President’s Award (2004)

  • Winner of the Bentley Systems prestigious international award (civil Design) for 3D and 2D rockfall hazard analysis and design using the Microstation suite of geospatial software packages (2004)

  • Runner-up in SAICE’s National Award for Excellence in Civil Engineering (2004)

Entilini