Cape Town’s Chapman’s Peak Drive is one of the most scenic roads in the world. Carved into the mountainside a century ago, it winds for 9 kilometres along the coastline between Hout Bay and Noordhoek. The original construction of the road was a remarkable feat of engineering – and since then, the feats have continued, with modern efforts to stabilise the route and protect it from rock falls and other damage.
The construction of a road linking Hout Bay and Noordhoek was ordered by Sir Frederic De Waal, the first administrator of the Cape, in the early 1900s. Although the cliffs and ravines along the proposed route were steep and unstable, De Waal ordered that plans for the road go ahead when construction began in 1915.
The winding road, which has 114 curves, was built over a seven-year period with the use of convict labour. Construction began at the Hout Bay end first, with construction of the Noordhoek side beginning in 1916.
To create a stable road on an almost vertical cliff face, the road surface was cleverly built on top of a 630 million year old Cape granite contour. The roadside was carved out of the more workable Malmesbury series sediments.
Opening of Chapman’s Peak:
Chapman’s Peak Drive was opened to traffic on 6 May 1922 by the Governor of the Union of South Africa, His Royal Highness Prince Arthur of Connaught.
The road wasn’t named in honour of any living person, but rather after the mountain around which it skirts. The mountain itself got its name in the early 17th century, when the skipper of a British ship was sent ashore to scout for supplies. The skipper, named John Chapman, later recorded the area of modern-day Hout Bay as Chapman’s Chaunce (Chance), and the name stuck.
Important safety issues:
Despite its extraordinary views, Chapman’s Peak Drive is not without danger. Mud slides and rock falls have long posed a safety hazard for motorists using the road.
Between 1998 and the end of 1999, rock falls along the road resulted in four deaths and several serious injuries. As a result, the road was closed to the public in January 2000.
A triumph for SA engineering:
In order for Chapman’s Peak Drive to be re-opened after its January 2000 closure, the risk to road users had to be minimised. In 2002, this task was handed to Entilini Concessions in the form of a 30-year contract for the rehabilitation and operation of Chapman’s Peak Drive.
Designing appropriate safety measures:
The rock fall safety measures implemented on Chapman’s Peak Drive were based on the best international practises available. Many of these were engineering “firsts” for South Africa.
These design measures required sophisticated three-dimensional computer modelling of the topography of the mountain, cliffs, and boulders located above the road to predict rock fall patterns.
Coupled with high-resolution aerial photographs, this allowed for the prediction of the locations of rock falls, along with the trajectory, bounce height and energy of falling rocks. These predictions assisted engineers to determine the size and shape of the rockfall protection structures required.
The use of cranes, helicopters and abseilers:
No fewer than 11 mobile cranes were used in the construction of the structural safety measures; which include a half tunnel, catch fences, and canopy structures.
Due to the road being too narrow for the cranes to be moved up and down, helicopters were used to lift grouting and drilling equipment to heights of 65 metres and more. Abseilers were also deployed to complete construction in otherwise inaccessible areas of the mountainside.
Safety measures includes the following:
1. Half tunnel
Chapman’s Peak Drive’s 155-metre half tunnel was the first half tunnel used in South Africa. The tunnel was cut into the mountainside at road level to form an overhang. The road was then shifted underneath this protective canopy, which is supported by 95-tonne rock anchors and 150-millimetre thick steel-reinforced shotcrete lining.
In two locations where the cliffs extend more than 400 metres above the road, concrete canopies were constructed over the road, to ensure that any fallen material ends up in the ocean below.
One of these canopies is a 40-metre long, curved cantilever canopy, which arches over both lanes on a tight bend. This canopy is secured to the cliff face at both ends by 100-tonne rock anchors and supported in the centre by 11 large pre-stressed counterfort ribs.
The other canopy is a portal canopy, built in the location of the highest predicted rockfall energies. This canopy is supported by a row of circular sloping columns on the front edge and by columns from behind.
3. Catch fences
Sophisticated Swiss-designed catch fences were implemented to trap fallen rocks along Chapman’s Peak Drive. These 1,6-kilometre fences consist of interlocking rings of high tensile wire and are anchored to the rock with steel wire ropes. The fences have capacities of between 500 kJ and 3500 kJ, and vary in height from 4 to 6 metres.
Thanks to the measures used to ensure the safety of motorists on Chapman’s Peak Drive, the road was re-opened in 2003.
The project went on to win several awards, including:
• SAACE National Award for Engineering Excellence (2004)
• SAFCEC National President’s Award (2004)
• 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).