Chapman’s Peak Drive

The narrow road winds tightly around the mountain, rock face above and steep slope below, dropping sharply into the sea. The view across the shimmering bay to the peaks on the far side is truly breathtaking.

Chapman’s Peak Drive is one of the most impressive scenic drives in South Africa. “Hewn out of the stone face of sheer mountains,” as its inaugural plaque says, the Drive is spectacular. It is also occasionally dangerous. The road on the Cape Peninsular was originally constructed in 1922. It is a tourist attraction and an important arterial road for commuters, business interests and for strategic reasons.

Motorists need to pay close attention. The lanes are narrow, the curves sharp and rock falls have long been a hazard. After a local resident died in a rock fall accident in December 1999, the local authority was forced to keep it closed.

The effect of the closure soon became evident. Tourism and other business suffered. Local residents and users, from running and cycling enthusiasts, to families on Sunday afternoon outings, complained. They put considerable pressure on the authorities to reopen the road, but this could only be done if safety were greatly improved — a design and engineering challenge.

The improvements had to be structurally effective, environmentally sound and had to preserve the scenic beauty of the Drive. The area is also ecologically sensitive. The narrow lanes restrict the movement of heavy equipment and wet and stormy winter weather can cause construction problems and delays. The geo-physical, climatic and aesthetic constraints of the project called for some creative and innovative engineering.

The Chapman’s Peak Design and Construction Joint Venture consortium won the contract to undertake the R150 million ($30 million) project. When the road reopens — scheduled for 2004 — it will have the (dubious) distinction of being the first toll road in South Africa.

Steep challenge

Project manager Marcus Minutelli describes the difficult terrain and the challenge of the project. The road is nine kilometres, three kilometres of which pass under steep cliffs with many loose rocks and boulders at heights of up to 280 metres. Rain loosens the boulders and fire kills the vegetation that helps to anchor them. High winds regularly rake the slopes.

With the help of experienced Swiss engineers and abseilers, the team did topographical and various surveys under three conditions: summer, winter and after fire. They produced a 3D rock fall model and used energy and velocity studies to predict the most likely trajectory and flow path of dangerous boulders. Results showed some 6,000 large boulders, 2,000 of which needed securing.

Rock catchers

Catch fences will be constructed to deal with most of the boulders, but three locations need different solutions and their construction poses a challenge. According to Minutelli, helicopters and absailers will deliver materials and construction staff to sites on the steep slopes.

Catch fences of Swiss design will be installed on the slopes to absorb the shock of falling rocks and trap them. They consist of interlocking rings of high tensile wire secured with tie back cables anchored into the rock. The fences are designed to absorb energy by deforming up to 5 metres when a rock hits. A brown plastic coating will cover the fences to reduce corrosion and blend into the landscape.

A 155-metre “half tunnel” will cut into the base of Chapman’s Peak, the most dangerous part of the road. One side of the “half-tunnel” will be open on the sea side, and the rock roof will provide protection against falling rocks.

In the areas where rocks tend to accumulate on the road, concrete canopies will be constructed that allow rocks to roll over the road and further down the embankment.

When the road opens in 2004, it will be safer than ever before. Tourists and other users hope that the rehabilitated road will largely retain its original character, with significant changes confined to the sections with the canopies and the half-tunnel. The road will remain unlit, and stone-faced barrier walls will be used for guard rails. Motorists will, however, notice some changes. The new speed limit will be 20 to 40 kilometres an hour, new signage will provide information and cameras will monitor traffic. Seismic monitoring will alert authorities when rock falls occur, allowing them to warn motorists and close the road.CCE

Rosalind Cairncross, P.Eng. is a contributing editor of Canadian Consulting Engineer.