We take an in-depth look at how South Africa’s roads are designed and constructed. It's more complicated (and expensive) than you may think...

It is true to say that motoring as we know today would not be possible without the paved road infrastructure at our disposal. It is a crucial part of modern motoring that is often taken for granted. So, what does it take to design and build a national road? We talked to John Craig, a professional engineer and Western Cape regional manager with consulting firm Knight Piésold; and visited the N7 highway between Cape Town and Malmesbury where associate engineer at ERO-Engineers Freddie Henning showed us how a road gets built. Next time you enjoy your favourite set of bends, spare a thought for the people who made your journey possible.

It's expensive and time-consuming to build a road...

Up to 15% of total budget: Cost of consultants for design and supervision of construction

45 days per km; R40 million/km: A new, single-carriage national road (incl. junctions and bridges)

R2 million/km: Resurfacing an existing single-carriageway national road

Up to 400: Workers needed for a new 30 km single-carriage

Up to 100: Road-building vehicles needed for a new 30 km single-carriage

Who builds what?

South African road responsibilities are divided into three jurisdictions: municipalities (mostly city centres); provincial (city surrounds); and national (national road network). Each has its own set of regulations. Whereas the money for municipal and provincial roadworks comes mostly from rates and taxes, the South African National Roads Agency (Sanral) is funded mostly by the fuel levy. When a road building or resurfacing project is identified, it is advertised to attract tenders from consultancy firms, which will design the roads, and contractors, who will build them.

Regulations

Ever considered the radius, bank angle and gradient of a turn when rounding a bend? Well, that’s the road-design engineer’s task. This person must fit a road between two points, taking into account the design requirements of Sanral’s extensive Geometric Design Guidelines document, as well as an environmental study that must be conducted, plus land topography. Computer software has made the task simpler, but an engineer still needs years of experience and skill to efficiently thread a road through a given landscape. According to Craig, design speed is the main initial requirement. All other requirements hinge on the speed at which vehicles will travel. National roads are set mainly at 120 km/h and the regulations provide many tables stating the minimum requirements for radii and bank angle (also called super elevation; see graphic below on turn radii).

These requirements are based on the lateral forces experienced by a vehicle. For example, the friction coefficient of a sportscar tyre and the road is close to 1,0. That means an expert driver would feel a maximum lateral acceleration of 1 G when rounding a flat turn. However, roads are used by various vehicles in changing conditions and are operated by drivers of different skill levels and reaction times. Therefore, the calculations assume a conservative friction coefficient of about 0,2. A tendency for the regulations to become even more conservative stems from studies on driver comfort and has resulted in the lateral acceleration value being lowered to roughly 0,2, as the previous slightly higher values were considered excessive for some.


Basic road-building steps

Once the road design has been approved, the road-building contractors take over (the consultancy firm responsible for the design retains an advisory role). To create a road, the following needs to happen:

Preparation

This step involves marking the road reserve (80 metres wide in case of a national road) using specialised measuring equipment. Next, the road reserve needs to be cleared of vegetation; earth-moving equipment is used to clean up the area according to the specifications of the design document, including cutting through hills or filling depressions. Before any ground work starts, an environmental-impact study is conducted and any indigenous plant and animal species on the endangered list are rehabilitated elsewhere. According to Henning, much fynbos and many tortoises were moved during the N7 project.

Materials

The materials that are used to construct the different layers of the road are mainly transported to the building location. In some cases, a quarry is created close by to help supply fill material.

Layer by layer

Road building consists of a layering process and this differs between climatic regions. Each layer has to be built to a high specification and water is extremely important during the compacting phase. The damp layer needs to dry out before the next layer can be put down, which is why you will see stretches of road under construction that seem to be devoid of workers and machines for long periods. Layer material is classed according to a G scale, where a higher number denotes a higher-specification product. To ensure it meets the required specification, there is a laboratory on site to test the material in each layer.


Road markings

Markings are painted on the road in accordance with the South African Road Traffic Signs manual. These requirements are based on mathematical and geometric calculations to ensure safe behaviour. According to Craig, the markings are conservative and, if in doubt, a solid centre line is the default choice.

Traffic control

When an existing road is resurfaced, the traffic is rerouted. This, too, is finely regulated because a crash resulting from incorrect signage may mean legal action against responsible parties.

Future of road building?

In South Africa, our labour-intensive method of road construction is unlikely to change dramatically because it creates many jobs. In Europe, however, there is automated technology that can build roads far quicker and with minimal human supervision. These road trains set down the layers of the road in sequence and, as bitumen is used as the ingredient for most layers, there is no need of compacting or sealing afterwards. The cost of construction is higher, but it’s quicker and therefore less disruptive.


Our thanks to Knight Piésold’s John Craig (www.knightpiesold.com) and Freddie Henning from ERO-Engineers (www.ero-engineers.co.za).

Author: Nicol Louw