We use them every single day; they’re how we get to work and how the things we buy get to the shops – and yet we seldom take a moment to appreciate the network of roads that connect us all – until we encounter a pothole, that is! Few construction projects need to be able to stand more wear and tear than a busy road, and there are dozens of factors that need to be taken into account when a new road is planned. Let’s take a look at some of the different techniques, materials and processes involved in constructing these most hardworking of structures.
Planning and ‘setting out’:
Like so many aspects of modern construction, things aren’t quite as simple as they used to be! In the case of major roads and highways, a feasibility study and environmental impact assessment need to be carried out before any kind of work can begin. This might sound like a headache, but it can save an enormous amount of time and resources further down the line. Having a good grasp of the type of soil on which the road is to be built (known as the subgrade), what the drainage is like and how steep the slope is can all affect what type of road is built and what materials will be used. And once everything has been approved and finalized, it’s time to break ground!
Earthworks and soil stabilization:
Now it’s time to clear the way for construction to begin. This involves removing any vegetation and topsoil to reveal the subgrade and create an even surface known as the founding layer level. Graders, bulldozers and tractor shovels are often used for this process. In a process known as mounting, diggers and bulldozers carve out the shape that the final roadway will take. Drains, sewage pipes and culverts to divert runoff and stormwater may also be laid at this point.
Once the subgrade is accessible, testing needs to be done to determine if stabilization and additional drainage will be required, and if the water table is likely to affect the road stability as it changes with the seasons.
In particularly challenging soil situations, geocell engineering might be used to improve the ground bearing capacity to the point where it can tolerate the loads it needs to. Geocells are honeycomb-like cells made from a nano-polymeric alloy known as Neoloy that are infilled with soil. Much like a pail of sand, the geocell confinement improves the strength of the infill , and helps to spread the vertical load across a wider area, thus making the road far stronger and more durable. Soil may also be compressed with rollers, and ‘sand wicks’ put in place. These are essentially sand-filled boreholes that give excess groundwater and runoff place to dissipate to.
Of course it’s not just the subgrade that needs to be stabilized. If the road route goes through hilly terrain, any cut slopes created by the excavation process need to be stabilized as well.
Once the subgrade has been prepared, each of the three road layers will be placed, graded and compacted, starting with the subbase, base (or formation) and wearing layer or surface (the hard layer which vehicles will travel on). This process will depend on what the final construction method or surface is set to be.
Common types of road surface:
Unpaved Dirt or gravel roads are the most common forms of road construction – over 80% of the roads in the world are unpaved. But, of course, they are not suited for roads that will be getting a lot of heavy traffic. On dirt roads, only compaction via rollers is done, whereas gravel roads receive a layer of, you guessed it, gravel on top. If the soil has been stabilized properly, these roads can actually be remarkably durable, and so are an excellent option for rural regions where costs need to be kept to a minimum.
Bitumen or asphalt roads involve a ‘paving’ mix process combining an aggregate such as crushed stone, and the binding agent bitumen, which is a byproduct of petroleum distillation. Up to four layers of asphalt can be placed on top of each other depending on the expected traffic volume of the road, and the batches of hot asphalt are paved on-site by specialized paving equipment. The major advantage of asphalt is that it makes strong and durable roads, and any damaged surface or potholes can be repaired relatively easily.
Concrete road surfaces are composed from cement (which acts as the binding agent), aggregate, chemical admixtures and fly ash, and may be reinforced using mesh or steel. Concrete roads are very strong and long lasting, and they are a good option for particularly steep roads – but there are also more expensive to construct.
Amazingly, people have been laying roads for thousands of years – as early as 4,000BC! Even today, the process is still evolving, with new materials (such as recycled aggregates and binding agents) and new processes being tested all over the world.