The first Roman roads were stone paved, built in North Africa and Europe for military operations. Road construction techniques were gradually improved by the study of road traffic, stone thickness, road alignment, and slope gradients, developing to use stones that were laid in a regular, compact design, and covered with smaller stones to produce a solid layer.
Very broadly, the construction of roads can be described by three processes:
This is carried out following the dimensions specified in layout drawings.
A commonly used setting out procedure is the profile board method. A series of boards that show the exact level 1 metre above the completed construction level are placed at intervals along the proposed line of the road. A profile board with a fixed height, called the traveller, is used for controlling the excavated levels between these profile boards. By placing the traveller in the sightline between two level boards, it can be seen whether or not the excavation has been carried out to correct levels and adjusted accordingly.
The level of each profile board is controlled using a line level which is a short spirit level hung from a nylon string. The line operator moves the string up or down until the bubble is centred.
Junctions, hammer heads, turning bays and intersecting curves are laid out in a similar manner.
Earthwork is one of the major works involved in road construction. It involves the removal of topsoil, along with any vegetation, before scraping and grading the area to the finished ‘formation level’. This is usually done using a tractor shovel, grader or bulldozer. Below the formation level, the soil is known as the ‘subgrade’. It is essential that the strength of the subgrade is tested prior to earthwork beginning.
Most earthworks are formed by cut-and-fill, and the type of ‘fill’ material must be considered, not only in terms of its physical properties, but on the conditions in which it is to be used, and the methods of compaction.
Depending on its quality, compressible subsoil may be removed or stabilised. If the cost of full or partial excavation of subsoil is uneconomical and would be likely to result in consolidation, sand wicks or sand drains may be used. Sand wicks are sand-filled boreholes beneath the road embankment that give greater stability to the soil by decreasing the length that water has to travel in a drainage path, so dissipating water pressure. Sand drains alongside the road are used to intercept ground water.
The strength of the subgrade can be achieved by using the following techniques:
- Removal of poor material in cuttings and replacing with selected fill.
- Compacting subgrade to a high dry density.
- Providing adequate subsoil drainage.
- Soil stabilisation methods such as the use of cement, bituminous materials or chemicals.
For more information, see Types of soil.
The subgrade strength will decrease as moisture content increases so protection may be required if it is to left exposed for any length of time.
Protection covering can be either:
- Medium gauge plastic sheeting with 300 mm laps.
- Sprayed bituminous binder with a sand topping.
 Paving construction
Once the subgrade has been prepared and drainage or buried services installed, the paving construction can begin. Paving can be either flexible or rigid. There are pros and cons to each type, with one being selected over the other depending on the specific needs of a project.
Rigid pavements tend to have lower maintenance costs, a longer design life and higher flexural strength; but flexible pavements tend to have lower construction costs and have a higher ability to expand and contract with temperature and so do not need expansion joints.
Flexible paving consists of materials applied in layers directly over the subgrade to which the traffic loads are distributed. To prevent permanent deformation, and therefore an uneven running surface, the thicknesses of individual layers must be capable of distributing such loads. The subgrade is compacted with the sub-base on top of it. On top of this is laid the surfacing which is made up of the base layer and the wearing course.
The wearing course is the upper layer of bituminous material, often denser and stronger than the base layer. The thickness depends on the material specification and the amount of wear that is expected. Desired properties are good non-skid capabilities, minimal glare and acceptable durability.
The main materials that are used are hot rolled asphalt (HRA), dense bitumen macadam (DBM), dense tar macadam (DTM) and porous asphalt (PA). PA is especially suitable as it is an open-graded material that is designed to allow rapid drainage of surface water, thereby reducing spray as well as tyre noise.
The base will typically have a minimum thickness of 60 mm and is usually made of dense bitumen macadam or asphalt. It is laid with the appropriate crossfalls and gradients.
This is placed in a layer usually not exceeding 150 mm over the subgrade after waterproofing is complete. Various materials can be used but it is common for crushed stone or dry lean concrete (such as 1 : 15) laid and compacted by heavy rollers.
Rigid paving consists of a reinforced or unreinforced insitu concrete slab laid over a thin granular base course. The rigidity and strength of the pavement enables the loads and stresses to be distributed over a wide area of the subgrade.
Rigid paving is made up of the following layers (from top to bottom):
- Sub-base of thick crushed stone. Usually to a thickness of 80 mm.
- Anti-friction membrane normally made of polythene sheeting. Also prevents grout loss from freshly laid concrete.
- Insitu concrete paving slab. Reinforcement in the form of either steel fabric or re-bar may be used.
- Asphalt or similar topping if required.
Longitudinal and transverse joints are required in rigid paving between the slabs, limiting the stresses applied due to subgrade restraint (friction between the pavement and subgrade), and providing room for expansion and contraction movements. The spacing of road joints is determined by:
- Thickness of the slab.
- Whether there is reinforcement in the slab or not.
- The expected traffic load and flow rate.
- The temperature at which concrete is laid.
For more information, see Road joints.
For more information, see Highway drainage.
For more information, see Kerbs.
 Related articles on Designing Buildings Wiki:
- Bituminous mixing and laying plant.
- Bridge construction.
- Ground conditions.
- Ground investigation.
- Highway drainage.
- Infrastructure and Projects Authority.
- Insitu testing of soils.
- Land surveying.
- NEC contracts - road development and management schemes.
- Overview of the road development process.
- Railway engineering.
- Road improvement scheme consultation.
- Road joints.
- Road traffic management.
- Runway construction.
- Setting out.
- Types of soil.
- Types of road and street.
- Types of roller.
- Underpass construction.
 External references
- ‘Introduction to civil engineering construction’ (3rd ed.), HOLMES, R., The College of Estate Management (1995)
- ‘Building Construction Handbook’ (6th ed.), CHUDLEY, R., GREENO, R., Butterworth-Heinemann (2007)
Featured articles and news
CEOs and high-level executives explain who they expect to be the most successful players in the future of construction.
What are package contracts and how are they broken down? Find out in our introductory article.
Identifying sustainable shoreline protection solutions in the face of rising sea levels and storms in the US.
Budget documents state modern methods of construction will be favoured for public infrastructure schemes from 2019.
A walk-through exhibition of an emergency humanitarian shelter is officially opened at BRE's Innovation Park.
How to work safely on a construction site during winter.
Housing is the big winner in Chancellor Philip Hammond's Autumn Budget.
The winner of our BSRIA competition, Tomorrow's challenges in today's buildings, is.... Bob Hendrikx. A big thank you to everyone that took part.
Committee of MPs accuses government of dealing billpayers a 'bad hand' over the guaranteed power price.
In 1992, the Joint Fire Code was first published. What influence does it still have on construction sites today?
"Companies will have to adapt or go out of business" - how are virtual reality and big data disrupting digital construction?
International Well Building Institute and BRE collaborate on multiple levels to advance human health through better buildings.
"The industry has tried moving away from prescriptivism to focus on performance, but maybe that’s no longer working".