How Our Roads Are Built

Roads are a relatively ancient invention for the modern human. The first mention of road building came was over 6000 years ago when the people of Mesopotamia built roads in the cities of Ur & Babylon. The Sumerians formed mud bricks and after drying, these would be laid in bitumen. Bitumen is the natural sticky black substance in asphalt (1). The first roads in Europe however were built of timber and stone. Centuries would pass before asphalt was used in Europe and America.

Today in the UK, we have a road network totalling approximately 262,300 miles which, if set in a straight line, would circle the earth 10 and a half times. Over 95% of our British roads are made up of asphalt (2).

Simple road construction can be split into two categories;

· New Road Construction

· Maintenance of existing roads.

Today, we will discuss how a new road would be built.

Very broadly the construction of new roads can be described in the following 3 processes;

· Setting Out

· Earthworks

· Paving Construction

Setting Out

The most commonly used setting out procedure is the profile board method. In essence, it’s a series of boards that show the exact level 1 metre above the completed construction level which are placed at intervals along the proposed road.

A separate profile board with a fixed height called a traveller is used for controlling the excavated levels between the original profile boards. The traveller is placed in the sight line between the two profile boards so it can be seen whether or not the excavation has been carried out to the 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.

Earthworks

Topsoil is removed, along with any vegetation, before scraping and grading of the area to a finished formation level. Below the formation level, the soil is known as the subgrade.

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.

“It’s essential that the strength of the subgrade is tested prior to the earthworks beginning.

The required thickness of the pavement is determined by the subgrade strength, so it is important to make the subgrade as strong as possible.

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 (3).”

Paving Construction

Once the subgrade has been laid and all relevant services and drainage is installed, the construction of the paving can commence.

Paving material can fall into one of two categories:

· Flexible Paving

· Rigid Paving

Flexible paving consists of materials placed in layers on top of the subgrade. The first layer is named the base course. It is constructed to a depth of 100 to 150mm is generally built up of construction aggregate. The aggregate is spread and compacted to a minimum of 95% relative compaction, thus providing the stable foundation needed to support either additional layers of aggregates or the placement of an asphalt concrete wearing course which is applied directly on top of the base course.

The wearing course is often denser and stronger than the base layer. The thickness is dependant on the specification outlined by the engineers which is based on the amount of wear, durability and maintenance. The surface course should have good non-skid capabilities and minimal glare

The main materials that are used are hot rolled asphalt (HRA), dense bitumen macadam (DBM), dense tar macadam (DTM) and porous asphalt (PA). Porous asphalt is mainly used 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.

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;

· Subgrade

· Sub-base

· Anti-friction membrane normally made of polythene sheeting

· In-situ concrete paving slab

· Asphalt or similar topping

Rigid paving is required when installing longitudinal and transverse joints.