Roads & Runways

Setting Out:
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.

Earthworks:
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.

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.

The construction of runways is similar to that of roads in that the type of pavement required depends on the loads needing to be carried, although the stresses applied by aircraft can be very high and variable, up to 8 times greater than those on roads. Pavements must facilitate safe aircraft ground operations, and in order to achieve this they must meet a number of performance requirements:

1) Rideability,
2) Good friction and drainage characteristics,
3) Strength and stability sufficient to withstand shear stresses induced by heavy wheel loads and high tyre pressures,
4) Durability,
5) Resistance to fuel spillage and jet blast,
6) Low maintenance requirements.