Every road built to stand up to traffic needs a well-laid road bed. The purpose of this bed is to bear the weight of the road surface and help control environmental factors. Modern roads facilitate the movement of heavy and constant traffic. The surface must be properly supported for the road to stand up to continued use without major repairs. Modern road beds generally have several layers to perform their intended design function.
The bottom layer of any road bed is always the existing soil or rock where the road will be laid. It is levelled to the specific needs of the road surface that will be laid on top of it. Levelling in this section makes it easy to grade each successive layer for a smooth road surface in the final phase. Where rock is present, blasting is often the first step in the levelling process. This section of the road bed is called the sub-grade.
The next step in creating a road bed is laying the sub-base. This step is optional in some cases. If the rock or soil is properly levelled and can support the weight, a sub-base layer is unnecessary. Filling depressions in this layer will consist of smaller aggregates such as silica sand and gravel pieces. It may be necessary with some soil surfaces to add this layer for the entire length of the road bed.
The final layer before the road surface is called the base course. This directly supports the road surface and is always used when laying a modern road. Many base courses use a combination of dried aggregates such as gravel and silica sand. The road surface chosen is then laid on top of this course. In places where the road surface is above the surrounding landscape, an aggregate of gravel is often used on the road side. This allows rain water to pass directly into the ground and help prevent erosion on the hillside.