Road construction and base layer construction — from soil to base layer

A road looks simple when it is finished — asphalt or stone on an even surface. But under the surface there is a whole structure of layers each with a function. Build wrong from the bottom and it shows up years later as ruts in the asphalt or subsidence in the pavement and then the damage is done.

§The soil determines the structure

The first thing you as a civil engineering structure worker must consider is the subsoil. The education requires that, based on a given soil type, you can choose and carry out the right base construction for a road or civil engineering task. Sand, clay and filled soil bear and behave very differently: sandy soil drains well but can be loose, while clay holds together better but is more sensitive to frost and moisture. The assessment of the subsoil is therefore not a formality – it determines how strongly you must build up, and whether the soil needs to be replaced at all.

§The layers in a road construction

A typical road or square construction consists of several layers on top of each other. At the bottom is a subbase layer that protects against frost and distributes the load down into the underlying soil. On top comes the base layer — often stabilized gravel — which gives the construction its bearing capacity. Then follows a leveling layer that makes the surface even and precise and finally a wear layer: either a bituminous surface (asphalt) or paving stones and tiles depending on the task.

• 01Frost protection layer — protects against frost and distributes load into the ground
• 02Base layer (e.g., stabilized gravel) — provides the structure with its load-bearing capacity
• 03Levelling layer — makes the surface even before the wear layer
• 04Wearing course – asphalt or paving stones, what traffic and water meet first

§Traffic load determines thickness

How thick each layer should be depends on how much traffic the road or square should carry. A pedestrian path requires a completely different structure than a driveway for trucks or a public road. For this purpose, the construction industry uses the Road Directorate's road rules and manuals for dimensioning pavements, which specify how thick layers of stable gravel and bituminous materials are required at different traffic classes and soil conditions.

§Drainage, beds and frost

A road structure only holds if water is efficiently conducted away from the construction. Ditches and drainage along the road prevent surface water from seeping down and softening the base layer from within and the shoulders on the sides should support the edge of the pavement so it does not break down under traffic load. In parts of Denmark where the soil is particularly frost-sensitive additional considerations about frost protection are included — either by building sufficiently thick frost-safe layers or in special cases by using frost protection insulation under the construction so freeze-thaw cycles do not lift and crack the road over time.

§From offset to finished road

The entire structure depends on your leveling and layout: elevations, falls and lines must be set correctly before the first layer is excavated, and you must be able to calculate quantities of gravel, stable material and paving so you neither run out nor have surplus lying around. It is the combination of the correct soil assessment, correct layer structure and precise layout that determines whether the road or square will last for decades.

Even the best materials and the most accurate placement cannot save a structure where quality control fails along the way. Therefore dimensions and load-bearing capacity are typically checked continuously with measurements and in some cases load tests so any weak points in the foundation are detected and corrected while something can still be done about it — not only after the surface layer is already on top.

“No one sees the load-bearing layer when the road is finished – but it's what carries all traffic over it.”