Sizing of fastening — step by step

A pavement is dimensioned so it is strong enough to carry the planned traffic — and yet not over-dimensioned. This requires two things: knowing the traffic class (what must the surface withstand?) and the raw soil bearing capacity (what can the base withstand?).

NfA 1992 operates with three levels of raw soil bearing capacity: good medium and soft. In practice this is assessed based on soil type and any geotechnical survey. Clay soil and humus-rich soil are typically weaker; gravel and sandy soil are typically stronger.

With traffic class and raw soil bearing capacity, the total pavement thickness can be looked up (NfA 1992, section 3.1, measurements in cm in compacted state):

The total thickness is divided into load-bearing layer, levelling layer and surface. The surface thickness is determined by material type — e.g. sett 12 cm, macadam 8 cm, paving stone 6 cm, tiles 5 cm. The levelling layer is typically 30 ± 10 mm. The load-bearing layer thickness makes up the rest of the total pavement (minus any foundation protection).

• 01Standard A: base protection normally not necessary.
• 02Standard B–D on soft raw soil: base layer + possibly geotextile to separate raw soil and base layer and prevent mixing.
• 03Geotextile / filter fabric

The total pavement is distributed across the layers from bottom. NfA 1992 (section 3) specifies minimum thicknesses for the base layer according to Standard, and the finishing layer is kept at 30 ± 10 mm. The surface thickness is determined by the material, and the remainder — what is left up to the total pavement — is executed as a stabilising base layer.

The bearing layer thicknesses are minimum values. On heavy traffic or soft subsoil, the bearing layer is increased, and the difference between the bearing layer's minimum and the total pavement is taken in the subbase. The subbase layer has both a draining and a separating function.

Base course and foundation layer are unbound mixtures whose specifications follow DS/EN 13285 (Unbound mixtures — specifications). The material's strength comes from the grading curve: a continuous grading, where fine particles pack the spaces between coarse particles, provides high load distribution capacity and stability in the compacted state.

• 01Stable gravel (SG) for base course: continuously graded, typically 0/31.5 mm (0/32), with good load distribution capacity (DS/EN 13285; Road Directorate's guidance on stable gravel).
• 02Ground stabilisation gravel (BG) for ground stabilisation: coarser, well-draining material that separates raw soil and bearing layer and drains water.
• 03The requirement for grain curve, fine content and purity is documented by the supplier — request a declaration according to DS/EN 13285 if the project requires it.
• 04Crushed concrete and crushed brick can be used as unbound base layer for light traffic according to the Road Authority's AAB — with own requirements for grading and cleanliness.

On soft or fine-grained subgrade soil, a geotextile (fibre mat) is placed between subgrade soil and subbase. It separates the layers so that subgrade material does not work its way up into the base layer (contamination), and acts as a filter that allows water to pass through but retains soil particles. The properties for road and traffic areas are specified in DS/EN 13249. The mat is selected according to robustness class (mechanical strength during placement and compaction) — the rougher the subgrade and the harder the compaction, the higher the robustness class. Select the class according to the project's AAB, not by price alone.