A durable terrace or path is built up in layers: formation level, capping layer, base course, levelling layer and surface. NfA 1992 specifies layer thicknesses by traffic class (Standard A–D). Here is how the layers fit together.
Introduction
A pavement structure is the layered build-up beneath a surfacing. The layers distribute the load from, say, footsteps, bicycles or cars down into the subgrade, so that the surface does not settle or crack. The order from the bottom up is: levelled subgrade formation, an optional capping layer, base course, levelling/bedding layer, and finally the surfacing itself.
The layers from the bottom up
- 01Planum: the graded subgrade. Evenness under future paving: maximum 4 cm gap on a 3 m straight edge (areas after subgrade removal: maximum 15 cm — NfA 1992, section 3).
- 02Capping layer: laid on soft subgrade to separate and drain — not needed for Standard A (foot traffic only).
- 03Bærelag (stabilgrus): det bærende lag. NfA 1992: B/C = 12 cm i komprimeret stand; A = 10 cm og D = 20 cm er typiske/vejledende minimumstykkelser.
- 04Levelling/bedding layer: typically 30 ± 10 mm (min. 20 mm); bedding gravel for paving blocks 3–5 cm.
- 05Surface: paving slabs, paving stones, cobblestones, setts etc. — each with its own thickness.
Standards A–D — complete paving build-up
NfA 1992 specifies the total pavement thickness according to traffic class and the bearing capacity of the subgrade (three levels per class):
| Standard | Application | Total fastening (good → soft subsoil) |
|---|---|---|
| A | Paths, patios (foot traffic only) | 15 / 25 / 35 cm |
| B | Pavements, cycle paths, driveways | 40 / 50 / 60 cm |
| C–D | Residential/local roads → larger secondary roads | 50 / 70 / 80 cm |
If clayey road gravel is used as the surface, the total pavement build-up increases by 10 cm (NfA 1992). Each type of surface layer has its own thickness — e.g. cobblestones 12 cm, setts 8 cm, paving blocks 6 cm, slabs 5 cm, clinker pavers 4 cm, grass paving stones 16 cm.
Materials in each layer
The layers are not just "gravel" — each layer has its function and its material. Base course and base stabilisation are unbound mixtures whose specifications follow DS/EN 13285.
- 01Base course gravel (BG): coarse, well-draining material that separates raw soil and bearing layer and drains water. Used on soft or fine-grained raw soil.
- 02Stable gravel (SG) for base course: continuously graded (typically 0/31.5 mm) so the fine particles pack the gaps between the coarse ones and give high load distribution capacity (DS/EN 13285).
- 03Setting/bedding layer: a thin, uniform gravel layer (typically 30 ± 10 mm) on which the stones are placed. For paving stones, 3–5 cm bedding gravel is often used.
- 04Geotextile (fibre cloth): laid on levelled raw soil under base layer, where layers must be separated (DS/EN 13249).
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