Sub-base construction and compaction of the base course

Determine what the paving must carry (Standard A = pedestrians only, B = pavement/driveway, C–D = vehicular traffic) and look up the base course thickness. NfA 1992 specifies a crushed-gravel base course in the compacted state: Standard A = 10 cm, B and C = 12 cm, D = 20 cm. The total pavement build-up also depends on the bearing capacity of the subsoil — look it up in NfA, never guess.

Set out and excavate to the levelled subgrade — the depth corresponds to the total pavement build-up plus the surface. Remove soft, organic or topsoil-rich ground that cannot bear loads, and keep the formation at the correct fall so water is already directed away from buildings at this stage.

Lay a 3 m straightedge on the subgrade. Under a future pavement, the gap may be at most 4 cm on the straightedge; on areas that are only subgrade, up to 15 cm is accepted (NfA 1992, section 3). If you level the formation now, you save gravel in the base course later.

Even subgrade = even and thinner gravel consumption. Take time over the formation level.

If the subsoil is soft or poorly draining, geotextile/fibre fabric is laid directly on the graded formation before the gravel goes on. The fabric separates subsoil and base course so that subsoil material does not work its way up and contaminate (mix into) the gravel — one of the most common reasons why paving settles unevenly later.

On soft subsoil (Standard B–D), a sub-base layer is laid under the base course to drain and even out. For Standard A (pedestrian only), a sub-base is not normally needed. Follow the total pavement build-up from NfA — the sub-base is what remains once the surface, bedding and base course are subtracted.

Loose gravel takes up more volume than when compacted. So order approx. 15–35 % extra stabilised gravel relative to the finished, compacted volume (indicative, depends on material). Use the calculator »Gravel / base course volume« and enter the compaction factor and density yourself (stabilised gravel approx. 1.5–1.6 t/m³, supplier guidance).

Spread and compact the stabilised gravel in thin layers — no more than approx. 15–20 cm per layer with a plate compactor, thinner layers with a hand tamper (guideline). Too thick a layer is not compacted right through and causes local settlement. Dampen the gravel lightly if needed so it packs better.

Check the fall with a spirit level and straightedge after each compaction — the fall is built up from below and must be in place before the bedding layer is laid. Walk over the sub-base: it must feel firm and not give underfoot, with no soft spots.

Do you wear hearing protection when using the plate compactor? It typically produces 90–99 dB(A), and hearing protection must be used from 85 dB(A) (Arbejdstilsynet).