Loose soil and gravel settle when compacted. Here are NOVA 2015's requirements for the degree of compaction, how to check it on a small project, and why you must order more material than the finished layer takes up.
Introduction
When a layer of gravel or topsoil is laid loose and then compacted with a plate compactor or roller, it takes up less space than the loose material. If you order only the finished, compacted volume, you will end up short of material.
How compact must it be? (degree of compaction)
The degree of compaction tells you how densely the material is packed compared with the densest packing you can get with the same material in a laboratory test. The test is called the Proctor test. So 95% means the gravel is packed to 95% of what the laboratory could achieve. NOVA 2015 uses two kinds of test: standard Proctor (ramming) for clay soil and vibratory Proctor for gravel and sandy soil.
| Material | Average / minimum per test | Test |
|---|---|---|
| Stabilised gravel | > 95 % / 92 % | Vibratory Proctor |
| Frost protection gravel | > 95 % / 92 % | Vibratory Proctor |
| Subsoil, sandy | > 95 % / 92 % | Vibratory Proctor |
| Subsoil, clayey | > 96 % / 93 % | Standard Proctor |
| Stone and macadam | Cannot be measured — compact with heavy machinery until the stones are locked in place | — |
The figures are measured with a nuclear density gauge (an instrument lowered into the layer). Crushed concrete, brick and asphalt must have the same degree of compaction as stabilised gravel. The requirements for subsoil apply down to 2 metres below the finished surface.
The figures apply to roads. For small pavements with no risk of overloading you may go 2 percentage points lower, and for terraces 4 percentage points. NOVA itself states that there is no documentation for these values.
Checking on a small project
On a road, compaction is measured several times per control section. On a small project, the check may be based on process control: how big the machine is, how many times you pass over the layer, and simple tests. You compact until the plate compactor, the wheels or the tool no longer leave marks in the surface (NOVA 2015, pp. 66-67). Compaction works best when the gravel has the right moisture content and the machine suits the layer thickness.
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