Soil compaction is the pressing together of loose soil to remove the air and voids between the grains, so the ground becomes denser, stronger and better load-bearing. Compaction is one of the most common operations in earthworks and largely determines whether a subsoil stays stable.
Why compact?
Loose, freshly placed soil contains a lot of air. Without compaction, that soil would later settle under the load of a building, road or paving, causing subsidence and cracking. Compaction provides:
- Higher bearing capacity — the ground can carry more load
- Less settlement — the soil is already pressed together and sinks less later
- More stability — less risk of shifting or washing out
Methods and equipment
The choice of plant depends on the soil type and layer thickness:
- Plate compactor — for sand and small areas, such as under paving slabs and blocks
- Vibrating roller — for larger areas such as road formations and embankments
- Rammer (jumping jack) — for narrow trenches and corners
- Drop weights (dynamic compaction) — for greater depths on heavy projects
Compaction is done in layers. A rule of thumb is a layer thickness of about 20 to 30 cm per compaction pass; layers that are too thick are not properly compacted at the bottom. Sand compacts best when slightly moist (optimum moisture content). On important works the density achieved is checked, for example with a Proctor test or plate load test.
Related terms
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