Filling sand is fine sand with a very uniform, fine grading — in Dutch klapzand or plaatzand. It is a backfill material: fine for filling trenches, excavations and voids that carry no load, but not suitable as a bearing layer under a structure.
In dry weather fine sand feels surprisingly firm. That is misleading: as soon as it gets wet or is shaken by vibration, that apparent firmness disappears. In some Dutch regions the term also refers to loamy sand that clearly “sticks” more than ordinary sand.
Where it is used
- Backfilling — trenches, service trenches and the gaps beside foundations where no bearing function is required
- Raising ground level — local raising of garden areas and unloaded parts of a site
- Only marginally under paving — possible, but sharp sand and crushed sand are far better suited
Why not as a bearing layer?
A bearing layer under a floor, terrace or foundation needs sand that compacts and whose grains interlock. Fine, uniform sand does neither:
- Poor compaction — the grading is too one-sided, so the sand settles again even after compaction
- Wash-out — fine sand is easily washed away by rain and under paving; settlement and tilted slabs follow
- Water uptake — wet fine sand can lose most of its bearing capacity
Practical tips
- Choose sand by function — sharp sand for the laying course, crushed sand or recycled aggregate for the sub-base under paving, fill sand for volume
- Compact in layers — never backfill in one thick lift; work in layers of roughly 20 to 30 cm and compact each one
- Ask for the grading — where there is doubt about a load of sand, the particle size distribution or the supplier’s product sheet decides, not the regional name
Related terms
- Sand
- Crushed sand
- Soil compaction
- Paving bond
- Soil type
- Ground investigation
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