Bulking and swell

Frequently asked questions

Why is there more spoil than hole?

Because undisturbed ground is a settled structure and a heap is not. In the bank, particles have been packed by centuries of overburden and water into an arrangement with comparatively few voids. Digging breaks that arrangement apart: the same solids are now loosely piled with air between them, so the volume goes up — modestly for sand, which was already loose, and substantially for stiff clay or rock, which was tightly bound and breaks into lumps that cannot repack themselves. Typical figures run from a small percentage for sand up to half as much again for clay and more for blasted rock. The consequence for a job is entirely practical: excavation is measured in the BANK, because that is the hole being dug, but everything that leaves site — lorry loads, skip counts, tip charges — is measured LOOSE. Ordering disposal from the excavation quantity under-orders it by the bulking factor every time.

If it bulks when dug, why does backfill run short?

Because two different changes happen in sequence and only the first is an expansion. Digging loosens the material, and compacting it puts it back into a dense arrangement — but the machine and the layer thickness do a better job than the original geology did in some soils and a worse one in others, so the compacted volume can be less than the bank volume it came from. That second change is what American usage calls SHRINKAGE, and it is measured from bank to compacted while swell is measured from bank to loose. Nothing has been lost: the same solids occupy less space because there is less air between them. This is the arithmetic behind the routine surprise on a cut-and-fill exercise, where a balanced cut and fill on the drawing needs imported material on site. Anyone reconciling earthworks has to carry three volumes rather than one, and to say which of the three any figure is.

Is bulking of sand the same thing?

No, and the shared word causes real errors in concrete. BULKING OF SAND is a surface-tension effect: a film of moisture on damp sand grains holds them apart, and sand at a few percent moisture can occupy a quarter more volume than the same dry sand — with the expansion peaking at a moderate moisture content and collapsing again once the sand is saturated, because water then fills the voids instead of separating the grains. It has nothing to do with excavation and everything to do with batching. Measuring aggregate by VOLUME from a damp stockpile therefore puts materially less sand into a mix than the proportion intended, which weakens it, and the effect is largest exactly where site sand is normally kept: damp, uncovered, part-used. It is the main reason concrete specifications are written by mass rather than by volume, and why a site-batched mix from a shovel count is not the mix on the drawing.