Compaction is invisible work. Once the slab is on top of it, nobody can see whether the hardcore beneath was placed in 200mm layers at the right moisture content or tipped in one lift and levelled with a bucket. The building finds out later.
What compaction is doing
Soil and granular fill contain air voids. Compaction drives the particles together and forces the air out, which increases density, increases bearing capacity and shear strength, and — crucially — removes the capacity for further settlement under load. Poorly compacted fill continues to settle for years, and it settles unevenly.
It also reduces permeability, which matters under floor slabs and behind retaining structures.
The three things that decide the result
Layer thickness. A plate compactor influences roughly 200–250mm of granular material. A roller reaches deeper. Place 500mm of murram and run a plate over it and you get a well-compacted top 200mm sitting on 300mm of loose fill. This is the single most common failure, because thick layers are faster and the surface looks identical.
Moisture content. Every material has an optimum moisture content at which it reaches maximum density for a given compactive effort. Too dry and the particles will not slide past each other; too wet and water occupies the space you are trying to close. On site the practical test is the hand test — a squeezed ball of material should hold together and not leave water on the palm — backed by laboratory Proctor results where the work justifies it.
Number of passes. Compaction is not one pass. Depending on plant and material it is typically four to eight passes with overlap. Two passes at speed to get the plant off site is not compaction.
Material selection
Good fill is well graded — a range of particle sizes so the small ones fill the voids between the large ones. Uniform-sized material cannot achieve high density however much you compact it. Excavated black cotton clay is never acceptable as structural fill; nor is material containing topsoil, vegetation or rubbish, all of which decompose and leave voids.
Where we import, we specify well-graded murram or crushed hardcore and we check what actually arrives against what was ordered, because a cheap load is usually cheap for a reason.
Testing without over-engineering it
On a house, disciplined layer thickness, visible moisture control and a supervisor counting passes are enough. On commercial slabs, access roads, hardstandings and anything carrying vehicles, we test — field density tests at a defined frequency, compared against a laboratory maximum dry density, with a target of 95% or better depending on the application.
The tests cost little. Their real value is that they change behaviour: a gang that knows a density test is coming places thinner layers.
Where poor compaction shows up
- Ground floor slabs that crack and dish within two years, sometimes with a visible hollow ring underfoot.
- Paving and aprons that settle away from the building, tipping surface water back towards the wall.
- Backfilled service trenches that sink into a channel across a driveway.
- Retaining walls that move because the backfill behind them consolidated.
Every one of those is a repair to a finished surface, which is always more expensive than the extra half-day of compaction would have been.
OK Estate Construction and Supply Limited carries out earthworks, filling and compaction across Uganda and South Sudan. Talk to our team.