How to compact soil: step by step, by hand or with a machine

Before you start: clear it and know your soil

Remove roots, rubbish, mud and large stones: none of it compacts and it leaves voids that sink later. If you're filling, use even material without big clods.

Then look at what soil you have, because that decides the machine. Sand, gravel and crushed base (granular soil) compact by vibration, with a plate. Clay and silt (cohesive soil) compact by impact, with a rammer. If a squeezed handful of damp soil forms a sticky ball, it's cohesive; if it crumbles, it's granular.

Moisture: neither dust nor mud

Moisture matters as much as the machine. Dry soil won't settle and kicks up dust; with too much water it moves like jelly and never firms up. The site test depends on the soil. In clay, silt or clayey earth, squeeze a handful: if it holds as a ball without dripping water, it's right; if it falls apart as dust, it needs water; if it drips, it has too much. In sand or gravel don't expect a ball: even at the right moisture it crumbles when you open your hand, because that's what granular soil does. There, go by colour and feel: at the right point it looks dark and leaves your palm damp without releasing water; if it runs loose and pale like dry sugar, it needs water; if water squeezes out or it puddles, it has too much.

• If it's dry, water it evenly with a hose or watering can and wait for the water to soak in before compacting.
• If it's soaked, let it dry out or swap the material: compacting mud doesn't work.
• Sign from the machine: if the ground pumps or sinks, it's too wet; if it stays loose however many passes, it's too dry.

Lifts and passes with a machine

The rule broken most often: soil is compacted in lifts, never the whole fill at once. A thick lift looks firm on top and stays loose underneath; months later, the floor sinks. These are the loose-lift thicknesses and passes for each machine. If your project calls for another thickness, the spec rules:

• Work in straight strips, from the edges towards the centre, overlapping each strip on the last.
• Full throttle on both machines: at half throttle the plate skids and the rammer gets damaged.
• Next to pipes, walls and manholes, thinner lifts and the rammer.

By hand: when it works and how

Tamping by hand, with a hand tamper or mallet, works for small fills that won't carry weight: around a post, a planter, the bottom of a manhole. The method is the same as with a machine, in miniature: soil at the right moisture, thin lifts, even blows overlapping each one on the last, and a check before the next lift.

For a floor, a sidewalk, a slab or a pipe trench, hand tamping doesn't reach the density the job needs, and there's no way to know how much reached the bottom of the lift. That's machine work: a plate for open areas and granular soil, a rammer for trenches and clay.

How to know it's compacted

On site, the most reliable sign is that the machine stops sinking and the surface no longer changes from one pass to the next. With the rammer, when it starts bouncing instead of sinking, the soil is dense. If the job calls for a compaction percentage, a lab tests your material: compact with that test in mind, not with how it looks.

• It barely leaves a mark → that lift is done: spread the next one.
• It keeps sinking after the passes → the lift is too thick or the moisture is wrong.
• The plate bounces and won't move forward → it's probably clay: switch to the rammer.