SIMAQ School · Course

Soil compaction

The soil decides. Learn which machine to use by soil type, and how lifts, passes and moisture work.

Which compactor do I need?

Answer two things and we'll tell you the exact machine and why.

1 · What kind of soil?

The fundamental

Compacting expels air, not water

Soil is three things mixed: grains, water and air. Grains don't compress and neither does water. The only thing you can drive out is air — and everything else follows from that.

Too dry

With no water to lubricate, grains lock by friction and resist rearranging. However many passes you give, air voids stay trapped between them.

On point

A film of water on each grain cuts friction just enough for them to slide and nest. Same machine, same effort, maximum density.

Too wet

Water has filled the voids. Since it doesn't compress, now it's water stopping the grains from closing. The soil pumps under the roller instead of densifying.

The curve that rules your contract

Plot dry density against moisture and a hump appears: the Proctor curve. Its peak is the most that soil can give, and the moisture where it happens is the optimum. When a spec demands “95% of Proctor” it means that peak — and if moisture is off, that 95% doesn't exist that day.

Material moisture9%

Moisture on point100%of max Proctor

Here the soil gives everything it can. Passes now actually turn into density.

Note the difference between the two soils: granular peaks higher and earlier, with a sharp hump — easy to overshoot. Cohesive peaks lower, later, and flatter: more forgiving on moisture but never as dense. That's why a gravel base is specified differently from a clay embankment.

The theory

Vibration or impact: the soil decides

Granular → vibration

Sand, gravel and crushed base settle with high-frequency vibration: particles rearrange and air escapes. Vibratory plate or smooth roller.

Cohesive → impact

Clay and silt don't respond to vibration: they need impact that kneads the soil. Jumping-jack rammer or padfoot roller. Mixing them up is the most common mistake.

Machine	Soil	How it compacts	Best for
Vibratory plate	Granular	High-frequency vibration	Base, pavers, open areas
Rammer (jumping-jack)	Cohesive / trench	Impact (kneading)	Trenches, corners, clay
Smooth roller	Granular	Weight + vibration	Roadway, embankment, large area
Padfoot roller	Cohesive	Deep kneading	Cohesive embankment, trench
Pneumatic roller	Mixed / asphalt	Pressure + kneading (rubber tires)	Thin lifts, asphalt pavement

Deeper

Energy types and depth

Each machine delivers energy differently — and reaches a different depth.

Depth of influence

Static

Static pressure and kneading, from the roller's weight alone.

Up to ~1 m

Depth of influence

Vibratory

Low amplitude, high frequency: ideal for granular and asphalt.

Up to ~2 m

Depth of influence

Impact

High amplitude, low frequency: kneads cohesive soil deep.

Over 2 m

How do you know if compaction is acceptable?

It's measured against the Proctor test (the soil's maximum dry density at optimum moisture). Sites require a degree of compaction —usually ≥ 95% of Proctor— verified with in-situ density tests: sand cone or nuclear gauge. If it falls short: more passes needed, the lift is too thick, or the moisture is off.

Your machine

How much lift each machine handles

Energy dissipates with depth. Above a certain lift the machine densifies the top and leaves the bottom loose — and that shows up months later, when the floor sinks.

Sand, gravel, base. Compacted by vibration.

Lift thickness and passes per machine in granular soil
Machine	Energy	Loose lift	Passes
SQRD L71 Ride-on roller 4.2 t	40 kN · 150 N/cmpublished	35–45 cm	4
SQRD L61 Ride-on roller 3 t	2×30 kN · ≈12.5 kg/cm	30–40 cm	4
SQ15 PRO Vibratory plate	13.0 kN · 5,500 VPMpublished	25–30 cm	4
SQRD L51 Ride-on roller 1.5 t	22 kN · ≈8.3 kg/cm	25–30 cm	4
SQ60 PRO 2 Rammer	13.7 kN · 34×29 cm shoe	20–25 cm	3
SQRPTRC L202 Padfoot (trench)	70 kN · 1.53 t	20–25 cm	4
SQRDPWB L102 Trench roller	17 kN · ≈5.7 kg/cm	20–25 cm	4
SQRDWB L34 Walk-behind tandem	17 kN · ≈6.2 kg/cm	20–25 cm	4
SQRD L41 Ride-on roller 0.8 t	20 kN · ≈5.7 kg/cm	20–25 cm	4

“Loose lift” is the material thickness BEFORE compacting; after the passes it drops 15–25%. Ranges are guidance by machine class, derived from published specs.

The test strip: the only truth

No table knows your material. Before committing, compact a strip with the same soil, machine and site moisture, and test density after 2, 4, 6 and 8 passes.

Density climbs fast on the first passes, then flattens. That knee is your pass count: going beyond it burns fuel and machine hours for no density.
If the curve flattens BEFORE 95%, passes aren't the problem. It's moisture, or the lift is too thick for that machine.
Repeat the strip when the borrow pit changes or after rain. Yesterday's soil isn't today's.

Interactive

Lifts, passes and moisture

Move the controls and watch the density change.

Lift thickness25 cm

Recommended: 20–30 cm per lift.

Passes4

Soil moisture

Density reached

85%

Almost — tweak lift, moisture or passes

Target ≥ 95%

Compacted

Compact in 20–30 cm lifts: any thicker and the bottom never reaches density no matter how many passes. Watch moisture (optimum is the Proctor value). And passes have diminishing returns: the first does a lot, the sixth almost nothing.

The machine for your soil