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 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.

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