Every project needs a stable foundation, and proper compaction is often the answer. But compaction is tricky even for pros. Mistakes are common, so it’s natural to feel some uncertainty when compacting soil, asphalt or any other material.

The following tips can help you choose the right compaction method and avoid settlement, cracks, water seepage, swelling and other issues caused by poor compaction.

1. Use the right compaction method

Different materials require different compaction methods and therefore different equipment. Match the equipment to the material for best results.

Cohesive soils: When compacting cohesive soils such as silt and clay, you need to break the tight bonds that hold the soil particles together. This usually requires an impact or static force (the deadweight of the machine). For small areas, consider a vibratory rammer, which delivers impact force as well as vibratory force. For larger areas, consider a roller with a padfoot drum, aka sheepsfoot roller, to knead the soil. Vibratory plate compactors aren’t the best choice for cohesive soils.

Granular soils: Sand, gravel and cobbles are made of larger particles that slide against each other when compacted. The best way to compact granular soils is with a vibratory or static force. Consider a vibratory plate compactor or a roller with a smooth drum. A rammer isn’t ideal for granular materials.

Asphalt: This mix of bitumen and aggregate reacts like a hot, molten soil. Rammers, plate compactors and rollers with a smooth drum are good compaction choices. Double drum rollers work well for compacting asphalt and subbases on small or medium-size jobs and are ideal for parking lots, driveways, road repairs and other patchworks.

Want to know more? Read about choosing a compactor or roller.

2. Monitor the material conditions

Moisture matters when compacting soil. Water helps particles slide into place during compaction, but too much moisture saturates the soil, leading to under-compaction and potential settlement issues.

With some soils you can test the moisture by hand. Squeeze a handful in your fist. If the material crumbles without forming a ball, or it shatters when dropped, it’s likely too dry, so you may need to add water. If it stays in a sticky ball, it’s likely too wet. To remove moisture, disk the ground and let it air dry.

Getting the moisture level right will let you compact with fewer passes. You may need lab tests to determine the optimal moisture content.

For asphalt, compact while the mixture is hot. Cold asphalt tends to under-compact. Stay alert to ambient temperatures and gusting winds, which can cool asphalt quickly.

3. Get the lift right

Getting the lift (soil layer thickness) right is critical to successful compaction. Thick layers won’t compress as easily as thinner layers, and you may end up with under-compaction no matter how many passes you make. On the other hand, if layers are too thin, the project will take all day.

The ideal lift depends on several factors, including the material type, the moisture level and the compaction equipment being used. It may take trial and error to find the lift that lets you achieve the required density with the fewest passes.

5. Start with a test section

Try the compaction equipment on a small area to see how the material reacts to the machine. You might vary the machine speed, the rammer height if using a rammer and the vibration frequency if using a compactor with vibration. Smaller soil particles will need a higher vibration frequency. Larger particles will need a lower frequency, which may mean a larger machine.

4. Work in layers

Work layer by layer, noticing how the machine reacts, which hints at how dense the material has become. Make sure each layer is adequately compacted before moving on to the next one, but don’t over-compact. Over-compaction could lead to cracks or break down the material, making it weaker instead of stronger.

For many construction projects, a quality assurance consultant will be required to monitor, test and verify the compaction results.

Successful compaction is a science that requires knowledge and experience and you should always consult with an expert regarding the specific circumstances, applicable rules and regulations related to your site and to your situation.

For help finding compaction equipment for your project, give our team a call at 844.873.4948 or visit our online marketplace.