Soil Compaction 101: Effective Road Building Preparation

It’s road-building season again. It’s a process that probably looks simple to folks not associated with the construct ion industry. But insiders know better. If you don’t get the roadway foundation just right, things can go very wrong down the road, so to speak. Proper soil compaction is critical.

Why is soil compaction so important?

The point of tamping down the underlying soil is to increase its density by removing air pockets. Dense, uniform soil can carry a heavier load than loose soil, without risk of shifting under the weight. Compaction also reduces water seepage, which can cause problems including frost damage in the winter. Considering the weight of cars and commercial trucks that regularly travel our roadways, it’s easy to see why load-bearing capacity matters.

Improperly prepared underlayment invites problems. Asphalt or concrete can become difficult to maintain, and ultimately it can buckle or bulge or crack. Structural failures mean the roadway will need to be repaired or replaced, at great expense. Proper soil compaction protects the integrity of the finished roadway.

Contractors have to consider several factors to ensure compaction goes well.

  1. Moisture content

Different soils contain different amounts of moisture. To achieve ideal compaction, the soil cannot be too dry or too wet. Neither soggy nor dry soil is stable enough to give you the results you need. There are three basic soil types:

  • Cohesive, or clay
  • Granular, or sand
  • Organic

Organic soil is used for planting and is not appropriate for road-building. Cohesive and granular soils can be compacted. As the name implies, cohesive particles tend to stick together, whereas sandy soils tend to crumble. It’s easy to tell the difference at the jobsite by simply scooping up a handful of soil, compressing it in your fist and then letting it fall to the ground.

If it’s too dry, it will seem powdery even after you’ve squeezed it, and it will crumble as it falls. If it’s too wet, it will retain its balled-up shape when it hits the ground. It may leave your hand wet or muddy. Soil that will provide good compaction will be damp enough to form a shape when you squeeze it but fall into a few pieces when it hits the ground.

When soil has the proper moisture content, you can not only achieve optimal compaction, you can get the work done faster.

  1. Choosing and maintaining compaction equipment

There are several types of equipment you can use to compact soil:

  • Rammers and trench rollers work best on cohesive soils
  • Walk-behind rollers, ride-on rollers and forward/reversible plates work best on granular soils.

Whatever type of equipment you use, maintenance is crucial. Because soil compaction must be done before the rest of the road project can proceed, equipment breakdowns can bring everything to a halt. These machines work hard. Small compaction equipment can weigh as much as 20% of the force it puts exerts. In other words, a 185-lb. rammer compacting an area 11 feet square puts out about 3,000 pounds of force.

Stick to your manufacturer’s maintenance schedule – that typically requires an annual inspection and service. If you rent compaction equipment, ask how often your rental shop or dealership maintains their equipment.

Operator training

Soil compaction is such an art, even veteran operators can get it wrong. The key is to make enough passes over an area to create the desired density, without performing more passes than necessary. That just wastes time.

For trench backfill, you need to add the right amount of soil and then apply the right amount of force. If there is too much soil, it will not compact properly.

Planning each soil compaction project will bring you the best results. Know what you’re dealing with in the way of soil. Choose the right machine for the job. And choose an experienced operator. That way you can be confident your roadway construction job is getting off to the best start possible.