Best Practices

Proof Rolling

What Is Compaction Proof Rolling?

Proof rolling is a method of testing how well a subgrade has been compacted. Once compaction is complete, contractors will proof roll the material before installing the overlying asphalt or pavement layers. This test or proof is a visual assessment to help determine whether the subgrade will enable the construction of the subsequent layers of material.

Why accurate subgrade compaction is critical

Subgrade compaction is the method of grading, shaping, and compacting the natural underlying materials before adding an aggregate base or pavement. It improves the load bearing ability of the ground by increasing the density of the soil or base. Different soil types have various optimum moisture contents and densities, with sandy soils, for example, typically achieving higher densities than silts or clays do.

Due to the variations in optimal densities for different types of material, proof rolling is a necessary step to ensure your subgrade is adequately compacted.

When are proof rolling methods used?

Most engineers recommend an optimum proctor density range of 95-100% for pavement bases and trench backfill under pavement. Testing samples of the subgrade in a lab is often used to ensure the optimal density is met. However, when the material isn't uniform either in gradation or composition, coming up with an accurate proctor sample is difficult and requires time consuming lab testing from various locations of the subgrade. In these situations, testing may not be used and proof rolling becomes an even more important method for determining if the subgrade is compacted sufficiently.

How to proof roll compacted subgrade

The most common proof rolling methods are driving either a smooth drum roller or fully laden dump or water truck onto the subgrade. The roller or truck moves slowly over the ground as an engineer walks alongside it to see if there is any visible deformation. The engineer will note how much deflection there is in the subgrade during the proof roll, with the desired outcome being no signs of deflection on the material's surface. What they don't want to see is a subgrade that heaves under the applied load, as this usually indicates there is a high level of moisture or high organic content in the material.

If the subgrade fails the proof roll, there are several options for contractors to move forward. First, if the underlying material is wet you can let it dry out and then proof roll again. In other cases, large rock sizes in the ground prevent the subgrade from interlocking. If the material is suitable, the contractor could blend in finer crushed rock to improve interlocking. For clay subgrades that fail proof rolls, the most common method for improving the compaction is layering with cement or lime.

When possible, use compaction testing

It's important to remember that proof rolling is not a very technical method and simply a visual assessment. Your subgrade should also pass all the required compaction tests when possible, to ensure it can support the weight of the aggregate, asphalt, and pavement layers you add on top of it.

If you have any questions about proof rolling, contact our team at Miller-Bradford & Risberg.

Want to complete compaction jobs in fewer passes? BOMAG rollers incorporate the BOMAG ECONOMIZER to continuously measure the degree of compaction.