Subgrade compaction is the act of grading, shaping, and compacting the natural subgrade materials prior to placing an aggregate base or pavement. It mechanically increases the unit volume (density) of the soil or base. Different soil types have different optimum moisture contents and densities. Sandy soils require lower moisture contents and can typically achieve higher densities than silts or clays do. Also silts and high-fine clays are more prone to moisture retention and frost heave making them generally less optimal for performing well as a pavement subgrade.
Compaction achieves the following:
- increases load bearing capacity
- helps reduce future rutting/settling
- reduces voids which increase susceptibility to moisture changes/freeze thaw
- help ensure that freeze/thaw movement is uniform.
Subgrade compaction is an act that is sometimes overlooked in private, non-inspected, construction projects but (depending on the soil type and condition) can be critical in future performance of pavements.
It’s important to note that for subgrade areas that are found to be man made fill, overly moist, or otherwise questionable, subgrade compaction may actually make the situation worse by disturbing, consolidating, or working moisture up to the surface. Soils with higher than normal moisture content will ‘pump’ underneath tires and tracks of grading equipment, compaction equipment, trucks, and paving equipment. In these cases staying off of the subgrade and leaving it virgin may be the better option ( if undercutting and replacement with premium material is not in the budget).
A common test used in conjunction with subgrade compaction is the Proctor Density Test. This test classifies the existing material and defines the desired maximum density of a particular soil type and it’s optimum moisture content. A nuclear density gauge is a piece of testing equipment typically used on-site to check the density of a sub-base or subgrade. It will also indicate the moisture content.
Most engineers recommend an optimum proctor density range of 95-100% for pavement bases and trench backfill under pavement. Testing the subgrade for passing compaction is less common than for premium aggregate bases, but it is still sometimes done. The primary reason testing isn’t done on a subgrade is that the material typically isn’t uniform either in gradation or composition, so to come up with an accurate proctor sample is difficult and requires time consuming lab testing from various locations of the subgrade.
On a small scale subgrade compaction can be done with a simple plate compactor. On a medium scale, subgrade compaction can be done with a plate tamp attachment for a backhoe or excavator. On a large scale, drum rollers are utilized. The different types of rollers include: smooth drum, vibratory, pneumatic, and sheepsfoot. A static application is a non-vibratory pass that reduces disruption of adjacent structures and components vs. vibratory actions.
On a large scale subgrade compaction is typically done with a small 1-2 man crew equipped with a dozer and a roller. For sandy or gravely subgrades, a basic roller will often do. For silty and clayey soils, a sheepsfoot roller or a low amplitude vibratory roller is typically more productive.