For some difficult soils, the subbase may be placed atop a layer of geogrid.

Most subgrade improvement is accomplished by compacting the soil. In extreme situations, when the soil is particularly bad or the loads high, soil stabilization can be used. In this process, portland cement, calcium chloride, or lime are mixed into the soil then it is compacted. The subgrade soil can also be excavated and mixed with gravel then compacted.

Soil compaction is the act of squeezing out as much air and moisture as possible to push the solid soil particles together—this makes the soil more dense and typically the higher the soil's density, the higher its bearing capacity. Well-compacted soils also do not allow moisture to move in and out as easily. So, compaction accomplishes the following:

Soil compacts more easily at the proper moisture content. Wacker Neuson

  • reduces the amount that the soil will compress (settle) when the slab is on it
  • increases the amount of weight we can put on it (bearing capacity)
  • prevents frost damage (heave) if the soil under the slab freezes
  • reduces swelling and contraction

How much a soil can be compacted is measured by a geotechnical (or soils) engineer by placing the soil in a cylinder and beating on it—seriously. The standard or modified Proctor tests (each uses different weights to compress the soil) determine the relationship between soil density and moisture and tell us the maximum reasonable soil density that can be achieved in the field.

What we are trying to determine with the Proctor test is the moisture content in the soil that will make it easiest to compact and result in the highest density—remember that density is directly related to compaction. Too little moisture and the soil is dry and doesn't compress easily; too much moisture and you can't easily squeeze the water out. To get the best compaction, the optimal moisture content will typically be in the range of 10% to 20%. So when you hear that according to the specification the soil needs to be at 95% of the maximum modified Proctor density, you will know that you need the moisture content to be about right in order to get to that level of compaction.

A soil density-moisture curve defines the optimum moisture content and the maximum density achievable in the field.

If you aren't going to get Proctor tests done, there are some simple field tests for getting a rough idea of bearing capacity and moisture content:

  • For moisture content use the hand test. Squeeze a ball of soil in your hand. If it's powdery and won't hold a shape, it's too dry; if it molds into a ball then breaks into a couple of pieces when dropped, it's about right; if it leaves moisture on your hand and doesn't break when dropped, it's too wet.
  • Clay that you can push your thumb a few inches into with moderate effort has a bearing strength in the range of 1000 to 2500 psf
  • Loose sand that you can just barely push a #4 rebar into by hand has a bearing capacity of 1000 to 3000 psf
  • Sand that you can drive a #4 rebar into about 1 foot with a 5-pound hammer has a bearing capacity over 2000 psf

Also, remember that it's not just the soil (the subgrade) that needs to be compacted. Any subbases or base courses, which will typically be granular materials, also need to be well compacted in the proper lift thicknesses.

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