Subbase or Subgrade: Improving Soil Conditions

Subbases need to be uniform to support the structures above them.

The base course or underlayer is placed by a paving machine
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A solid subbase is a key to a successful building project. A slab on ground and pavements normally are designed to be supported by a solid subbase, which needs to be uniform by nature to support the structure above it. A subbase will go on top of the subgrade, which typically is native soil or improved soil that has been compacted. It is important for a subbase to have solid edges and joints to prevent cracking and joint spalling.

How is a Subgrade Defined?

A subgrade is made up of native soil that has been compacted to withstand the loads above it. It is a layer required in many structures such as pavements and slabs, although it needs to have certain characteristics. A subgrade might need special drainage structures to let water if it is composed of impermeable soil, and it should be graded to within plus or minus 1.5 inches of the specified elevation.

There is no consistency in regards to the terms of subbase and subgrade, but normally the subgrade is the native soil while the subbase is the layer of soil or aggregate on top of the subgrade.

How Thick Should a Subgrade Be?

A subgrade should be thick enough to withstand the loads acting upon it. The heavier the loads, the thicker the subgrade should be, as you might have subgrades as thick as 12 inches and event thicker, normally for highways. Nonetheless, the subgrade material is normally cheaper than the surface material and could prevent water from showing up to the surface.

Subbase Layer Advantages

A solid subbase layer should offer the following advantages:

  • Provide strength and support to the overlying pavement
  • Provide drainage and frost protection
  • Prevent settlements to pavement and slab on grade
  • Be reusable if you decide to change your pavement surface
  • Keep workers out the mud
  • Create a workable surface prior to the placing of the finished pavement
  • Reduce construction costs

Typical Subbase Materials

Some of the most used subbase materials include:

  • Recycled concrete
  • Granular fill
  • Manufactured aggregate
  • Crushed rock
  • Lean concrete
  • Recycled materials, such as crushed concrete or brick

Improving Subgrade Material

When the subgrade material is not adequate to support the necessary loads, then additional work should be done to make the material suitable for the construction. Normally, subgrade material is improved by installing geotextiles. They are used to prevent mixing of soft or inadequate soil that might affect the structural capacity of the subgrade. Geotextiles must have specific mechanical and hydraulic properties to ensure they have the right characteristics for their intended use.

Recommendation and Tips When Working With Soils for Subbase

When you are preparing and working with your subbase material, these are some important issues to keep in mind:

  • Silts cannot be compacted in thick layers, but granular soils can be. However, silts can be compacted at their optimum moisture content.
  • Avoid using swelling soils as they can expand and contract with moisture variations, affecting the integrity of the slab and pavement above them.
  • Subbase and subgrade characteristics can be controlled using chemicals or other materials.
  • Subbase materials preferably might consist of naturally occurring, coarse-grained soils or blended and processed soils.
  • When possible, avoid using materials which have more than 15 percent fines.
  • Lift thickness is dependent on the type of material, the compaction equipment used, and the method of construction.