Subbase or Subgrade: Improving Soil Conditions

A subbase needs to be uniform by nature to support the structure above it

Sub base
••• Photo bluesnap Pixabay

A solid subbase is a key to a successful project. A slab on ground and pavements normally are designed to be supported by a solid subbase or subgrade. A subbase needs to be uniform by nature, not strong, so it should have the capability to support the structure above it. However, it is important to have solid edges and joints, to prevent cracking and joint spalling.

How is a subgrade defined?

A subgrade can be defined as a native soil 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 out as they might be composed of impermeable soil and 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 in 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 in 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 will offer the following advantages:

  • Provide strength and support to the overlying pavement
  • Provide drainage and frost protection
  • Prevents settlements to pavement and slab on grade
  • It can be reused if you decide to change your pavement surface.
  • It will keep workers out the mud.
  • It can create a workable surface prior to placing the finished pavement
  • Reduces construction costs

Typical Subbase Materials

Some of the most used subbase materials are:

  • Recycled concrete
  • Granular fill as indicated in ACI 302 'Concrete Floor and Slab Construction'
  • Manufactured aggregate
  • Crush rock
  • Lean concrete
  • Recycled materials such as crushed concrete or brick

Improving Subgrade Material

When the subgrade material is not adequate to support the loads, then additional work should be done to make the material suitable for the construction. Normally, subgrade material is improved by installing geotextiles. The geotextile is used to prevent mixing 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. The following items should be considered when working with your subgrade:

  • Characteristics and properties of the subgrade material
  • Depth to bedrock and depth to the water table.
  • Compaction that could potentially be achieved in the subgrade.
  • CBR values of uncompacted and compacted subgrades.
  • Presence of organics in the subsoil or other detrimental material
  • Susceptibility to detrimental frost action or excessive swell.

Recommendation and Tips When Working With Soils for Subbase

When you are preparing and working with your subbase material be careful about the following:

  • Compaction is achieved when working with fill only, cut material is the existing one and normally will get more compact than what it actually is.
  • Silts cannot be compacted in thick layers, 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.