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What is a Subbase or Subgrade


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A solid subbase is the 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 a Subgrade is 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.

How Thick a Subgrade Should 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 subgragdes as thick as 12 inches, 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 finish 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 are 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 propertiesto ensure they have the right characteristics for their intended use.


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