Subgrades and subbases for slabs – suitable materials and techniques that provide a solid foundation

The soil support system beneath your concrete slab is critical to a successful job. Brisbane’s experienced concrete suppliers understand that a well-compacted subgrade and subbase is the key to a solid concrete slab. Therefore, if you place concrete on an unstable soil support system, you will either waste the concrete or end up with a thin floor. Placing concrete on an inconsistent foundation results in cracking because the ground is not solid and stable.

What is a subbase / subgrade for slabs?

The subgrade is the soil that is usually compacted when preparing for slab installation. Thus, you find this soil at the bottom of the slab system. The subgrade may refer to the native soil or the improved soil delivered from a different source.

The subbase refers to the layer placed on top of the subgrade. Thus, the subbase features an easy- to-trim granular fill that remains stable and supports the slab. Therefore, the subbase and subgrade comprise the soil-support system for the slab. So, how do you ensure suitable materials and techniques for a solid foundation?

Here are essential considerations to bear in mind when working on subgrade or subbase.

  • Slits cannot be compacted in thick layers. Only granular soils can compact appropriately in thick layers. However, note that slits can be compressed at the optimum moisture content.
  • Soiling soil can expand and contract with moisture variations. The contraction and expansion of swelling soils affect the slab integrity. Therefore, avoid using swelling soils when working on subbase and subgrade.
  • Talk to Brisbane concrete suppliers for the right chemicals and materials for controlling the subbase and subgrade.
  • The subbase materials consist of naturally occurring coarse-grained soil or blended and processed soils.
  • Where possible, avoid the use of materials with more than 15% fines.
  • The subgrade and subbase thickness depends on the type of materials, compaction equipment, and construction method. However, the bottom line is that the subgrade should be thick enough to withstand the load on it.

What are the advantages of adding the subbase /subgrade layer?

  • The subbase or subgrade layers provide strength and support to the overlying pavement.
  • They provide proper drainage and ensure adequate frost protection
  • A compact subbase and subgrade prevents settlement to the pavement and the slab on a grade.
  • Subbase and subgrade compaction keeps the construction workers out of the mud and creates a workable surface before placing the concrete slab.
  • Overall, a well-laid subbase and subgrade reduces construction costs.

Subgrade soil is classified under four broad categories.

  1. Gravel—a coarse material featuring little or no fines and hence provides cohesion of materials.
  2. Moorum — the product of decomposition and weathering or rocks, similar to gravel.
  3. Silts— finer than sand and brighter in colour compared to clay, while they exhibit little cohesion.
  4. Clay—a type of soil that exhibits stickiness and features high strength when dry.

The types of subbase for flooring come in four categories:

  1. Cementitious— a lean concrete with a lower cement content that binds stone aggregate and sand.
  2. Granular—a popular alternative for cementitious
  3. Hill earth—a unique material used as a subgrade.
  4. Polyurethane foam insulation— a unique situation subbase material encountered in cold storage applications.


We have looked at the different subgrade and subbase materials. Now you know to ensure your site is compact before laying concrete. Talk to Reocrete for concrete delivery. We are theleading Brisbane concrete suppliers.