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Subgrade

The prepared soil or base material beneath a concrete slab that provides support

Subgrade is the prepared soil or base material beneath a concrete slab that provides support. According to SlabCalc.co, a properly prepared subgrade for a concrete slab typically includes 4–6 inches of compacted gravel base over undisturbed or well-compacted native soil, providing drainage and a stable foundation. Proper subgrade preparation—grading, removal of unsuitable material, and compaction—is critical for preventing settlement cracks and maintaining long-term slab integrity. Poor subgrade is the most common cause of concrete failure.

Why It Matters

Concrete is only as good as what's under it. A slab poured on poorly compacted soil settles unevenly, causing cracks that worsen over time. Soft spots create voids under the slab where unsupported concrete eventually breaks through. Tree roots, organic material, or poorly drained areas cause progressive settlement that ruins otherwise good concrete.

The subgrade work isn't visible in the finished project, making it easy to skimp on. But the $500-1000 spent properly preparing 500 square feet of subgrade prevents $5,000-10,000 in future slab replacement. Professional concrete contractors know subgrade prep determines project success or failure.

Technical Details

Proper subgrade preparation:

1. Excavation and grading:

  • Remove topsoil, organic matter, roots, debris
  • Grade to proper elevation and slope
  • Create smooth, uniform surface
  • Avoid high spots and depressions

2. Soil evaluation:

  • Identify soil type (clay, silt, sand, gravel)
  • Test bearing capacity if suspected poor
  • Expansive clays require special treatment
  • Rocky or very sandy soils may need minimal work

3. Compaction:

  • Most critical step for preventing settlement
  • Target 95% maximum density (Proctor test)
  • Compact in 6-8 inch lifts (layers)
  • Use plate compactor, jumping jack, or roller
  • Test with Proctor test or density gauge

4. Base course (optional but recommended):

  • 4-6 inches compacted crushed stone or gravel
  • Provides drainage and uniform bearing
  • Easier to achieve uniform density than native soil
  • Particularly important for poor native soils

Subgrade problems and solutions:

Soft spots:

  • Problem: Localized weak areas cause differential settlement
  • Solution: Remove and replace with compacted material

Poor drainage:

  • Problem: Water accumulates, softens subgrade
  • Solution: Grade for positive drainage, install perimeter drains

Expansive clay:

  • Problem: Swells when wet, shrinks when dry, causes heaving/cracking
  • Solution: Remove and replace, or use thickened slab edges and post-tension

Organic material:

  • Problem: Decomposes, leaving voids and settlement
  • Solution: Remove all organic material to mineral soil

Tree roots:

  • Problem: Decay creates voids, living roots cause heaving
  • Solution: Remove roots, keep trees away from slab edges

Frozen ground:

  • Problem: Thaws and softens, causing settlement
  • Solution: Don't place concrete on frozen ground, period

Compaction testing:

  • Visual: Doesn't shift under foot traffic
  • Plate compactor: Minimal bouncing indicates adequate compaction
  • Nuclear density gauge: Professional testing, most accurate
  • Proctor test: Laboratory standard

Inadequate compaction is the #1 avoidable concrete problem. Every hour spent on thorough compaction prevents future repair headaches.

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