GeoStructCiv

Pavements

CBR, subgrade strength and why your pavement is thicker than you expected

CBR (California Bearing Ratio) is a simple test that measures how strong the soil under a pavement is. Because pavement thickness is designed directly from that number, a weak subgrade can quickly add gravel, stabilisation and cost.

5 min read

What CBR measures

A soil sample is compacted and soaked, then a plunger is pushed into it. The resistance compared with a standard crushed rock gives the CBR percentage. Soaked clays in South East Queensland often test low.

Why it changes thickness

The lower the CBR, the more granular material is needed to spread the traffic load. Heavy trucks and frequent loading increase it further.

Ways to reduce cost

  • Test enough locations to avoid designing for the worst result everywhere
  • Consider lime or cement stabilisation of the subgrade
  • Improve drainage so the subgrade stays drier
  • Use geogrids or geotextiles where they reduce gravel thickness

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