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Civil Engineering QUESTION #2132
Question 1
In transportation engineering, the California Bearing Ratio (CBR) test is used to:
  • Measure the compressive strength of subgrade soil for designing airport runways only
  • Evaluate the bearing capacity of subgrade and sub-base materials relative to a standard crushed stone material — the CBR value is used as input to empirical pavement thickness design methods (IRC, AASHTO) to determine required pavement structure depth✔️
  • Measure the permeability of road base materials for drainage design
  • Determine the optimum moisture content of subgrade soils for compaction
Correct Answer Explanation
CBR = (load to cause 2.5mm or 5.0mm penetration in test soil / load for same penetration in standard crushed stone) × 100%. It was developed by California Division of Highways and adopted worldwide for flexible pavement design. High CBR (>80%): excellent subgrade; medium (20–50%): fair; low (<5%): poor, requiring thick pavement. It is used in IRC:37, AASHTO, and most national pavement design guides.