GEOTECHNICAL

California Bearing Ratio (CBR)

Subgrade strength index for pavement design

Also calledcbrcalifornia bearing ratiocbr testsubgrade strength
Related on InfraLens
CODES
IS 2720IRC 37
Definition

California Bearing Ratio (CBR) is a soil-strength index used primarily for pavement design and earthwork compaction control. Standardised in IS 2720 Part 16:1987, CBR is the ratio (expressed as a percentage) of the load required to produce a specified penetration in the soil to the load required for the same penetration on standard crushed stone (the 'California' reference material). The test uses a cylindrical mould (150 mm diameter, 175 mm height) compacted to specified density, with a 50 mm diameter plunger advanced at 1.25 mm/min while load is recorded. CBR at 2.5 mm penetration is the standard reported value; at 5.0 mm is also recorded.

For pavement design under IRC 37:2018 (flexible pavements) and IRC 58:2015 (rigid pavements), CBR is the key subgrade input. Higher CBR = stronger subgrade = thinner pavement structure required. Typical values: very weak silty/clayey soil 2-5%, ordinary subgrade 5-10%, good subgrade 10-25%, very good (gravel-bearing) 25-50%, granular sub-base 80-100%, well-graded crushed stone aggregate base 80-100%. Subgrade CBR is normally tested in the laboratory on remoulded soaked samples (90% MDD per IS 2720 Part 8), simulating the worst-case wet condition the in-service pavement will face.

CBR is also widely used to specify earthwork acceptance for embankments — minimum CBR 8% at 95% MDD per IRC SP 84:2019 four-laning standards. The test has well-recognised limitations: it does not directly measure shear strength or modulus, the soaked-CBR penetration is not representative of actual loading patterns, and laboratory CBR can over-predict field strength by 30-50% if compaction or moisture differs. Modern practice in tier-1 highway projects increasingly uses Resilient Modulus (Mr) testing and dynamic cone penetrometer (DCP) for direct field-strength measurement.

Typical values
Very weak clay subgradeCBR 2-5%
Ordinary black-cotton/red-soil subgrade5-10%
Good silty sand subgrade10-25%
Gravel-bearing soil25-50%
Granular sub-base (GSB)≥ 30% (IRC 37 spec)
Wet mix macadam (WMM) base≥ 80% (IRC 37 spec)
Crushed stone aggregate80-100% (the reference)
Where used
  • Flexible pavement design per IRC 37:2018 — subgrade and sub-base CBR
  • Rigid pavement design per IRC 58:2015 — subgrade CBR for k-value correlation
  • Embankment construction QC per IRC SP 84 — minimum 8% at 95% MDD
  • Airfield pavement design per IRC SP 84 + ICAO standards
  • Soil stabilisation effectiveness — CBR before vs after lime/cement treatment
Acceptance / threshold
Per IS 2720 Part 16: CBR test on soaked sample at specified compaction density. For pavement design per IRC 37, the design CBR is the average of 8 representative samples minus 1 standard deviation. Embankment compaction acceptance per IRC SP 84: CBR ≥ 8% at 95% MDD.
Site example
Site reality: a 23 km Madhya Pradesh state highway project tested subgrade soaked CBR at 6.5% (from 8 samples). Design CBR per IRC 37 protocol (mean − 1σ = 6.5 − 1.2 = 5.3) was used, calling for 600 mm GSB + 100 mm DBM + 40 mm BC pavement. The contractor argued for using mean (6.5) directly to save cost. Engineer correctly insisted on the IRC 37 statistical procedure. Two years later when the road performed without rutting, the conservatism proved worthwhile — five neighboring contractors who had used mean values had pavement failure within 18 months.
Frequently asked
What is CBR test in civil engineering?
California Bearing Ratio (CBR) is a soil strength index measured by penetrating a 50 mm plunger into a compacted soil sample at 1.25 mm/min and recording the load. CBR (%) = load required for 2.5 mm penetration in soil ÷ load for same penetration in standard crushed stone × 100. Standardised in IS 2720 Part 16:1987.
What is design CBR for highway pavement?
Per IRC 37:2018: design CBR is the average of 8 representative subgrade samples minus 1 standard deviation. Indian highway subgrades typically have design CBR 4-10%; very weak soils <4% require sub-grade replacement or stabilisation. The design CBR enters the pavement-thickness chart (IRC 37 catalogue) along with traffic to give pavement layer thicknesses.
How is CBR test performed?
Per IS 2720 Part 16: (1) compact soil in standard mould (150 × 175 mm) at MDD with OMC, (2) soak sample for 96 hours under surcharge 4.54 kg, (3) install in test machine, (4) advance 50 mm plunger at 1.25 mm/min, (5) record load at 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0 mm penetration, (6) CBR = load at 2.5 mm ÷ standard load at 2.5 mm × 100. Standard load at 2.5 mm = 13.24 kN; at 5.0 mm = 19.96 kN.
Related geotechnical terms
Safe Bearing Capacity (SBC)
Max safe pressure soil can take. 100-500 kN/m² typical.
Standard Penetration Test (SPT)
Soil density test in borehole per IS 2131
Soil Classification
Per IS 1498 / Unified Soil Classification System
Geotextile / Geomembrane
Synthetic fabrics for drainage, filtration, soil reinforcement
Dewatering
Removing groundwater from excavations during construction
Pile Foundation
Deep foundation transferring load to firm strata via piles
Retaining Wall Types
Walls retaining earth — gravity, cantilever, counterfort, diaphragm
Settlement
Vertical downward movement of a foundation under load. Total ≤25 mm for raft, ≤50 mm for footings (IS 1904).