This document describes the California Bearing Ratio (CBR) test, which is used to determine the strength of soils and granular materials for pavement design. The CBR test involves compacting a soil sample and measuring the penetration of a piston under increasing loads. The CBR value is the load required to penetrate the sample 2.5mm or 5mm divided by a standard load value. Higher CBR values indicate stronger soils suitable for supporting pavement layers. The document outlines the apparatus, test procedure, interpretation of results, and classification of subgrade strength based on CBR values.

1. VISHWAKARMA GOVT.
ENGINEERING
COLLEGE,CHANDKHEDA
Prepared By :-
Gamit Kaushal A.

2. CALIFORNIA BERING RATIO (CBR) TEST
 This is a penetration test developed by the
california division of highway.
 This is most widely used method for the
design of flexible pavement.
 The thickness of the different element
comprising a pavement is determine by CBR
value.

3. Apparatus
a) Cylindrical mould of inside dia. 150 mm & height 175 mm
with a detachable collar & a base plate,
b) Spacer disc of 148 mm in Dia. & 47.7 mm height,
c) Surcharge weight of mass 2.5 kg,
d) Penetration plunger of 50 mm dia. & height 100 mm,
e) Loading m/c of capacity of 500 kg,
f) Compaction rammers,
g) Dial gauges, Filter paper, Mixing tools, Measuring cylinder,

5. TEST PROCEDURE
 Test specimen can be prepared either by static compaction or
by dynamic compaction.
• In static compaction soil is compacted in loading m/c, while in
dynamic compaction soil is compacted by specified rammers.
• Dynamically compacted specimen can be prepared by light
compaction or by heavy compaction.
Dynamic Compaction Light compaction Heavy Compaction
Rammer Weight 2.6 kg 4.89 kg
No. of layer 3 5
Free Fall 31 cm 45 cm
Blows per layer 56 56

6. • Fix the collar to the top of the mould & the base plate to its
bottom.
• Insert the spacer disc over the base with the central hole of
disc at lower side & put disc of course filter paper on the top
of spacer disc.
• Take about 5 kg of soil passing 20 mm sieve but retained on
4.75 mm sieve.
• Add predetermined quantity of water such that w/c of soil is
either equal to OMC or FMC & mix throughly.
• Compact the mixed soil in the mould using either the light or
heavy compaction.
• After compacting remove the collar ,base plate ,& filter paper.
• Note down weight of soil along with mould.

7. • Then place the mould assembly on the penetration test m/c
& fix the penetration plunger.
• Allow the plunger to penetrate at rate of 1.25 mm / min.
• Record the load reading at penetrations of 0, 0.5, 1, 1.5, 2,
4, 5, 7.5, 10 & 12.5 mm.
• Draw graph of penetration v/s load.
CBR Value =
Test load corresponding to 2.5mm or 5 mm * 100%
Standard load for 2.5 or 5 mm penetration
Standard Load Value
Penetration
( mm )
Unit Load
( kg / cm^2 )
Total Load
( kg )
2.5 70 1350
5.0 105 2055

8. • The curve is mainly convex upward although the
initial portion of the curve may be concave
upwards. A correction is then applied by drawing a
tangent to the curve at the point of greatest slope.
The corrected origin will be point where the
tangent meets the abscissa.

9. Interpretation & Recording
• CBR value are usually calculated for penetration of 2.5 mm &
5.0 mm
• Generally the CBR value of 2.5 mm will be greater then that at
5.0 mm & in such Cash the formar (2.5 mm CBR value ) should be
taken as design CBR.
• If CBR for 5.0 mm exceeds that for 2.5 mm, the test shall be
repeated.
CBR value. Subgrade strength
​ 3% & less. Poor
​ 3% - 5% Normal
​ 5% - 15% Good