DHARM710 GEOTECHNICAL ENGINEERING
17.5 CALIFORNIA BEARING RATIO
The strength of the subgrade is an important factor in the determination of the thickness
required for a flexible pavement. It is expressed in terms of its ‘California Bearing Ratio’,
usually abbreviated as ‘C.B.R.’.
The CBR value is determined by an empirical penetration test devised by the California
State Highway Department (U.S.A.), and derives its name thereof. The results obtained by
these tests are used in conjunction with empirical curves, based on experience, for the design
of flexible pavements.
The test is arbitrary and the results give an empirical number, expressed usually in per
cent, which may not be directly related to fundamental properties governing the shear strength
of soils, such as cohesion and angle of internal friction. However, attempts have been made
recently to correlate CBR value with the bearing capacity and plasticity index of the soil.
The California bearing ratio (CBR) is defined as the rate of the force per unit area
required to penetrate a soil mass with a standard circular plunger of 50 mm diameter at the
rate of 1.25 mm/min to that required for the corresponding penetration of a standard material.
The standard material is crushed stone and the load which has been obtained from a
test on it is the standard load, this material being considered to have a CBR of 100%.
The CBR value is usually determined for penetrations of 2.5 mm and 5 mm. Where the
ratio at 5 mm is consistently higher than that at 2.5 mm, the value at 5 mm is used. Otherwise,
the value at 2.5 mm is used, which is more common.
The CBR test is usually carried out in the laboratory either on undisturbed samples or
on remoulded samples, depending upon the condition in which the subgrade soil is likely to be
used. Efforts shall be put in to simulate in the laboratory the pressure and the moisture condi-
tions to which the subgrade is expected to be subjected in the field.
17.5.1Determination of CBR Value
The C.B.R. Test as standardised by ISI [IS: 2720 (Part-XVI)-1979—Laboratory Determination
of CBR] is as follows:
The apparatus consists of a cylindrical mould of 150 mm inside diameter and 175 mm in
height. It is provided with a detachable metal extension collar 50 mm in height and a detach-
able perforated base plate 10 mm thick. A circular metal spacer disc 148 mm in diameter and
47.7 mm in height is also provided. A handle for screwing into the disc to facilitate its removal
is also available. A standard metal rammer (IS: 9198-1979) is used for compaction for prepar-
ing remoulded specimens. The apparatus is shown in Fig. 17.1.
One annular metal weight and several slotted weights weighing 24.5 N (2.5 kg) each
(147 mm in diameter with a central hole 53 mm in diameter) are used for providing the neces-
sary surcharge pressure.
A metal penetration plunger, 50 mm in diameter and not less than 100 mm long, is used
for penetrating the specimen in the mould. If it is necessary to use a plunger of greater length,
a suitable extension rod may be used. Dial gauges reading to 0.01 mm are used to record the
penetration.