Civil Engineering Reference
In-Depth Information
The California Bearing Ratio test
Subgrade strength is expressed in terms of its California Bearing Ratio (CBR) value. The CBR value is
measured by an empirical test devised by the California State Highway Association and is simply the
resistance to a penetration of 2.5 mm of a standard cylindrical plunger of 49.65 mm diameter, expressed
as a percentage of the known resistance of the plunger to various penetrations in crushed aggregate,
notably 13.2 kN at 2.5 mm penetration and 20.0 kN at 5.0 mm penetration.
The reason for the odd dimension of 49.65 mm for the diameter of the plunger is that the test was
originally devised in the USA and used a plunger with a cross-sectional area of 3.0 square inches. This
area translates as 1935 mm
2
and, as the test is international, it is impossible to vary this area and therefore
the plunger has a diameter of 49.65 mm.
Laboratory CBR test
The laboratory CBR test is generally carried out on remoulded samples of the subgrade, and is described
in BS 1377-4:1990. The usual form of the apparatus is illustrated in Fig.
14.10.
The sample must be com-
pacted to the expected field dry density at the appropriate water content. The appropriate water content
is the
in situ
value used for the field compaction. However if the final strength of the subgrade is required
a further CBR test must be carried out with the soil at the same dry density but with the water content
adjusted to the value that will eventually be reached in the subgrade after construction. This value of
water content is called the
equilibrium moisture content
.
The dry density value can only be truly determined from full-sized field tests using the compaction
equipment that will eventually be used for the road construction. Where this is impracticable, the dry
density can be taken as that corresponding to 5% air voids at a moisture content corresponding to the
omc of the standard compaction test. In some soils this will not be satisfactory - it is impossible to give
a general rule; for example, in silts, a spongy condition may well be achieved if a compaction to 5% air
voids is attempted. This state of the soil would not be allowed to happen
in situ
and the laboratory tester
must therefore increase the air voids percentage until the condition disappears.
Fig. 14.10
The California bearing ratio test.
