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7.2.3 Determination of CBR using plasticity index
Where it is not possible to determine the CBR of a given soil directly, an alter-
native methodology involving use of the soils plasticity index and a knowledge
of certain service conditions can be used to derive a CBR valuation for cohe-
sive soils (Black & Lister, 1979).
In order to derive the plasticity index of a soil, its liquid and plastic limit must
be obtained.
Liquid limit
The liquid limit is the moisture content at which the soil in question passes from
the plastic to the liquid state. It is derived using the cone penetrometer test. In
it, a needle of a set shape and weight is applied to the surface of a soil sample
placed in a standard metal cup and allowed to bear on it for a total of 5 seconds.
Example 7.1 Contd
At 2.5 mm penetration:
Soil =8.2 kN
Aggregate with 100% CBR =13.02 kN
Therefore
CBR =(8.2 ¥100) ∏13.02
=63%
At 5.0 mm penetration:
Soil =13.0 kN
Aggregate with 100% CBR =19.9 kN
Therefore
CBR =(13.0 ¥100) ∏19.9
=65.3%
Taking the larger of the two values:
Final CBR =65.3% Æ65%
Note: CBR values are rounded off as follows:
CBR ≤30% – round to nearest 1%
CBR > 30% – round to nearest 5%
