Question

An initial geotechnical survey found that the road alignment is situated on a Sandy Clay material

(CL), that is well-drained. What is a typical CBR value to adopt, in -lieu of the final geotechnical survey?

Answer 1

CBR TEST

The CBR test is a simple strength test that compares the bearing capacity of a material with that of a well-graded crushed stone (thus, a high-quality
crushed stone material should have a CBR of 100%). It is primarily intended for, but not limited
to, evaluating the strength of cohesive materials having maximum particle sizes less than 0.75
inches.

The CBR is a comparative measure of the shearing resistance of soil. The test consists of
measuring the load required to cause a piston of standard size to penetrate a soil specimen at a
specified rate. This load is divided by the load required to force the piston to the same depth in a standard sample of crushed stone. The result, multiplied by 100, is the value of the CBR.

California Bearing Ratio (CBR). This method is probably the most widely used for determining the strength and stability of compacted soil . A
subgrade generally requiring a CBR of 10 or greater is considered good and can support
heavy loading without excessive deformation (see Section 6E-1, Subgrade Design and
Construction, for additional information). some typical values of CBR soils
are shown in Table

So CBR value of sandy clay(CL) ranges from 5-15.In the final geotechnical investigation report it must be stated that the soil being well drained has a cbr value greater than 10 % due to high shear strength. Hence it with proper compaction methods, the soil will be suitable for sub-grade soil in road construction.