Highway Engineering

194 Highway Engineering

7.2 Soils at subformation level,

7.2.1 General

Unless the subsoil is composed of rock, it is unlikely to be strong enough to

carry even construction traffic. Therefore it is necessary to superimpose addi-

tional layers of material in order to reduce the stresses incident on it due to

traffic loading.

The in-situ soil would suffer permanent deformation if subjected to the high

stresses arising from heavy vehicle traffic loading. The shear strength and stiff-

ness modulus are accepted indicators of the susceptibility of the soil to perma-

nent deformation. A soil with high values of both these characteristics will

be less susceptible to permanent deformation. Both are usually reduced by

increases in moisture content. Knowledge of them is essential within the pave-

ment design process in order to determine the required thickness of the pave-

ment layers.

Since it is not always feasible to establish these two parameters for a soil, the

California bearing ratio (CBR) test is often used as an index test. While it is not

a direct measure of either the stiffness modulus or the shear strength, it is a

widely used indicator due to the level of knowledge and experience with it that

has been developed by practitioners.

7.2.2 CBR test

The CBR test acts as an attempt to quantify the behavioural characteristics of

a soil trying to resist deformation when subject to a locally applied force such

as a wheel load. Developed in California before World War II, to this day it

forms the basis for the pre-eminent empirical pavement design methodology

used in the UK.

The test does not measure any fundamental strength characteristic of the soil.

It involves a cylindrical plunger being driven into a soil at a standard rate of

penetration, with the level of resistance of the soil to this penetrative effort being

Surfacing +

Roadbase

Foundation

Cement bound subbase

Subgrade

Concrete slab

Formation level

Figure 7.2Layers within a typical rigid highway pavement.