200 Highway Engineering
7.3 Subbase and capping,
7.3.1 General
The subbase and capping together act as a regulator of the surface of the sub-
grade below and protect it against the effects of inclement weather. They, along
with the subgrade, provide a secure platform on which the upper layers of the
highway pavement can be built.
The determinant of the thickness of this section of the pavement is the
strength of the underlying subgrade. Its design is independent of the cumula-
tive traffic incident on the upper layers of the pavement over its design life. For
subgrades in excess of 5% CBR, the required subbase depth is no greater
than 225 mm, down to a minimum of 150 mm at a subgrade CBR of 15% (HD
25/94).
Granular and cement-based subbases are recommended for flexible pavements
while only cemented subbases are recommended for rigid-type pavements (HD
25/94). In the case of unbound subbases, their grading should be such that it
constitutes a dense layer of relatively high stiffness modulus, relatively imper-
meable to water though not of necessity free draining. Their laboratory CBR
should be a minimum of 30%.7.3.2 Thickness design
The thickness of the subbase/capping layer is dependent on the CBR
of the subgrade and is determined in accordance with HD 25/94 using
Fig. 7.5.
Figure 7.5 illustrates two separate designs, one with subbase only where
capping is not required (denoted by the heavy dotted line) and one comprising
subbase combined with capping (denoted by the heavy continuous line).
The following four broad categories apply:(1) No subbase is required if the subgrade is composed of hard rock or of a
granular material with a CBR of at least 30%, provided the water table is
not at a high level.
(2) In the case of subgrades with a CBR greater than 15%, a subbase thick-
ness of 150 mm is required (in practical terms this constitutes the minimum
subbase thickness for proper spreading and compaction).
(3) Where the CBR of the subgrade lies between 2.5% and 15%, two options
are available:
(a) use 150 mm of subbase over a layer of capping material, the thick-
ness of which depends on the subgrade CBR,or