Highway Pavement Materials and Design 201
(b) a layer of subbase varying between 150 mm (at 15% CBR) and 350 mm
(at 2.5% CBR) in thickness.
(4) For all pavements where the subgrade CBR is below 2.5% and for rigid
pavement construction on materials with CBR less than 15%, 150 mm of
subbase must be used on top of capping. The thickness of the capping layer
will reach 600 mm where the CBR of the subgrade dips below 2%.
(5) Where the subgrade CBR is substantially below 2%, the material will often
be removed in favour of more suitable material. The depth of this imported
material would typically be between 500 mm and 1000 mm deep. Though
this material may in reality be quite strong, it will be assumed to have a
CBR of 2% and will thus require a 600 mm capping layer.
7.3.3 Grading of subbase and capping
Type 1 granular materials are usually employed in subbases for flexible type
pavements. In situations, however, where the design traffic loading for the pave-
Subbase
400
300
200
100
0
600
500
400
300
200
100
1 3 2 4 5 8 10 15 20
Subgrade CBR (%)
Capping
thickness
(mm)
Subbase
thickness
(mm)
LEGEND
Subbase + capping
Subbase only
Capping
Figure 7.5Design of subbase and capping thickness.
