(8.3)
where
Fm=number of commercial vehicles per day in one direction (mid-term year)
t =mid-term year minus 1945.Step 3
Calculation ofN, the cumulative number of standard axles
N=Tn¥D (8.4)Subgrade strength
The CBR test is taken as a direct measure of the strength of the in-situ sub-
grade material. Despite concerns regarding the limited accuracy of this test, it
is utilised on the basis that it is widely used and accepted by both theorists and
practitioners.Subbase and capping layer
In terms of the overall structural strength of the pavement, the subbase is an
extremely important layer. If the design life traffic volume is less than 2 msa, the
CBR of the subbase must exceed 20%. If it is greater than 2 msa, the minimum
CBR of the subbase rises to 30%. Use of a capping layer (with a lower specifi-
cation than the subbase material) will allow a thinner layer of the more expen-
sive subbase to be used in the pavement.
Table 8.1 indicates the thickness requirements for both subbase material alone
and combinations of subbase and capping for different CBR values of the
underlying subgrade material.D= ttF
+-
+
Ê
Ë
ˆ
̄
Ê
Ë
ˆ
()∏ ̄
035
093 0082
026
092 0082
10
39 1550
.
..
.
..
.
.
234 Highway Engineering
Layer CBR of subgrade
< 2 2345 +
Subbase thickness (mm) 615 400 310 260 225
Subbase +capping
Comprising- Subbase thickness (mm) 150 150 150 150 225
•Capping layer thickness (mm) 600 350 350 350 —
Table 8.1Thickness of subbase and capping layers (Crown copyright 1984)Roadbase and surfacing
Figures 8.3 to 8.5 are illustrations of the required thicknesses for bituminous,
wet-mix macadam and lean-mix concrete roadbases plus surfacings as detailed
in LR1132.