Highway Engineering

Highway Pavement Materials and Design 199

(6) ‘A’ denotes the situation where the pavement is 300 mm thick (thin pave-
ment construction)
(7) ‘B’ denotes the situation where the pavement is 1.2 m thick (thick pavement
construction).

If full information is not available for Table 7.3, certain assumptions can be
made. The worst service condition of ‘high water table’ can be assumed, together
with the assertion that construction is being carried out in accordance with stan-
dard specifications, taken as ‘average’ construction conditions in Table 7.3. If
the pavement thickness varies between the two values of 300 mm and 1.2 m, the
final CBR can be derived by interpolation between the values given in Table 7.3.
Where full information is unavailable, general CBR values of the type given
in Table 7.4 can be used (HD 25/94) (DoT, 1994).

High water table Low water table

Poor Average Good Poor Average Good

PI ABABA BABABAB

Heavy clay 70 1.5 2 2 2 2 2 1.5 2 2 2 2 2.5

60 1.5 2 2 2 2 2.5 1.5 2 2 2 2 2.5

50 1.5 2 2 2.5 2 2.5 2 2 2 2.5 2 2.5

40 2 2.5 2.5 3 2.5 3 2.5 2.5 3 3 3 3.5

Silty clay 30 2.5 3.5 3 4 3.5 5 3 3.5 4 4 4 6

Sandy clay 20 2.5 4 4 5 4.5 7 3456 68

10 1.5 3.5 3 6 3.5 7 2.5 4 4.5 7 6 > 8

Silt — 1 1 1 1 2 2 1122 22

Sand

Poorly graded — ————20 ———————

Well graded — ————40 ———————

Sandy gravel — ————60 ———————

Table 7.3CBR values for different soil types and conditions

Soil type PI CBR (%)

Heavy clay 70 2

60 2

50 2

40 2/3

Silty clay 30 3/4

20 4/5

Sandy clay 10 4/5

Sand

Poorly graded — 20

Well graded — 40

Sandy gravel

Well graded — 60

Table 7.4CBR

estimates where

information is poor