Geotechnical Engineering

DHARM

720 GEOTECHNICAL ENGINEERING

Corrected 5 mm penetration

Corrected 2.5 mm penetration

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Corrected zero

Penetration, mm

Pressure, kg/cm

2

70

60

50

40

30

20

10

0

45.8

23.5

Fig. 17.8 Pressure versus penetration curve (Example 17.2)
Note. Since the initial portion of the curve is concave upwards, correlation is applied to
the penetration; the corrected zero being 2 mm.

The test pressures at corrected values of penetration of 2.5 and 5.0 mm are obtained as
23.5 and 45.8 kg/cm^2 , respectively;

C.B.R. at 2.5 mm penetration = 23.5/70 = 33.6%
C.B.R. at 5.0 mm penetration = 45.8/105 = 43.6%
Since the value at 5 mm is greater, it is to be reported as the CBR assuming that it will
be consistently higher even when the test is repeated.

Example 17.3: A road expected to carry medium traffic is to be constructed in an area where
the CBR value of the subgrade is 10%. The base material chosen has a CBR value of 50%. No
sub-base is to be provided. Determine the thickness required for the base course and for the
surface course.

CBR value of subgrade = 10%; CBR value of base material = 50%
Using Fig. 17.5, for medium traffic, thickness of construction required for CBR 10% = 25
cm, and thickness required for CBR 50% = 10 cm, nearly.

∴ Thickness of the surface course (over the base) = 10 cm.

Thickness of the base course (over the subgrade)

= Total thickness over the subgrade – Thickness of surface course = (25 – 10) cm

= 15 cm.