Geotechnical Engineering

DHARM

714 GEOTECHNICAL ENGINEERING

9810

8829

7848

6867

5886

4905

3924

2943

1962

981

0

100

90

80

70

60

50

40

30

20

10

0

Unit load, kg/cm

2

No correction

required

Corrected 2.5 mm

penetration

Corrected

5 mm penetration

Corrected for concave

upward shape

Unit load, kN/m

2

5.0 7.5 10.0 12.5

Penetration, mm

Fig. 17.3 Load vs penetration curves for CBR test

where PT = Corrected unit (or total) test load corresponding to the chosen penetration from the
load-penetration curve, and

Ps = standard unit (or total) load for the same depth of penetration as for PT taken from
Table 17.2.

The CBR values are usually calculated for penetrations of 2.5 mm and 5 mm. Generally,
the CBR value at 2.5 mm penetration will be greater than that at 5 mm penetration and in
such a case the former shall be taken as the CBR value for design purposes. If the CBR value
corresponding to a penetration of 5 mm exceeds that for 2.5 mm the test shall be repeated. If
identical results follow, the CBR corresponding to 5 mm penetration shall be taken for design.

The CBR value shall be reported correct to the first decimal place.

Table 17.2 Standard load

Depth of Unit standard load Total standard load

Penetration (mm) kg/cm^2 kN/m^2 kg kN

2.5 70 6,867 1370 13.44

5.0 105 10,300 2055 20.16

7.5 134 13,145 2630 25.80

10.0 162 15,892 3180 31.20

12.5 183 17,952 3600 35.32