California Bearing Ratio Test - Soil Properties: Testing, Measurement, and Evaluation - page 406

100

90

80

70

60

50

0.1-in. penetration

39.5
1000

40

CBR = X 100 = 4.0%

0.2-in. penetration

30

56.7
1500

CBR = X 100 = 3.8%

20

Design CBR = 4.0%

10

0 0.1 0.2 0.3 0.4 0.5

Penetration (in.)

Figure 24-6 Curve of Penetration Stress versus Penetration

These values fill in the second row of the form on page 408. Similar
calculations furnish required values for succeeding piston loads and
penetration stresses for completing the remaining rows on the form.
The curve of penetration stress versus penetration is plotted and
shown in Figure 24-6. Since the curve is not concave upward initially,
no zero point correction is required.
The bearing ratio at 0.10-in. penetration is determined by dividing
the penetration stress for 0.10-in. penetration (from the curve of pene-
tration stress versus penetration) by the standard penetration stress of
1,000 psi and multiplying by 100:

CBR at 0.10 - in. penetration 39.5

1,000 100 4.0 %

The bearing ratio at 0.20-in. penetration is determined similarly using
the penetration stress for the 0.20-in. penetration and a standard pen-
etration stress of 1,500 psi:

CBR at 0.20 - in. penetration 56.7

1,500 100 3.8 %

Inasmuch as the bearing ratio at 0.10-in. penetration is greater than
that at 0.20-in. penetration, the ratio at 0.10-in. penetration should be
reported as the design CBR.
Hence, in this example, the CBR is 4.0%.