CORRELATION with CALIFORNIA BEARING RATIO
There has been considerable interest in correlating the Clegg Impact Value (CIV) with the California Bearing Ratio (CBR), using the 4.5 kg Clegg Impact Soil Tester from a 0.45 m drop-height with the highest value blow of four successive blows used for the CIV.
The current formula used is the most recent from studies and the most adaptable for all soil types. It is the combination of lab tests and field tests to arrive at this equation.
CBR = 0.1691 (CIV)^1.695
The first such correlation was presented by Clegg at the Third ANZ Geomechanical Conference, Wellington, New Zealand, 1980. This correlation was based on laboratory tests in the Department of Civil Engineering at the University of Western Australia. The relationship was expressed by the equation
CBR = 0.07 (CIV)^2
Since that time considerable data has become available from Australian, New Zealand and United Kingdom sources. These cover a wide range of soils for both laboratory and in situ conditions. The results of about 200 tests are shown plotted on the attached figure. It can be seen that the previous general relationship is confirmed but with a slight correction suggested at the lower end of the scale, i.e.
CBR = (0.24 (CIV) + 1)^2 (r = 0.957)
This revised equation appears appropriate for general use. However, since CBR is particularly subject to high variability, even within one organization, one soil type, etc., correlations from individual sources may vary from the general equation. To avoid a change of standards it is appropriate therefore that each organization should consider establishing its own relationship for specific materials and conditions, particularly where there is strong reliance on CBR for design purposes.
It should be noted that these data are for both samples at essentially the same density and moisture content and prepared in the same manner. Attempts have been made to correlate unsoaked CIVs with soaked CBRs but with little success. Also the question of the effect of surcharge may need to be considered.
Further information as to the sources of the data is available on request.