Clegg Hammer - a real blast from the past and I can't ever remember using to try and correlate CBR values.
I guess the first question should be why are you wanting to use a Clegg hammer to determine CBR?
The best documented method I am aware of where a hammer type arrangement is used to 'assess equivalent CBR values' is using the TRRL Lightweight Dynamic Probe. This does allow an evaluation of the CBR with depth as a profile, and if this is the kind of assessment you are looking to make, I would suggest that short of actually doing the 'true CBR' test this would be the preferable option.

There is a direct correlation between the Clegg Hammer (IV) and CBR, the conversion factor can usually be found in the manufacturers handbook.
A search of the internet will usually produce several correlations on soils from different parts of the world, some with slightly different results. However conversion factors are likely to be very close to the original value of:

Equivalent % CBR = 0.07(IVxIV)

I have successfully correlated the Clegg/Lab CBR for Tropical Residual Soils over a wide range of values, from below 10% to 50% CBR.

The Clegg is the perfect tool for site if you want to identify soft areas of fill. A useful reference document for control of fill by the Clegg is ASTM D5874-95.

Soiledup,

I can say there is increased interest in a direct readout feature that converts the Clegg Impact Value reading (IV) to %CBR. We are regularly providing instruments that do this now - the LCD display shows %CBR 2 seconds after the IV is displayed. This isn't meant to sound like an advert but only to help answer your question.

Thank you and regards,


Simon Deakin

okay - is this equivalent to the soaked CBR?  Likely not, unless you are in the wet season.  One thing you have to keep in mind is that for road design it is the soaked (worst condition) CBR that is of greatest interest.  Doing a CBR in the dry season and needing the strength in the wet season are two different animals.

Excellent and very helpful. I am after correlation between CBR and Clegg hammer; which I got it. Thanks you hstanger and Soilup.