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Readings of the Clegg Impact Soil Tester

Readings of the Clegg Impact Soil Tester

Readings of the Clegg Impact Soil Tester

thread158-373316: CBR % from Clegg Readings

The question was asked about converting the readings of the Clegg Impact Soil Tester, also known as the Clegg Hammer, to an equivalent percent CBR. I have attached Dr. Clegg's 1986 Newsletter 2 on this topic. Note that the equations in the newsletter are applicable to the 4.5 kg Clegg Hammer, in terms of output in CIV (Clegg Impact Value), also known simply as IV for Impact Value, which is how the term appears in ASTM D5874. If CIV is derived from a procedure different than that given for IV in ASTM D5874, the correlation equation needs to be altered (through empirical findings preferably, rather than theoretical), in that the output of the Clegg Hammer, while still being in terms of a value based on the peak deceleration of the drop-hammer upon impact with the surface, will no longer be the same CIV due to the laws of physics, as well as the particular conditions encountered. For example, the output of the 20 kg "Heavy" Clegg Impact Soil Tester has the descriptor of CIV/H, to indicate that the output value was derived from the Heavy Clegg Hammer. (More on all this, e.g. to do with the test procedure, other masses of Clegg Hammers, along with their drop-heights and descriptors, is given in ASTM D5874). A question that often comes up: is it possible for there to be a %CBR greater than 100? Yes, in that 100% CBR relates to a particular material at a particular condition, but it doesn't represent the absolute that is possible. From Dr. Clegg's own research and experience and feedback from others, a CIV of around 38-40 as obtained using the "Standard" 4.5 kg Clegg Hammer equates to a field CBR of around 100 percent, generally speaking (for unsoaked, no surcharge conditions). In the state of Western Australia, a specification that was put out by the Main Roads Department of that state includes testing for a "Characteristic CIV" of not less than 55 prior to sealing a road (so, a value above 100% Field CBR, if 38-40 CIV is taken to represent that percent CBR); that is, the road is tested after compaction is completed and it has dried back (with nine tests per lot, search online for "specification 501 pavement. - Main Roads" and then search once saving as a pdf for "CIV" to find out more) so that, or until, the material exhibits this Characteristic CIV of 55 at the very least, as measured on the top of the basecourse (where the CIV may be said for basecourse materials to be commonly in the range of around 30-34 at the as-compacted moisture with the material compacted using heavy compaction - so therefore less than 100% CBR in the as-compacted moisture state). The best paper I have seen to-date in relation to developing correlations between %CBR and the 20 kg Clegg Hammer (again, with the output descriptor as CIV/H or IV/H), is entitled: “Development of a Correlation Model between a 20-kg Clegg Hammer and Field CBR for Measuring Subgrade Strength in Forest Roads in Western Oregon” by Justin Pattison, Kevin Boston P.E., R.P.F & Marv Pyles P.E. (2010), published online 2013.

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