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...
I have seen similar results on Tropical Residual soils where the dispersing agent was not strong enough to completely break the bonding between the particles. However, the abrupt change in the grading curve did not happen at 75 microns, but lower at about 30-40 microns.
A very good indicator as...
Firstly has the material been oven-dried prior to testing? If so, try the test on material which is undried and wetted up to the required moisture points, then left to mature for several hours, preferably overnight. I wonder what air voids are remaining in the samples at your highest moisture...
