The ideal material is very clean sand and gravel.  When the percentage of fines exceeds about 6 to 10 percent, the ability to measure the degree of compaction using normal geotechnical testing decreases substantially.  However, surface settlements in these soil types often indicate that significant improvement is still being accomplished, even where test results have not changed.  On one of our projects, static cone penetration tests were repeated several years after completion of the project, and the results increased by 500% as compared with the tests conducted within weeks of completion.