Strength And Modulus Changes Associated With Compaction Of Residual Soils And Weathered Rock

Burt G Look

Density testing has been applied widely in earthworks quality testing. A common inference from density testing is that as the dry density ratio (DDR) increases, the strength and modulus also increase. This is a well-established principle as under compaction can result in collapse, low strength and settlement of the placed fill. However, there is no clear relationship that a specified DDR will result in a design strength of or modulus. At high levels of compaction of residual soils and weathered rock materials with heavy vibratory rollers, when strength or modulus measurements are correlated back to density testing, a poor correlation often results. This is due to factors such as the depth of influence is different, and with the quality and compaction being combined into one parameter (say modulus). Data from field trial embankments with 3 different materials using residual soils and weathered rocks were tested using a range of alternative testing equipment. Compaction shows an increase in the DDR with the number of passes, but a decrease in modulus and friction angles with 8 No. of passes as material breakdown occurs with heavy vibratory rollers. This breakdown varies with the type of roller. DDR is shown to be a non-reliable indicator of strength and modulus at high compaction, if paired data matching was used. A compaction DDR of 95% is shown to have an associated friction angle variation of over 5 degrees depending on the type of material compacted. Similarly, the modulus can vary by factor of 2 at a given DDR depending on the material type and compaction roller used. This observation at high stiffness (DDR > 98%) should not detract from the benefits of compaction as overall and at (DDR < 98%) the strength and modulus increase with number of passes.