Effect of Gravel on Pumping Behavior of Compacted Soil

Pumping can occur when clay is compacted wet of optimum. Rather than densifying, the clayey soil squeezes out from underneath the compaction equipment. Adding gravel to the clayey soil creates a granular skeleton that increases the undrained shear strength. The results of this study show that even a partial gravel skeleton can prevent pumping of a clayey soil. Pumping occurred during the modified proctor compaction test when the undrained shear strength was less than 14 kPa (290 psf) and the initial tangent modulus was less than 1,600 kPa. To prevent pumping, the tested clayey soil (sand and montmorillonite mixture) had to contain at least 63% rounded gravel or 54% angular gravel. A lower percentage of angular gravel prevented pumping because it more effectively interlocked and created the necessary gravel skeleton at a lower density than the rounded gravel.