EFFECT OF GRAVEL ON PUMPING BEHAVIOR OF COMPACTED SOIL

Pumping can occur when clay is compacted wet of optimum. Rather than d ensifying, the clayey soil squeezes out from underneath the compaction equipment. Adding gravel to the clayey soil creates a granular skelet on that increases the undrained shear strength. The results of this st udy show that even a partial gravel skeleton can prevent pumping of a clayey soil. Pumping occurred during the modified proctor compaction t est when the undrained shear strength was less than 14 kPa (290 psf) a nd the initial tangent modulus was less than 1,600 kPa. To prevent pum ping, the tested clayey soil (sand and montmorillonite mixture) had to contain at least 63% rounded gravel or 54% angular gravel. A lower pe rcentage of angular gravel prevented pumping because it more effective ly interlocked and created the necessary gravel skeleton at a lower de nsity than the rounded gravel.