Postdensification penetration resistance of clean sands

Freshly consolidated or densified clean sands exhibit a substantial increase in stiffness under drained aging conditions. Laboratory measurements of dynamic shear modulus after primary consolidation and field measurements of penetration resistance after ground densification by blasting, vibrocompaction, or dynamic compaction, in clean sands show a significant increase with time. A possible mechanism for this time effect in clean sands is the continued rearrangement of sand particles during secondary compression. Improved frictional resistance to deformation develops with time with a gradual increase in interlocking of particle surface roughness and increased geometrical particle interference through more efficient packing. Empirical equations are proposed that result in satisfactory predictions of the postprimary consolidation shear modulus and of postdensificatibn cone-penetration resistance. The equation for the time-dependent cone resistance includes the assumption of a decrease in compressibility and an increase in effective horizontal stress during secondary compression. In the absence of continued cone-penetration testing subsequent to compaction, the proposed equation may be used to extrapolate from measurements that are carried out immediately after ground modification.