DYNAMIC PRESTRAINING OF DRY SAND

Dynamic prestraining is the application of vibratory shearing strains to soil that is acted upon by static confining pressure. Tests on hollow, cylindrical Ottawa sand specimens, in a torsional resonant column apparatus, demonstrate that material behavior may be significantly changed by prestraining. In general, prestraining at shearing strains between 0.001% and 0.01% can double the dynamic shear modulus and damping, strikingly reduce the static compressibility, and cause fatigue failures. Density changes cannot account for this. No changes occur for prestraining at shearing strains less than 0.001%. The effects are dependent on confining pressure, void ratio, shearing strain amplitude, and cycles of vibration. Millions of cycles may be necessary to produce these changes. Test results indicate that density or void ratio measurements may not reveal the total change in these parameters imparted to dry sands by vibratory loading. This is important in determining the dynamic response of foundations and the compaction improvement of sand soils.