Effect of cyclic pre-straining on the dynamic behavior of very dense sand

Earthquakes, including the main shock, fore-shocks and after-shocks, are among the most devastating natural disasters. Most building structures that are damaged due to a main shock then experience more damage when they are subjected to aftershocks. Any strong shock may decrease the strength of a steel or concrete structure. However, depending on its magnitude, a foreshock may either increase or decrease the shear strength of an earth-fill structure. A loose granular specimen usually gains some shear strength when subjected to pre-loading. This behavior is mainly due to an increase in relative density and experience of shear history. For a very dense granular material, it is unclear whether fore-shocks and main shocks increase or decrease its shear strength. However, the answer to this uncertainty is vital when evaluating the safety factor of a well-compacted granular earth-fill dam after an earthquake. The objective of this study is to study the effects of cyclic pre-straining on the dynamic behavior of saturated and very dense sand specimens. The major findings from the laboratory study are the following: (a) under small pre-strain, the microstructure remains essentially unchanged, but an increase in density leads to an increase in the soil strength; and (b) under large pre-strain, the microstructure is completely altered to a new form. In this case, if the loading amplitude is identical to the one in the previous prestrain, the soil strength increases; however, if the loading amplitude is less than the previous pre-strain, the strength decreases in the next stage. Such a decrease occurs even though the relative density increases.