Threshold Strain for Postcyclic Shear Strength Change of Mississippi River Valley Silt Due to Cyclic Triaxial Loading

Abstract

Abstract Soil microstructure is affected by cyclic loading, leading to changes in postcyclic shear strength. It is generally observed that the postcyclic shear strength of a given soil increases substantially after reconsolidation above a threshold strain for that soil. In this study, we investigated the threshold strain for change in the postcyclic shear strength of reconsolidated soil by conducting cyclic triaxial tests on reconstituted Mississippi River Valley silt with a plasticity index (PI) of 5.8. To this end, the effects of the cyclic stress ratio, overconsolidation ratio, effective consolidation pressure, and PI on the threshold cyclic axial strain were assessed. The postcyclic shear strength decreased slightly when the cyclic axial strain (εa,c) was less than 0.4 %, but it increased substantially when the cyclic axial strain was greater than 0.4 %. The threshold cyclic axial strain was determined as 0.4 %, which was close to the yield cyclic axial strain, equal to the strain at the intersection of two envelopes in εa,c versus p′ space. The threshold strain did not vary substantially with either the cyclic stress ratio or the overconsolidation ratio, but it decreased with the effective consolidation pressure. Within the PI range of 5.8 to 9.4, there was no obvious change in threshold cyclic axial strain. When we combined data from other researchers, it was found that the threshold shear strain increases from 0.1 % to 3.0 % with an increase in PI. The average line for threshold shear strain (γth) for postcyclic shear strength change was found to be almost parallel to that for cyclic pore water pressure in PI–logγth space.