Effect of Soil Plasticity on Cyclic Response

TLDR

Possible reasons for the observed influence of plasticity on cyclic response are discussed. The study investigates how the plasticity index affects cyclic stress‑strain parameters and seismic response of saturated soils for site‑response evaluations and microzonation. Ready‑to‑use charts derived from 16 experimental studies illustrate how PI influences modulus reduction and damping curves, and a parametric analysis evaluates seismic response of clay sites under 1985 Mexico City earthquake accelerations. Higher plasticity increases modulus retention and reduces damping, leading to more linear, less degrading cyclic behavior in soils.

Abstract

A study on the influence of the plasticity index (PI) on the cyclic stress‐strain parameters of saturated soils needed for site‐response evaluations and seismic microzonation is presented. Ready‐to‐use charts are included, showing the effect of PI on the location of the modulus reduction curve G/Gmax versus cyclic shear strain γc, and on the material damping ratio λ versus γc curve. The charts are based on experimental data from 16 publications encompassing normally and overconsolidated clays (OCR=1-15), as well as sands. It is shown that PI is the main factor controlling G/Gmax and λ for a wide variety of soils; if for a given γc PI increases, G/Gmax rises and λ is reduced. Similar evidence is presented showing the influence of PI on the rate of modulus degradation with the number of cycles in normally consolidated clays. It is concluded that soils with higher plasticity tend to have a more linear cyclic stress‐strain response at small strains and to degrade less at larger γc than soils with a lower PI. Possible reasons for this behavior are discussed. A parametric study is presented showing the influence of the plasticity index on the seismic response of clay sites excited by the accelerations recorded on rock in Mexico City during the 1985 earthquake.