EFFECT OF SOIL PLASTICITY ON CYCLIC RESPONSE

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/G(max) versus cyclic shear strain-gamma-c, and on the material damping ration gamma-versus-lambda-c 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/G(max) and lambda for a wide variety of soils; if for a given gamma-c PI increases, G/G(max) rises and lambda 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 gamma-c 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.