1st Croatian Conference on Earthquake Engineering 1CroCEE 2021

During earthquakes soil deposits are exposed to all kinds of seismic waves and are consequently subjected to three-directional cyclic loading and straining. In the geotechnical earthquake engineering practice, however, the analyses of such complex response of the deposits to seismic forces, the site response analyses, are typically reduced to the analyses of the effects of the vertically propagating plane shear waves through horizontally layered deposits. The popular geotechnical laboratory tests for simulation of such behavior are cyclic simple shear, cyclic triaxial, cyclic torsional and resonant column tests that all cyclically shear the soil specimen in just one direction, typically in series of cycles with constant amplitude of shear stress, c, or shear strain, c. The paper presents several fundamental aspects of the cyclic soil behavior obtained relatively recently in the cyclic simple shear tests, indicating that the basic research of cyclic and dynamic soil properties is still going on. The parameters and curves derived from cyclic simple shear tests that are used in popular computer models for site response analyses that are treated in this paper include: maximum shear modulus, Gmax, cyclic secant shear modulus, Gs, equivalent viscous damping ratio, , curve of the reduction of the first cycle Gs with c, second cycle  versus c curve, pore water pressure change with the number of cycles N in saturated soils, change of Gs with N, and the cyclic threshold shear strains for pore water pressure change, tp, and cyclic degradation, td.