NONLINEAR CYCLIC CHARACTERISTICS OF SOILS Fred (Feng) Yi 1 1 Senior Geotechnical Engineer, C.H.J. Incorporated, 1355 E. Cooley Dr., Colton, CA 92324; E-mail: fyi@chjinc.com Abstract: For this paper, research on nonlinear cyclic characteristics of soils was reviewed. A new model is presented consisting of a modified equation for an initial loading curve and a modified equation for constructing the hysteresis loop. The performance of the model was evaluated by simulating the frequently utilized relationships of shear modulus versus shear strain and damping ratio versus shear strain of various types of soils accumulated over the past decades. The outcomes indicated that the new model presented describes the measured relationships with excellent correlations. The new model can simulate not only the work-hardening, but also work-softening behaviors of soils. INTRODUCTION Since the late 1980’s, several unusually large earthquakes have occurred throughout the world. Those earthquakes not only destroyed innumerable houses, buildings and other structures such as bridges but also devastated the lives of thousands of people. In particular, after the Great Hanshin Earthquake, the previously accepted concept that the magnitude of displacement caused by an earthquake will be centimeters or decimeters was totally changed by the fact that measured displacements were on the order of meters. The lessons learned from those disasters require geotechnical earthquake engineers to make more accurate predictions of the behaviors of soil structures under large earthquake excitation. On the other hand, the dramatic development of semiconductor technology since the late 1980’s has allowed the personal computer to become an indispensable tool for geotechnical engineers. More and more numerical analyses, such as two and three dimensional linear and nonlinear stress and deformation analyses, are now performed on personal computers. Until relatively recently, such numerical analyses would have cost a great amount of money or, in many cases, would have been impossible. Although hardware capable of satisfying the calculation demands of geotechnical engineers is readily available, crucial technology to model the seismic behavior of soils has not kept pace, and, in fact, has fallen far behind. In this paper, a new constitutive model simulating cyclic characteristics of soils is proposed. It is intended not only to provide a better simulation of seismic behavior of soils but also to fully utilize the historical data accumulated during past decades. SIMULATION OF CYCLIC CHARACTERISTICS OF SOILS Although elasto-plastic theory has been utilized to simulate the nonlinear