0530 1 IST-DECivil, Technical University of Lisbon 2 IST-DECivil, Technical University of Lisbon SHEAR MODULUS OF SOILS UNDER CYCLIC LOADING AT SMALL AND MEDIUM STRAIN LEVEL J.A. SANTOS 1 And A. GOMES CORREIA 2 SUMMARY The main purpose of this paper is to investigate the stiffness of soils under cyclic loading at small and medium strain level. The influence of some important factors on the shear modulus of soils such as: the strain level, the plasticity index and the mean effective stress are discussed in detail. Behaviour of different kind of soils is also compared based on available laboratory data including some experimental results obtained by the authors. INTRODUCTION In many practical cases the ground response under seismic loading is evaluated using the well-known equivalent linear method in which compatible values of shear modulus and damping ratio are chosen according to the shear strain level in soil deposit. In this simplified approach the developed pore water pressure and the residual soil displacements cannot be calculated. However, if a given problem do not involve large strains (γ>10 -2 ) the equivalent linear method can be considered acceptable in a practical point of view. This work deals in the range of small to medium strain levels (10 -6 ≤γ≤10 -2 ). The use of this method of analysis needs reliable strain-dependent shear modulus and damping ratio curves. In this paper only the factors affecting the shear modulus of soils are treated and discussed in detail. The text is essentially divided into two parts. In the first part, the stiffness of soil at small strain level (γ≈10 -6 ) is analysed. In the second part, the investigation proceeds with the study of the shear modulus degradation at higher strain level until 10 -2 . INITIAL SHEAR MODULUS In recent years many studies were performed to investigate the behaviour of soil at small strain level. The initial shear modulus G 0 (for γ≈10 -6 ) is a very important parameter not only for seismic ground response analysis but also for a variety of geotechnical applications. A considerable number of empirical relationships have been proposed for estimating initial shear modulus for different kind of soils: [Hardin and Black, 1969], [Iwasaki and Taksuoka, 1977], [Marcusson and Wahls, 1972; Kokusho, 1972; Kokusho and Esashi, 1981 and Nishio et al., 1985 in Ishihara, 1996] and [Biarez et al., 1999]. All of these relationships are based on two experimental evidences: the shear wave velocity (v s ) is a linear function of void ratio (e) and depends on the mean effective stress (p') with a power of n/2, as proposed originally by [Hardin and Richart, 1963]: