Dynamic Properties of Sandy Soils at Large Shear Strains with Special Reference to the Influence of Non-Plastic Fines

Introduction

Strain dependent dynamic properties such as shear modulus (G) and damping ratio (D) are the two major parameters generally used for the analysis of engineering structures subjected to seismic loading. During seismic loading, the response of the soil mass is often dependent on these strain dependent dynamic properties and adequate information of these properties is very important for earthquake engineering problems. For many important problems, particularly those dominated by wave propagation effects, only low levels of strains is induced in soil and for other, such as stability of masses of soils, large strains are induced in the soils. Therefore, evaluation of the strain dependent properties had to carry out not only at low strains, but also at intermediate and high shear strain levels. In the recent past many investigations were carried out to understand influence of different parameters such as cyclic strain amplitudes, effective confining pressures, soil types, plasticity index, density, frequency of loading, number of loading cycles, overconsolidation ratio, degree of saturation and grain characteristics on the dynamic properties of soils (Richart et al., 1970; Seed & Idriss, 1970; Hardin & Drnevich, 1972; Iwasaki et al., 1978; Kokusho et al., 1982; Seed et al., 1986; Vucetic & Dobry, 1991; Ishibashi & Zhang, 1993; Stokoe et al., 2004). It has been reported that shear modulus and damping ratio is influenced mainly by cyclic strain amplitudes, effective confining pressures, soil type, and plasticity index. In addition, it has also been reported based on the torsional test (up to shear strain level of 0.015%) that dynamic properties, especially initial shear modulus (G_max) is influenced by the degree of fines (Chien & Oh, 2002).

In the present investigation, a comprehensive study has been undertaken to evaluate the strain dependent dynamic properties at large shear strain levels for the sand samples collected from the earthquake affected area of Ahmedabad of Gujarat state, India. A series of strain controlled cyclic triaxial tests, were carried out on base sand (natural sand) and clean sand. The shear modulus and damping ratios have been estimated based on the first cycle information. The effect of different parameters such as number of loading cyclic, relative density, confining pressures and non-plastic fines on the dynamic properties of the soils have been studied and reported.