Deformation properties of fine-grained soils from seismic tests

Geotechnical design requires the prediction of soil structure interaction, for which the deformation properties of the soil are needed. Little guidance can be found in the literature for estimating the soil modulus during undrained loading. Therefore, over-simplified methods are frequently used even for the analysis of complex problems. The concepts used to describe the deformation behavior of fine-grained, normally consolidated soils are presented and critically reviewed. The deformation properties (shear modulus) at small and large strain are discussed. Based on a comprehensive survey of seismic field and laboratory data, it is possible to predict the shear modulus at small strain and the variation of the shear modulus with increasing shear strain. A relationship is proposed which can be used to predict the variation of the normalized shear modulus as a function of shear strain. It can be shown that the strain rate at seismic small-strain testing is slow and comparable to that of conventional geotechnical laboratory tests. The starting point of the stress-strain curve (at low shear strain level) can be accurately established from seismic tests, and its end point (at high strain) by conventional shear tests. The variation of the shear modulus with strain can be determined from resonant column tests. A numerical model is presented which makes it possible to predict the variation of shear modulus as a function of shear strain. The practical application of the concept is illustrated by a case history, where good agreement was obtained between predicted and measured deformation properties.