Small-Strain Shear Modulus Behavior of Residual Soils in Tropical Regions under Pseudo-K0 Conditions

While the dynamic properties of residual soils, i.e., small-strain shear modulus, G0, and damping ratio, have been fairly well studied worldwide, the response of these soils under zero-lateral strain, K0, conditions has been less extensively investigated. This paper presents the results of a laboratory testing program designed to determine G0 in two residual soils derived from an igneous-metamorphic basement located in a tropical Andean region, tested under pseudo-K0 conditions in an electromechanical dynamic triaxial system. Shear waves were generated by bender elements in saturated samples under loading and unloading pseudo-K0 paths. The mineralogical and microstructural characteristics of the soils were examined using X-ray diffraction and scanning electron microscopy. The results show a relationship between G0 and mean normal effective stress, p′, that does not follow a power-law function. The resulting G0-values were compared, using a goodness-of-fit approach, against the predictions of well-established G0 expressions for fine-grained, coarse-grained, and residual soils. From these analyses, an equation that depends on void ratio, e, p′, and apparent overconsolidation ratio (OCR) is recommended to predict G0 in tropical Andean residual soils. The mathematical fitting constants were estimated via parametric analyses. It is concluded that, unlike sedimentary soils, all three fundamental variables (i.e., e, p′, and OCR) are required to calculate G0 in order to account for the complex microstructure of residual soils. Finally, it is highlighted that the classical Hardin equation, which is widely used for coarse-grained soils, is not recommended for calculation of G0 in residual soils.