Influence of in situ factors on dynamic response of Piedmont residual soils

The in situ chemical and physical weathering of igneous and metamorphic roc ks, indentified as the process of formation of Piedmont residual soils, is a fairly well understood phenomenon. However, the effect this weathering ha s on the physical, mechanical, and dynamic properties of the rock/soil is n ot understood fully. This study focuses on the dynamic shear modulus, G, an d material damping ratio, D, of this soil formation for low- to mid-level a mplitudes of vibration. The paper presents laboratory test results and corr elations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from differen t depths were tested in a Resonant Column (RC) device. The specimens tested were SM and ML soils according to the USCS classification. The low-amplitu de shear modulus and damping values were found to be similar to those repor ted in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolid ation ratio exert a noticeable influence on the dynamic response as a resul t of variations in confining pressure. The understanding of these effects w ill allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on thes e soil deposits. The response in free field soil deposits compared with tha t of soils experiencing added confining stresses due to foundation loading appears to vary significantly in these geologic formations. Threshold strai n and the variation of damping, D, with the normalized shear moduli, G/G(ma x), fall within the same range as those recently reported by other authors in similar soils.