Influence of In Situ Factors on Dynamic Response of Piedmont Residual Soils

Abstract

The in situ chemical and physical weathering of igneous and metamorphic rocks, identified as the process of formation of Piedmont residual soils, is a fairly well-understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock/soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to midlevel amplitudes of vibration. The paper presents laboratory test results and correlations 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 different depths were tested in a Resonant Column device. The specimens tested were SM and ML soils according to the Unified System (USCS) classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses caused by foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G/G sub max, fall within the same range as those recently reported by other authors in similar soils.