Effect of Fabric and Weathering Intensity on Dynamic Properties of Residual and Saprolitic Soils via Resonant Column Testing

Chemical weathering processes, such as decomposition and dissolution, are fairly well-understood phenomena as they relate to the formation of residual and saprolitic soils in the tropics. However, the effects that weathering intensity has on the physical characteristics and mechanical properties of weathered soil/rock materials, particularly their dynamic properties, are not yet fully understood. This paper presents the results of an experimental investigation conducted to assess the dynamic response of residual and saprolitic soils derived from a granodiorite rock in the central mountain range of Colombia and how this response relates to the soil microstructure and weathering intensity. Key dynamic properties, namely, shear modulus and material damping, were obtained via a fixed-free resonant column apparatus suitable for shear strain amplitudes ranging from 0.001 % to 0.1 %. Results from a short series of triaxial and oedometer tests substantiated the patterns of weathering intensity observed for each test soil. Soil fabric studies based on scanning electron microscopy observations, mercury intrusion porosimetry tests, and pore space distributions were also performed to identify the most relevant characteristics of the soil skeleton, as determined by the corresponding weathering intensity, that affect the dynamic response of each test soil. The residual soil, as the most altered/weathered material, was found to have a more rigid fabric, and thus greater stiffness, due to the presence of sesquioxides acting as cementing agents. In the saprolitic soil, a less weathered material, the soil fabric was dominated by distinct clay bridges formed between particle aggregations of partially decomposed primary minerals, resulting in less rigidity. The influence of confinement level on the shear modulus was found to be more pronounced in the saprolitic soil, which can be directly attributed to the changes in fabric of uncemented bonds during isotropic loading. Finally, and consistent with these general trends, the material damping of saprolitic soil was observed to be slightly less than that of residual soil, whereas the normalized shear moduli (G/Gmax) degradation curve was more pronounced in residual soil samples beyond a threshold value of shear strain amplitude.