Characteristics of weathered mudstone with X-ray computed tomography scanning and X-ray diffraction analysis

Large-scale deformation on a cut slope composed of mudstone from the Neogene was observed over time during excavation of the powerhouse area at the Rajamandala Hydroelectric Power Plant in Indonesia. This paper discusses the characteristics of the mudstone and the mechanisms of slope behavior. As a part of detailed research into the mudstone, X-ray Computed Tomography scanning (CT scanning) and X-ray diffraction (XRD) analysis were applied to core samples. The purpose of CT scanning was to visually observe inside the core samples in three dimensions. The CT scanning image, which was a set of numbered CT scans, exhibited a color tone corresponding to the soundness of the core samples and this can be an indicator of weathering grade. The extent of weathering of each core sample was classified quantitatively by the CT scanning image and CT number. The extent of weathering maintains objectivity in the engineering classification of rock mass, which is generally assessed by core sample observation and hammer impacts. XRD analysis is commonly applied to dried samples in order to clarify their mineral components, especially the content of swelling clay minerals such as smectite. This paper discusses, in addition, XRD analysis for field-wet samples, because it is considered that a number of interlayer water molecules in smectite may have changed while preparing dried samples. Smectite, because of the relatively high quantity of interlayer water molecules, could likely cause swelling and slaking.