Effect of Physico-Chemical Factors on the Disintegration Behavior of Calcareous Shale

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Calcareous shale, which is often encountered in construction projects, has a variety of physical and chemical properties that influence disintegration behavior of the rock mass. The diverse behavior of calcareous minerals in shale adds to the complexity of geotechnical investigations. Abundance of calcareous minerals is known to improve shale rock strength, although, when exposed to seasonal wetting and drying cycles, disintegration is rapid. The intent of this study is to statistically evaluate the disintegration behavior of calcareous shales in relation to their physico-chemical properties. Shale samples from fresh rock cuts and talus were tested using a multi-cycle slake durability index (Id), X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) elemental analysis, bulk density, natural water content, and porosity. A relationship was found between the degree of disintegration and the abundance of calcareous minerals, especially calcite and gypsum. Higher porosity and elevated water content were observed in highly weathered shales. Additionally, the efficiency of the Id test as an indicator of the degree of disintegration was evaluated. Grain-size distribution (GSD) analysis of disintegrated talus was compared with fragmental material of two-and five-cycle Id (Id2 and Id5) tests. The Coefficient of Uniformity (Cu) and Coefficient of Curvature (Cc) of Id5 materials showed satisfactory correlation with Cu and Cc of talus material. It is demonstrated that when compared to the conventional Id2 test, multi-cycle Id tests more precisely predict the disintegration pattern of shale and can be used to evaluate shale in terms of degree of disintegration.