Effect of Physico-Chemical Factors on the Disintegration Behavior of Calcareous Shale ARPITA NANDI MICK WHITELAW Department of Geosciences, 100 CR Drive, East Tennessee State University, Johnson City, TN 37614 Key Terms: Calcareous Shale, Multi-Cycle Slake Durability, Sevier Shale, Disintegration, Shape Para- meter, Rock Mechanics ABSTRACT 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, disinte- gration 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 relation- ship 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. INTRODUCTION AND OBJECTIVE More than 35 percent of Earth’s crust is made up of shale, a fine-grained argillaceous sedimentary rock, in which about half of the clastic grains are smaller than 0.06 mm in diameter (Blatt et al., 1980). Shale containing 20 percent calcium carbonate in the form of finely precipitated calcite or small organically fixed particles is called calcareous shale (Pettijohn, 1957). Other calcareous materials may include aragonite, dolomite, and gypsum. Since, in some shale, calcareous minerals usually constitute up to 20 percent of the material, their presence can have variable, yet significant effects on shale geotechnical properties (Sadisun et al., 2005). The presence of calcareous minerals in micro-fractures or as cement- ing materials in shale matrix is known to improve strength of the rock mass. However, exposed shale is often intensely fractured and weathered, and this can cause significant construction problems and damage to civil structures. When calcareous shale is exposed during construction, the action of natural or polluted water and seasonal wetting/drying cycles causes carbonate mineral dissolution and accelerates deterioration. Calcareous shale is characterized by wide varia- tions in physico-chemical properties such as mineral content, degree of induration, shrink-swell behavior, and presence of fractures, micro-fractures, and voids. Shales exhibit mechanical properties that range from low durability, fissile rocks to hard, massive, and compact types (Dick and Shakoor, 1992; Yilmaz and Hu ¨ seyin, 2002; Yasar and Erdogan, 2004; Santi, 2006; and Marinos and Hoek, 2006). The durability of such rock is measured by the rock’s resistance to disinte- gration caused by repeated wetting and drying cycles. Disintegration of shale into relatively small pieces upon exposure to different humidity-moisture envi- ronments, including submersion, is referred to as slaking. The resistance to disintegration is determined in the laboratory by the slake durability index (Id) test. The Id is an important parameter that controls project design in rock slope stabilization, embank- ment fill, foundation support, and underground mine stabilization. Due to the wide range of variability in Id of calcareous shale, quantification of disintegra- tion characteristics becomes difficult. Environmental & Engineering Geoscience, Vol. XV, No. 4, November 2009, pp. 273–285 273