Mechanical Behavior of Weathered Compacted Shales

Compacted crushed shale is widely utilized as a construction material in roadway embankments. However, compacted shales are highly problematic as they tend to degrade with time once being subjected to cyclic weathering events. With degradation, shale converts from a hard rock-like material to a soft fine-grained soil mass with lower shear strength and high deformability. Therefore, embankments constructed from compacted shale may suffer from excessive settlement and instability. However, accelerating weathering prior to compaction by wetting and breaking down the shales before placement can reduce the detrimental effects of deterioration during the service life of the embankment. In this study, a laboratory testing program was conducted to investigate the mechanical and physical properties of the compacted shales with respect to mineralogical characteristics of the shales and weathering period. The selected shale samples were collected from different regions within the state of Kentucky, and all had unique geological and mineralogical properties. Three shales were classified as mudstone and two as marlstone. Laboratory data indicated a direct relation between the index properties of the shales and their mineralogy (in particular, carbonate and clay content) of the shales. Mudstone shales emerge a concave isotropic normal consolidation line which indicates the structural damage occurring along loading, while Marlstone shales had a straight isotropic normal consolidation line which implies the shale particles stay intact during consolidation. For both shale classes, the shear strength of the shales increased with weathering time.