This paper present an experimental study on brittle-ductile transition and shear strength behavior of shales. The experimental study is based on triaxial tests on different shales sheared at different levels of effective confining stresses. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the apparent overconsolidation ratio of the shale. A relationship between normalized undrained shear strength and apparent overconsolidation ratio is established. Correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed. These correlations were used to adequately predict the normalized undrained shear strength of shales whose apparent preconsolidation stresses or apparent OCR values are not known. In the absence of material-specific data, the different correlations can be used to obtain preliminary estimates of the undrained shear strength and ductility/brittleness of shales.
1. INTRODUCTION
Shales are of great engineering importance since they are one of the most abundant materials on the uppermost layer of the earth's surface. Many constructions, particularly tunnels, are in shaly materials (Einstein, 2000). Many landslides have occurred in shales (e.g., Wright and Duncan, 1972; Skempton et al., 1989; Picarelli et al., 2005). Shales exhibit behavior that is intermediate of soft clays and hard cemented rocks (Johnston and Novello, 1994). They are formed in sedimentary basins by a process consisting of deposition, followed by a series of loading (from compaction) and unloading (from uplift and erosion), chemical and mineralogical changes, and other agents such as creep and aging. The entire process, which turns young clay sediments into fully compacted and cemented shales, is called diagenesis. The properties of the original sediment, the loading history and mineralogical changes influence the behavior and properties of shales (Bjørlykke and Høeg, 1997). Because of their diagenetic history, shales are more difficult to characterize and their behavior difficult to predict than clays. In turn, because of their complex behavior, shales pose greater challenge during construction than most types of soft rocks. The two important parameters related to the response of geomaterials under load are the shear strength, and the mode of shear failure. Shear failure occurs when loading creates shear stresses that exceed the shear strength. However, shear failure does not always result in shear fracturing for rocks. In addition to the failure criterion, fracturing is controlled by the ductility or brittleness of the material. The deformation can be brittle or ductile depending on the properties of the shale and the effective confining stress level. In the field, the failure mode can have important consequences on how to design against failure. Brittle deformation is characterized by dilative response and sudden failure at a well defined peak shear strength followed by strain softening down to residual shear strength. Brittle response can be accompanied by the formation of distinct shear failure surfaces. Ductile response usually produces less defined peak shear strength and strain softening, more diffused and large deformations, bulging in test samples, and less distinct shear failure surface.
