This paper present an experimental study on the shear behavior and the formation of shear fractures in shales. The experimental study is based on triaxial tests on shales sheared at different effective confining stresses. Triaxial tests with different loading conditions have been performed to establish the brittle-toductile transition. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the over-consolidation ratio. A relationship between normalized undrained shear strength and over-consolidation ratio is established. In combination with this relationship, a correlation between compressional wave velocity and apparent pre-consolidation stress, which accounts for both mechanical and chemical diagenesis, may be used as a tool to evaluate shear failure and fracturing in shales.


Shales are fine-grained soft rocks that contain 50 to 100% clay particles and are characterized by some degree of lamination and fissility parallel to the bedding plane. 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 cases of landslides have occurred in shales and clay-shales (e.g. Mesri & Cepeda-Diaz 1986, Stark et al. 2005). Drilling wells for hydrocarbon production must pass through shaly formations comprising the cap rock of a producing reservoir (Steiger & Leung 1991). Shales exhibit behavior that is intermediate of soft clays and hard cemented rocks (Johnston & Novello 1994). Shales 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 sediment into a fully compacted and cemented shale, is called diagenesis.

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