Closed-loop, servo-controlled experiments were conducted to investigate the evolution of the shear band in a sandstone under low confinement. The tests were performed with a plane-strain apparatus designed to allow the failure plane to develop and propagate in an unrestricted manner. Thin-section microscopy provided direct observations in and adjacent to the shear band. Two distinct regions of the failure surface were recognized from low-confinement experiments. The first region, called the primary fracture, cut diagonally through the center of the specimen and stopped before intersecting the surface. The second region featured a kink and began nearly at the termination of both ends of the primary fracture. The kinked region was inclined at a less steep angle to the direction of minimum compressive stress. Microscopic observations showed that the primary fracture was formed in shear and the secondary kinks were formed in tension.
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