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In experimental rock deformation, the stress applied on the cylindrical specimen parallel to the axis of the cylinder is called axial stress. If the specimen is unconfined, then the stress is biaxial with sigma2 equal to sigma3 and equal to 1 atmosphere or 1 kg per sq cm or 14 psi. If the specimen is confined, then the value of confined stress will change the strength of the specimen. As the confined pressure is increased, more axial stress is required to make the specimen fail, and the value at which this occurs is known as the compressive strength. This value is always given with respect to that of the confined pressure since it is solely dependent upon this, other variables being equal.