During triaxial compression test, a soil specimen experiences reduction of its height with increase in its diameter. New cross-sectional area is calculated assuming uniform increase in diameter. This condition is seldom met in actual soil specimens, because specimen undergoes non-uniform increase in diameter and very often with localized bulging in specimen affecting the accuracy of calculation of deviator stress, shears strength and volume change characteristics. The expandable jacket included in this invention consists of circular segmental metal plates wrapped around the soil specimen and elastomeric rubber bands or rings around the segmental plates to permit uniform radial expansion and maintain uniform diameter of the specimen during the test and thereby providing accurate values of deviator stress, volume change characteristics and shear strength of soil specimen. The calibration device for calibration of expandable jacket shall provide the magnitude of correction to be made in deviator stress.