Abstract: A device positionable in a borehole for determining rock mass deformability, comprises a generally cylindrical body having at least one pair of diametrically opposed, elongated expandable wall portions for pressure contact engagement with the borehole wall, the wall portions being radially outwardly expandable in diametrically opposite directions along a predetermined axis extending transversely to the longitudinal axis of the borehole. An elongated core is arranged in the body and extends longitudinally thereof. A chamber is associated with a respective one of the expandable wall portions and is defined between the respective expandable wall portion and the core, the chambers associated with the at least one pair of expandable wall portions being in fluid communication for receiving a fluid under pressure.