This paper presents an innovative double cell triaxial system developed for better understanding of the complex thermo-hydro-mechanical behaviour of soils. In addition to the conventional triaxial mechanical testing, the equipment is aimed to perform suction controlled tests under wide range of temperature and confining pressure. Suction controlled tests are implemented by the axis translation as well as vapor techniques to control the matric and total suction. The device is provided with two advanced features; the double-wall system which allows precise measurement of volume variation of the sample in unsaturated conditions, and the hydraulic and pneumatic circuits, which allow several testing layouts to apply. Both complexity of the mechanical components of the system, and their evolutions under thermal and mechanical loading impose several aspects to the calibration process. An analytical approach is presented for the calibration of the force-displacement and soil sample volume change of the system under high axial loading. The calibration has been done using solid PVC (Polyvinyl chloride) samples. Results show the efficiency of the approach in order to formulate the force-displacement and sample volume change relationships in closed form equations. This approach can be efficiently used to calibrate such complex experimental testing systems working under wide range of mechanical loading.