Operational principle,testing, and applications of the AWID-flat jack for absolute stress determinations using voltage measurements

Summary Stress measurements can be performed with the, measuring principle for a flat jack discussed in this paper without any material parameters of the flat jack being of influence. This means that no calibration measurements are required by this flat jack for absolute measurements and there is no dependence on temperature. It is called Absolut Widerstands Druckmesskissen or short AWID-Flat Jack.Basis of evaluation is a change in the electrical resistivity of the flat jack, which is caused by two metal sheets separating from each other when inflated with hydraulic oil as soon as the external pressure is reached.Besides theoretical considerations concerning the mode of operation of the flat jack, this paper presents laboratory measurements performed in an autoclave as well as in a tube filled with salt grit under a uniaxial press. Changes of stress can be measured if the flat jack is cemented into a borehole under initial stress. The absolute stress of the bedrock can be measured after a certain time of adjustment in rock capable of creep (salt, clay, etc.).The advantages of the AWID measuring system are confirmed by in situ-measurements in a salt pillar loadable with variable pressure.In a temperature experiment at the Asse salt mine (Federal Republic of Germany) where the salt was heated up to 200°C, the advantages of the AWID measurement system were confirmed.