Current laboratory techniques for the determination of Young's moduli or constrained moduli are fraught with restrictions: Typically in rock mechanics laboratories, the Young's modulus is determined on drill cores. For fault rocks this test procedure in many cases is not applicable due to the poor quality of the material. Oedometers, as characteristically used for soil testing are not adequate due to the small sample size and low testing stress levels. To enable testing of fault rocks under realistic conditions a large oedometer test apparatus was developed at the Institute for Rock Mechanics and Tunnelling, Graz University of Technology. The large oedo-meter has an inner diameter of 300 mm. The sample height can be varied between 60 and 100 mm. The main parts of the apparatus are the oedometer ring, a base plate, two filter plates, and a head plate. The test set up is designed to conduct tests with fixed ring as well as with a floating ring. The axial force is measured by a 2 MN load cell, which corresponds to 28.3 MPa. The ring is situated on three 50 kN load cells in order to get the amount of friction between specimen and ring. The vertical displacements are precisely captured by three displacement transducers. The oedometer ring is equipped with three strain gauges at different heights to determine the circumferential strains, allowing accurate calculation of the Poisson's ratio.


The knowledge of the deformation properties, especially of fault zones, is of tremendous importance for the selection of appropriate support measures and for minimizing risks in tunnelling and underground structures. In order to test fault rocks a large oedometer test apparatus (Fig. 1) was developed at the Institute for Rock Mechanics and Tunnelling at Graz University of Technology.

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