ABSTRACT

The process of compressive rock failure can be subdivided into 4 stages: Crack Closure (CC), Crack Initiation (CI), Crack Damage (CD) and peak strength (UCS). It is assumed that CI defines the lower limit of the long-term strength of rocks and is therefore an important stage in the complete failure process. Nevertheless, failure mechanisms and especially the determination of CI are not yet completely understood. This study aims to present various methods to detect CI and CD and to compare their results for a quartzite and a gneiss. For the latter, special focus was set on the foliated structure. Triaxial tests at minor confining pressures as well as uniaxial compression tests were conducted. In case of the triaxial tests one acoustic sensor was installed on the loading plate. In addition to passive recordings of acoustic emissions, we repeatedly transmitted acoustic pulses through the samples during uniaxial experiments. The arrival times and amplitudes of the recorded waves were evaluated and especially the latter leads to good results to determine CI. Orientated thin sections before and after a strength test were prepared to examine cracks and to identify the mineralogical composition by dint of a polarization and a scanning electron microscope.