Determination and detection of stress states in hard rocks play a fundamental role in the estimation of the deformation behavior during and after the design and monitoring phase of en-gineering geological structures. A possible alternative to the established strain measurements would be the finite deformation analysis. To show the dependence of changes of the principle stress direc-tions (σi) on the small-deformation behavior, laboratory uniaxial compression tests were performed under varying applied pressures on different rock types. Analyzed by the Intercept-Method, which generates deformation ellipses of the total deformation component, the results show a correlation between the shape of the deformation ellipse, the applied load, the microstructure and the particle sizes of the different rock types. Based on the creation of deformation ellipses strain changes can be observed, which display changes of the principle stress direction (si). Thus, the finite deformation analysis can contribute to the monitoring of rock deformation.

1 Introduction

The finite deformation analysis is a minimal invasive method of the visual quantification of micro-scale deformation behavior of hard rocks (e.g. Dunnet 1969, Fry 1979, Mulchrone 2007, Yamaji 2013). The determination of the deformation behavior of hard rocks and rocks masses are important for the estimation and the prediction of stress conditions in hard rocks during and after the design of engineering geological structures (e.g. underground cavities, bridges). In contrast to the established instruments for the characterization of the mechanical behavior of the rocks, the finite deformation analysis can be carried out without the installation of measuring instruments. Therefor strain ellipses displays detected micro-scale deformations of hard rocks (Ramsey 1967, Fry 1979).

We present analyses to study the change of the principle stress direction (σi) in dependence of the microscale deformation behavior (total or plastic deformation) and of the different grain sizes of two different rock types. Strain ellipses display the deformation and are generated with the Intercept-Method (Launeau & Robin 1996) on uniaxial compressed rock samples under different load levels. In addition, conclusions will be drawn for an applicability of the Intercept-Method for geotechnical issues and whether the data are sufficient if they are registered after compression tests (plastic), or if the data have to be obtained during a running compressing test (total).

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