Determination of mechanical properties of flysch using laboratory methods

The paper deals with the determination of the mechanical properties of heterogeneous rocks in the laboratory and proposes a methodology for predicting the rock mass strength of flysch formations consisting of siltstone–sandstone alternations in different proportions. In order to simulate such formations, composite specimens comprising superimposed disks of intact sandstone and siltstone material with different thickness ratios were prepared. Wave velocity (P and S), uniaxial and triaxial compressive strength and Youngʹs modulus of the composite samples were determined in the laboratory. According to the laboratory results, the uniaxial compressive strength and Youngʹs modulus decrease with the increase of the siltstone percentage in the specimens, and yield the strength of siltstone when the siltstone percentage in the rock is equal to 37%. Additionally, the triaxial compression tests on composite samples revealed a significant decrease of the mi parameter for a siltstone percentage of only 17%. A comparison is made between the results of the present study with similar laboratory and empirical references from literature. Finally, it was possible to suggest how the relation between uniaxial compressive strength, σci, and siltstone percentage, sl, may differ according to the ratio of the compressive strength of the two main rocks, weak and strong, which comprise the heterogeneous rock.