Loading-unloading tests under a conventional testing machine were conducted on Portuguese marble samples to detect interesting features of its mechanical behaviour, which are related with their involvement in underground mining methods. It was observed that decreasing the loading rates of the equipment led to obtaining stress-strain diagrams similar to those resulting from servo-controlled testing machines. It was also found that the effect of coupling between elasticity and plasticity exists in some of the specimens, although for other specimens no such result was obtained. These results show that the coupling effect is the feature that connected both testing conditions and failure modes of the specimens. Another finding was that irrecoverable deformations exist even when the unloading stress is much smaller than the compressive strength. For some samples, the tangent value of Poisson's ratio raises gradually with the increase of applied stress until the value is near 0.5, while Young's modulus remains approximately constant.
The production of ornamental stones is an important industrial activity of Portugal, aimed to supply local consumption and exporting considerable amounts. Marble is the most important of them, traditionally produced from open pit quarries. With the raising pressures of environment protection and the deepening of most quarries excavation levels, underground mining is becoming more attractive. Stress distributions around underground openings are usually more complex than those in open pit excavations, so conditions of unloading-reloading cycles in the rock masses often appear, thus justifying an investigation on marble mechanical behaviour, especially on their deformation characteristics under cyclic loading conditions. A simple classification according to the amount of the veins contained in marble rocks was implemented, creating three groups: samples without veins; those with few veins and a third class with many veins. Furthermore, rock colours can be chosen to divide marbles into two other types (white and dark), so there are altogether six types of marble samples. For the tests performed in this research, two types of marble samples were utilized:
Dark marble with many veins;
White marble with a few veins.
Cylindrical marble samples resulting from exploration boreholes performed in a Portuguese quarry were selected for the experiments. Their diameters were around 47.5mm and specimens were prepared with a height/diameter ratio of 2.5. Uniaxial compressive tests were conducted on the specimens under normal testing machine ELE Compact 1500, with a maximum loading capacity of 1500 kN. Six specimens were prepared for each rock type and electric strain gages were installed on the surfaces of the samples to measure deformations during the tests. A special recording system that can acquire both data of deformation and applied charges was utilized with direct computer connections. That system is composed of a multi-channel dynamic strain indicator with an analogue-digital converter (A/D converter) and a computer. The signals obtained from the strain gages are firstly amplified by the dynamic strain indicator, and then transformed into digital signals by the A/D converter, which are then sent to the computer for further processing.