Evaluating the variations of density and durability index of schist rock under the effect of freezing-thawing cycles

Rock weathering affects its durability and strength in mountainous areas during the time. Evaluating durability and strength variations of rocks under freezing and thawing (F-T) cycles is a key factor for stability analysis of bedrocks in engineering projects such as tunnel, mine and etc. In this study, the influence of F-T cycles on second durability index, fifth durability index, dry density and schist rock volume has been evaluated. For this purpose, 13 boreholes have been drilled in western wall of Angouran mine and required samples have been prepared. In overall, 75 F-T cycles have been conducted upon the samples and laboratory tests including durability index of second and fifth cycles, dry density and measurement of samples volume have been carried out in 0, 7, 15, 40 and 75 cycles of F-T. On this basis, new equations presented and suggested to calculate durability index and dry density of samples in different F-T cycles. The obtained results showed that durability index of second and fifth cycles and dry density both decreased exponentially. Also, volume of samples has been increased by increasing the number of F-T cycles. In addition, mineralogical characteristics of schist rock have been studied by optical microscope and XRD analysis. Results of these analyses showed that Chlorite, Biotite, Calcite and Quartz are main constructive minerals of the schist rock. Finally, intact and weathered schist samples (after 75 cycles of F-T) are studied by an electronic microscope to validate the results and evaluate the effect of weathering on the microstructure of specimen’s texture. Obtained results indicated that the texture of samples was dense before weathering in which spacing of the schistosity beddings was very low. However, after 75 cycles of F-T, the discontinuity spacing was increased and new cracks were created in the body of samples perpendicular to the longitudinal axis.