Experimental Pilot Thermomechanical Study of Lower Carboniferous Greywacke: Temperature Effect on the Rock Mass Properties Investigated up to 1,200°C

ABSTRACT Greywacke, i.e., a non- to anchi-metamorphosed immature sandstone that contains more than 15 % clay minerals, occurs in most orogenic belts, with depositional ages ranging from Precambrian to recent. Our study focuses on thermomechanical rock properties of greywacke sampled in Kobeřice quarry (Lower Carboniferous Culm Basin, Drahany Upland, Bohemian Massif). Both relatively fine-grained and coarse-grained (with higher abundance of lithic fragments and small pebbles) intact rock core samples were tested. The experiment was divided into two sets. The first set made of the coarse-grained greywacke gave insight into the changes of properties across the whole temperature range, up to 1,200°C. The second set of samples was used to study the behavior of the fine-grained greywacke in the lower temperature range, up to 400°C. The lower temperature analysis showed an unexpected phenomenon at 200°C to 300°C, manifested by decrease in the strength coupled with higher values of deformation characteristics. These trends reverted at 400°C. The described phenomenon may have been caused by changes in water content combined with clay minerals composition alternation. Alternatively, it could be also related to the rock “paleotemperature memory.” On the wider temperature range, the greywacke holds relatively well to the lower temperatures, with the strengths decreasing slowly after 400°C and more rapidly after 800°C. At 1,200°C, partial melting at grain boundaries accompanied by formation of larger pores in the sample center was observed. Results of thermomechanical studies can be crucial during the design of underground constructions or for assessment of the fire damage on a rock-based constructions.