Mechanics of thermal spallation and fracturing of dry rocks produced by surface heating: Fid-Bau Portal

Mechanics of thermal spallation and fracturing of dry rocks produced by surface heating

Guo, Yide / Dyskin, Arcady / Pasternak, Elena / Li, Xibing / Huang, Linqi

This paper reports the results of thermal spallation experiments on specially dried shale samples with 3 bedding orientations (0°, 45° and 90°) under 3 flame temperatures (899 °C, 1243 °C and 1559 °C) and the 3D thermal elastic finite element modelling. Under open flame heating, continuous spallation is observed with ejection of various spalls and popping sounds. After a period of spallation, tensile fractures are formed in the samples and grow perpendicular to the heating surface, except for a sample with bedding orientation of 90° under the low temperature. Increasing flame temperature promotes ejection, popping sounds and spallation rate, but reduces the spallation starting time, spallation duration, depression diameter and depression depth. The model shows that heating induces compressive stress in the surface layer and tensile stresses beneath it. The tensile stress is found to be sufficient to generate large tensile fractures. The ratio between the induced compressive and tensile stresses increases with increasing spallation depth but little affected by the flame temperature. The spallation compressive stress increases with temperature from 29 to 51% of the uniaxial compressive strength. This stress is shown to be sufficient to cause buckling of thin layers separated from the bulk of the rock. The size of the buckling layer is smaller than the size of the spallation zone leading to a mosaic pattern seen on the surface after spallation. The results are important for further understanding of the mechanism of thermal spallation of rocks as well as large scale spallation-like processes in the Earth’s crust.