Numerical investigation of micromechanisms of thermal strengthening in rock

Abstract

Thermal weakening of rocks is a well-acknowledged phenomenon. However, some experimental results reveal that some rocks are strengthened due to heating in a relatively low temperature regime (25°C to 175°C). In order to investigate the mechanisms of such strengthening phenomenon, a 2-dimensional grain-based model (GBM) which can mimic the thermal expansion and microcracking of polygonal grains is developed to examine the response of Carrara marble to thermal-mechanical loading from a microscopic point of view. Two sets of GBMs with different rock textures are designed and then tested under different confining pressures and temperatures varying from 25°C to 175°C. By examining the thermally induced microcracks, stress-strain curves as well as the distribution of microcracks in the numerical specimen, the micro-mechanisms of rock strength variation can be interpreted. The simulation results provide explanations for the thermal strengthening phenomenon in such a relatively low temperature regime. Few thermal microcracks, which tend to weaken rocks, are produced during heating in the rock. In contrast, compaction between grains occurs due to grain expansion upon heating, which can be regarded as the major causative factor that increases the rock strength. The tighter compaction in response to thermal treatment is a manifestation of the dependency of rock strength on rock texture. As a result, more heterogeneous rock texture may lead to a higher degree of strengthening. In addition, the confining pressure suppresses the grain expansion, and thus affects the stress-strain behavior associated with the strengthening phenomenon. The findings can provide important insights into geotechnical applications to achieve engineering safety and economy purposes. For example, in deep mining, we can take advantage of the naturally strengthened hot rock by correspondingly reducing the scale of artificial support, potentially leading to cost saving.