Rock strengthening or weakening upon heating in the mild temperature range?

Temperature has critical influences on rock strength. It is almost universally acknowledged from laboratory studies that rocks are weakened at elevated temperatures, as a result of the various "thermal damage" mechanisms. Most underground projects, such as geothermal energy extraction, nuclear waste repository and underground excavation for deep mining and caverns, are implemented at the depths ranging only from hundreds to thousands of meters where the temperature is far from reaching the high level for the sole operation of the thermal weakening factors. On the contrary, some experimental studies have reported that rock can be strengthened upon heating, rather than weakened in the mild temperature range from room temperature to 200 degrees C. Despite the profound scientific and practical significance in the geophysical and geological fields, whether rock will be strengthened or weakened in response to elevated temperature is elusive. The mechanisms behind the strengthening effect are also poorly understood. We carry out a comprehensive literature review on laboratory experimental results of the temperature effect on rock strength, with a focus on the mild temperature regime. We find that the dual effects of three major mechanisms and their interplay may either increase or decrease rock strength. Nevertheless, rock strengthening is more or less an intrinsic property depending on various factors. We also find that there exists a "transition period" at elevated temperatures from rock strengthening to weakening. This discovery is important for underground engineering projects, since the determination of a number of design parameters relies on the laboratory-measured rock strength. Misinterpreted temperature impact on rock strength in an engineering project potentially leads to structural and/or economical failure. For accurate evaluation of rock strength, future laboratory studies should concentrate on the mildly heated rock relevant to the in situ ground conditions.