Seismic response of granites with different grain sizes after thermal treatment: an experimental study

Abstract

Abstract Understanding the wave propagation behaviour in rock masses with different temperatures and geological conditions is of great significance for the stability and safety evaluation of deep rock engineering, e.g., enhanced geothermal system, nuclear waste disposal. However, the response and mechanism of ultrasonic waves through granites after thermal treatment are still poorly understood. In order to determine the combined effects of heating temperature and grain size on wave propagation across granites, a series of laboratory ultrasonic tests were performed with the pulse transmission method, combined with scanning electron microscopy observation. The testing results indicate that heating temperature and grain size have a combined impact on wave propagation across the tested granites. The wave velocity, transmitted coefficient, peak power spectral density and accumulative energy are generally negatively correlated with heating temperature regardless of grain size. The effect of grain size on wave propagation is more pronounced at low temperatures. Basically, the crack evolution is the main reason for the seismic response of granite after thermal treatment. A damage factor defined by the change of microcrack area in this paper is proposed and adopted to consider the combined effect of heating temperature and grain size. The peak power spectral density of the low-frequency wave and the transmission coefficient of the high-frequency wave are appropriate as the optimal wave indicators for evaluating the deterioration of granites at high- and low-damage stages, respectively. The findings in this study are of great importance for site selection and stability assessment in rock engineering activities under high temperatures, especially for the development of deep-seated hot dry rock.

Keywords

• Thermal treatment
• Grain size
• Wave propagation
• Microcrack evolution
• Damage