Abstract: To ensure the stability of the borehole surrounding rocks during geothermal energy extraction and improve the efficiency of the process, single radial compression mechanical tests were conducted on energy-storing granite with internal sleeves. Using multiple monitoring devices, including the VIC-3D non-contact full-field strain system, a static strain testing and analysis system, this study investigated the damage characteristics and internal energy evolution of the borehole surrounding rocks under complex geological conditions such as high temperature and thermal cycling. Additionally, a failure criterion was established. The results indicate that the load-displacement curve of the single radial compressed energy-storing granite with internal sleeves exhibits a distinct pre-peak concave segment and linear segment under various conditions. Post-peak behavior is influenced by the sleeve and rock heterogeneity, showing two patterns: a rapid steep drop, or a drop followed by recovery, or leveling off before dropping again. The final failure mode is fragmentation into three or four pieces. Damage initiates with cracks at the inner borehole wall, parallel to the loading direction, followed by cracks at the outer borehole wall, perpendicular to the loading direction. Adding sleeves reduces crack width; fracture morphology and crack growth are partly influenced by the bonding between the rock sample and the sleeve. Energy evolution analysis reveals that the total pre-peak energy grows exponentially. We analyzed the relationship between energy and sleeve thickness by examining the proportions of elastic and dissipated energy relative to total pre-peak energy. A sleeve thickness of 1.0−1.5 mm provides the optimal enhancement. Based on theoretical analysis, we developed a failure criterion for energy-storing granite with internal sleeves and validated its accuracy.

Key words: internal sleeves, energy-storing granite, single radial compression, mechanical characteristics, energy evolution, damage criterion