Abstract: In the field of underground engineering, the traditional isothermal cycle scheme employed in TBM-assisted thermal damage rock breaking technology is hampered by high energy consumption, which severely limits its economic viability and future application. To address this challenge, investigating the rock degradation effect induced by a novel variable temperature-water cooling thermal loading method is of significant value for promoting energy efficiency and practical implementation of this technology. Through uniaxial compression, Brazilian splitting, and penetration tests, this study systematically investigated the evolution in penetrability of extremely hard granite subjected to three cycles of variable temperature-water cooling at different high temperatures (T1, T2, T3). The analysis focused on the evolution of rock failure patterns, morphology, compressive and tensile strength, and penetration rate. The results indicate that as the initial temperature T1 increases, thermally induced microcracks evolve from a sparse to a networked distribution, with damage initiated solely through intergranular crack propagation. The failure mode transitions from tensile splitting to oblique shear, accompanied by a brittle-to-ductile shift in failure morphology, with 500 ℃ identified as the critical transition point. Both the rate of strength reduction and the rate of penetration increase peak within the T1 range of 500−600 ℃, beyond which they gradually decelerate and stabilize. When T1 exceeds 600 ℃, the rate of penetration increase diminishes compared to that at 600 ℃, although the absolute penetrability remains superior to the room-temperature control group. While the second high-temperature stage T2 (200−600 ℃) further reduces rock strength and increases microcrack density and crushed zone volume, its influence on failure mode and morphology is less pronounced than that of T1. A significant enhancement in penetration rate is observed specifically when T2 is 200 ℃. Therefore, for TBM excavation in extremely hard rock formations utilizing variable temperature-water cooling assistance, a cyclic scheme with an initial high temperature of 600 ℃ followed by a subsequent temperature of 200 ℃ is recommended.

Key words: variable temperature-water cooling cycle, extremely hard rock, rock strength, failure mode, penetration tests