Experimental study on damage evolution and failure precursor characteristics of granite under thermal shock cycles

Abstract

Investigating the damage evolution of surrounding rock under thermal shock cycles is crucial for ensuring the stability of engineering rock masses. This study performed Brazilian splitting tests on granite specimens under varying temperature and cycle conditions, employing acoustic emission monitoring, digital image correlation, and three-dimensional scanning technology. A systematic analysis was conducted on the patterns of damage evolution, failure precursor, and response mechanisms under combined thermal and cyclic loading. Experimental results show that both P-wave velocity and tensile strength degrade significantly with increasing temperature and cycle count, with temperature having a more pronounced effect than cycle count. Notably, damage evolution exhibits a dual-threshold behavior in which degradation accelerates markedly above 400 °C and stabilizes after 5 thermal cycles. Fracture surfaces evolve from initially planar to rugged morphologies, with peak-valley height differences at 600 °C being approximately three times greater than those at 200 °C. Furthermore, based on acoustic emission energy entropy analysis, we introduce a novel failure precursor indicator where the sustained increase and critical surge in average entropy serve as reliable early-warning signals for impending rock failure. These findings establish a solid theoretical basis and practical methodology for damage assessment and instability early-warning systems in high-temperature rock engineering.

1. Introduction

With the acceleration of urbanization and advancements in construction industrialization, the development and utilization of deep underground spaces—including deep foundations for super high-rise buildings, urban underground transportation tunnels, and large-scale geothermal energy wells—have become critical pathways to promote intensive urban development and contribute to carbon neutrality goals [1–3]. Granite and other rock types, serving as primary surrounding rock masses, are subjected to the combined effects of elevated temperatures and groundwater seepage during long-term service [4]. Taking urban deep geothermal development projects as an example, cold water is typically injected into reservoirs to extract geothermal resources through thermal conduction between high-temperature rocks and water [5,6]. This process exposes rock masses to repeated thermal shock cycles, with the rapid cooling phase being particularly critical as it generates significant tensile stress on the surface that can initiate and propagate microcracks [7,8], leading to considerable cumulative thermal damage that compromises the long-term stability and safe operation of geothermal wells [9]. Therefore, a comprehensive investigation into the evolution of damage accumulation in granite under thermal shock cycles and the establishment of corresponding early-warning mechanisms for instability are essential for ensuring the operational safety of deep rock engineering projects.

Recent systematic studies have investigated the physical and mechanical properties of rocks under thermal shock conditions. The primary physical characteristics of rocks include color, density, volume, wave velocity, and porosity [10–13]. Liu et al. [14] found that granite specimens exhibit marked color variations with increasing temperature, gradually transitioning from gray-brown to gray below 600 °C. Fan et al. [15] revealed that the attenuation rate of the longitudinal wave progressively increases with temperature. Gao et al. [16] documented gradual increases in both pore volume and porosity in granite specimens after thermal shock.