Water saturation dependent acoustic emission event structures and damage accumulation paths in limestone with a central circular through hole

Abstract

The instability of holed limestone in water-rich karst areas is controlled by both hole-induced stress concentration and water-induced weakening. Its damage evolution is reflected not only by mechanical degradation, but also by changes in acoustic emission event structures and energy release paths. To clarify the damage accumulation mechanism of holed limestone under different water saturation degrees, limestone specimens containing a central through-going circular hole were prepared and tested at five target saturation degrees of 0%, 25%, 50%, 75%, and 100%. Uniaxial compression tests were carried out with synchronous AE monitoring. Energy analysis, RA–AF-based cracking tendency identification, UMAP–HDBSCAN-based multi-feature event identification, and an event-type-weighted AE damage index, D AE , were combined to examine the effects of water saturation on mechanical degradation, AE event structures, and damage accumulation paths. The results show that, as the saturation degree increased from 0% to 100%, the average uniaxial compressive strength and elastic modulus decreased by 38.74% and 41.06%, respectively. Meanwhile, the peak dissipated energy ratio increased from 12.59% to 28.16%, and the failure mode changed from localized through-going brittle failure to progressive failure with multiple cracks. Conventional AE statistics indicate that the cumulative AE hit number and cumulative AE energy decreased with increasing saturation. In contrast, the pre-peak proportions of AE hits and AE energy increased, suggesting that damage activity shifted from concentrated release near the peak stress to distributed accumulation before the peak. Based on UMAP–HDBSCAN multi-feature identification and secondary physics-based merging, the AE hits were grouped into four event types: low-amplitude and low-energy weak events, short-duration high-frequency events, long-duration composite events, and high-energy release events. With increasing saturation, the proportion of high-energy release events decreased markedly, whereas the proportions of low-amplitude and low-energy weak events and long-duration composite events increased. The event-type-weighted D AE showed a staged evolution consistent with the dissipated-energy-based damage index, D U , and captured the transition of the damage path from “delayed activation–near-peak surge” to “early activation–continuous accumulation”. This work provides a laboratory-scale framework for linking water saturation, AE event structures, and pre-peak damage accumulation in cavity-like limestone, offering a basis for AE-based damage interpretation of water-affected holed rock. These findings provide a useful reference for damage identification and instability precursor analysis of cavity-bearing surrounding rocks in water-rich karst regions.