Damage evolution and failure modes of coal-concrete composites with varying height ratios under cyclic loading

Abstract

To ensure the safe implementation of underground reservoirs in abandoned coal mines, this study explores the mechanical behavior and failure mechanisms of coal-concrete composite structures under staged cyclic loading. Specimens with coal-to-concrete height ratios ranging from 0.5:1 to 3:1 were tested, with damage evolution continuously monitored using acoustic emission techniques. Results indicate that while the peak strength of pure materials decreases by approximately 1 MPa under cyclic stress compared to uniaxial compression, composite specimens exhibit strength enhancements exceeding 5 MPa. However, the peak strength of composite specimens decreases with increasing coal height, from 30 MPa at CR0.5 to 20 MPa at CR3.0. The damage state was assessed using the dynamic elastic strain energy index and Felicity ratio, which revealed that composite specimens are more prone to early damage accumulation. Spatial acoustic emission localization further reveals distinct failure modes across specimens with varying height ratios. To elucidate these differences, interfacial effects were incorporated into a modified twin-shear unified strength theory. The refined model accurately predicts the internal strength distribution and failure characteristics of the composite structures. These findings provide a theoretical basis for the structural design and safe operation of underground reservoir dams.

1. Introduction

Repurposing mined-out voids as underground reservoirs offers a forward-looking strategy with strong practical potential. This approach not only prevents the waste of subsurface resources but also facilitates the green transition of mining regions [1,2]. Moreover, it provides a solid foundation for validating and integrating underground pumped storage systems in coal mines (Fig. 1).

As a deeply buried, permanent hydraulic structure, the dam body of an underground reservoir is subject not only to sustained hydraulic pressure and in-situ rock stress but also to dynamic loads such as mining-induced seismicity and stress perturbations, making its stability critical. Numerous studies have investigated the stability of such dam structures. Several researchers have focused on brittleness evaluation in studying the deformation and failure behavior of coal pillars and proposed various quantitative indices [3,4]. Building on this, others have investigated the elastoplastic softening behavior of coal and developed corresponding constitutive models [5,6]. Regarding fatigue effects, Yu et al. [7], Wang et al. [8] and Wu et al. [9] investigated the damage behavior of coal under water exposure and cyclic loading, revealing that soaking duration significantly influences progressive damage, while wet-dry cycling markedly accelerates fatigue accumulation. Further studies have incorporated the hydro-mechanical coupling effects on dam materials under hydraulic pressure, establishing damage models and conducting numerical simulations to explore the dominant factors of coal degradation under multi-physical conditions [10]. Some scholars have also investigated the creep behavior of coal under triaxial seepage, identifying a three-stage pattern in seepage and strain evolution, and analyzed how varying pore pressures affect its long-term strength. For artificial dam bodies, most studies have employed numerical simulations to assess how structural parameters influence overall stability, whereas experimental investigations into damage and failure characteristics remain relatively limited [11–13].