Damage evolution mechanism of CA-UHPC in deep frozen shafts under hydro-mechanical coupling based on AE technology

Deep frozen shaft linings are subjected to a high risk of failure due to the coupled effects of elevated in-situ stress and hydraulic pressure. However, the damage evolution behavior of Coarse Aggregate Ultra-High-Performance Concrete (CA-UHPC), a promising high-performance lining material, under hydro-mechanical coupling conditions remains insufficiently understood. To address this knowledge gap, triaxial stress–seepage coupling tests were performed under various osmotic pressures, combined with Acoustic Emission (AE) monitoring and the Kneedle algorithm to quantitatively evaluate crack proportions, with a constant confining pressure of 10 MPa and osmotic pressures varied across five levels: 1, 3, 5, 7, and 9 MPa. The results indicate that the damage evolution process can be divided into distinct stages, each exhibiting systematic variations in AE parameters. Increasing osmotic pressure leads to the degradation of mechanical properties and a higher proportion of splitting tensile cracks, driving the macroscopic failure mode to evolve from shear-dominated, low-angle conjugate shear failure to steep-angle mixed tensile–shear failure. Mechanistically, the water-wedging effect weakens the cementitious matrix, while axial stress promotes crack propagation and coalescence into a continuous seepage channel, ultimately inducing structural failure. Notably, a pronounced increase in AE energy accompanied by a sustained decrease in the b-value is identified as a reliable precursor to structural instability. This study elucidates the progressive failure mechanisms of CA-UHPC under hydro-mechanical coupling, thereby providing a scientific basis for structural health monitoring and early warning systems in deep underground shaft linings.