Triaxial Compression and Unloading Acoustic Emission Characteristics of Coral Block

This study investigated the mechanical response and instability precursors of highly porous coral blocks from the South China Sea under complex stress paths through conventional triaxial compression tests, two types of triaxial unloading tests, and synchronous acoustic emission (AE) monitoring. The effects of confining pressure, unloading path, and unloading stage on strength, deformation, dilatancy, and failure behavior were examined. The coefficients of variation of dry density, saturated density, porosity, and P-wave velocity were 5.07%, 3.56%, 3.72%, and 5.77%, respectively, indicating relatively limited variability in the measured physical properties, although the influence of specimen heterogeneity cannot be fully excluded. Within the 0–2 MPa confining-pressure range, peak strength increased from 8.81 to 16.85 MPa, whereas axial strain at peak strength changed from 0.33% at 0 MPa to 0.63% at 1 MPa and then decreased to 0.40% at 2 MPa, indicating strong strength sensitivity but a nonmonotonic deformation response. During unloading, all specimens exhibited a transition from compaction to dilatancy. At unloading rates of 0.2 and 0.5 MPa/min, the absolute value of the volumetric strain evolution slope was higher under the increasing-axial-pressure unloading path than under the constant-axial-pressure unloading path, indicating that the path-related difference in dilatancy appears more pronounced under the present test conditions. AE activity increased progressively near peak stress during conventional compression, whereas unloading-induced AE events concentrated near macroscopic failure. Lateral strain anomalies generally preceded AE bursts, suggesting that lateral deformation appears to provide a more sensitive early-warning indicator under the present test conditions.