Moisture-induced softening characteristics and mechanisms of saturated hard rocks under compression

The water-rock interaction in rock mass engineering can lead to physical and mechanical deterioration of rock, that is, the moisture-induced softening phenomenon of rock. Currently, the studies on the moisture-induced softening properties of saturated hard rocks are not comprehensive, and it is still challenging to evaluate the triggering condition and extent of effects related to various water-rock interactions. By choosing the typical hard rocks with different mineral compositions and microstructures (i.e., Baoxin marble, Jinping marble, and limestone), the uniaxial compression tests, along with the acoustic emission (AE) monitoring, of three kinds of hard rocks under dry and saturated conditions are carried out. The mechanical properties of rock including uniaxial compressive strength (UCS), elastic modulus and failure patterns as well as the statistical characteristics of the dominant frequency of AE waveforms are analyzed. Further, the triggering condition and extent of the pore water pressure effects of these three kinds of rocks are evaluated. The results show that when the hard rocks are saturated, the UCS and elastic modulus both decrease, and the tensile cracks and tensile failure surfaces increase. The twin-peak feature of the dominant frequency of all AE waveforms is exhibited, which is independent of the hard rock types and soaking states. As the hard rocks are saturated, the number of AE waveforms in high dominant frequency bands reduces remarkably, while that in low dominant frequency bands increases significantly. The effect of pore water pressure, which corresponds to the low dominant frequency of AE waveforms, is the main factor responsible for the mechanical degradation of three kinds of saturated hard rocks, and its intensity depends on the mineral composition and pore structure of rock.