Experimental Study on the Effects of Water Saturation on the Microseismic and Acoustic Emission Characteristics of Sandstone in Different Stress States

To investigate the evolution characteristics of microseismic (MS) and acoustic emission (AE) of sandstone before and after water saturation in different stress states and the precursory information of rock failure, Brazilian splitting, direct shear, and uniaxial and biaxial compression tests were carried out on natural and saturated specimens of sandstone, and the whole failure process of each specimen was monitored by an MS system combined with an AE system. The results show that: (1) water increased the number and duration of MS waveform signals detected during the post-sigma(peak) stage, increased the overall level of the MS b value, and reduced the accumulative absolute AE energy (E-AE,E-ab) and single abrupt increases in E-AE,E-ab. These changes evince the effects of water on the strength and deformation of the sandstone and reflect the degradation and softening of the sandstone due to saturation. Furthermore, water was found to significantly affect the mechanical properties and acoustic characteristics of the sandstone, and the effect varied with the stress state of the sandstone. (2) A notable abrupt increase in the MS fractal dimension (D-t), a notable abrupt decrease in the MS b value, or the first post-sigma(peak) decrease in the AE D-t to a D-t,D-min less than 0.1 often suggests imminent rock failure. The overall average b value of MS signals during the failure process of a rock can be used as a criterion for determining whether it is in a saturated state. (3) Due to the difference in the frequency range, acquisition mode and activeness indexes of MS and AE signals, MS is more sensitive to large-scale fracture events and AE is more sensitive to small-scale fracture events. Combining the respective advantages of AE and MS signals and using both of them to monitor the failure process of rocks can facilitate a more comprehensive understanding of their intrinsic failure mechanisms and, therefore, help to provide more accurate predictions of rock failure.