Experimental study on the role of clay mineral and water saturation in ultrasonic P-wave behaviours across individual filled rock joints

Understanding wave propagation across rock joints is of great importance in rock engineering, petroleum engineering, and other geophysical applications. Previous studies have shown that wave behaviours across filled rock joints are highly affected by the properties of filling materials. However, how clay minerals and water saturation of the infilling affect wave attributes across filled rock joints is still poorly understood. In this study, ultrasonic tests were conducted on kaolinite- and bentonite-filled rock joints to investigate the roles of clay minerals and the water saturation of infillings on elastic P-wave behaviours across individual filled rock joints. The test results show that the infilling's water saturation and mineralogical compositions have combined effects on P-wave signatures of single rock joints filled with clayey soils. Particularly, for the kaolinite-filled rock joint, P-wave velocity and attenuation first increase and then slightly decline with the increasing water saturation. By contrast, for the bentonite-filled rock joint, an increase in water saturation causes gradual decreases in velocity and attenuation. The integrated interpretation of the experimental results indicates that the difference in heterogeneity saturation due to clay hydration is the first-order reason for the discrepancies in saturation-dependent P-wave attributes between kaolinite- and bentonite-filled rock joints. The findings of this study can help further understand wave propagation and attenuation across rock joints filled with fluid-saturated clayey soils.