Dynamic Stress Wave Behaviors Across Single Fluid-Filled Rock Fractures
Authors: Yang H. , Duan H. F. , Zhao Q. , Zhu J. B. 
Published: August 2024

ABSTRACT:
Dynamic stress waves are commonly encountered in geoengineering operations (e.g., rock blasting and hydraulic fracturing) and natural events (e.g., earthquake ruptures and volcanic eruptions), strongly influencing the deformation and failure of rock masses and rock infrastructures. Understanding dynamic stress wave behavior across rock fractures is essential for mining, tunneling, geothermal energy extraction, and hydrocarbon exploitation. The present study conducted considerable dynamic impact tests on synthetic fluid-filled rock fractures under different saturation conditions through a custom-made split Hopkinson pressure bar (SHPB) test system, aiming to quantitatively determine dynamic stress wave behaviors across fluid-filled rock fractures. The SHPB test data were processed to estimate transmission and reflection coefficients, and wave attenuation factors for quantifying dynamic stress wave responses of fluid-filled rock fractures. The experimental results show that increasing water content and decreasing fracture thickness lead to more wave transmission and less wave reflection. Wave attenuation decreases with rising water content within the range of 0% – 75% and reducing fracture thickness. A distinctive finding is that rock fractures fully saturated with water experienced more wave attenuation than those in close-to-saturation scenarios, which could be attributed to the energy consumption induced by the wave-induced fluid flow out of the filled joint (i.e., squirt flow).

1. INTRODUCTION
The interaction of seismic waves and fluid-filled rock joints has been a hotspot of geomechanics and geophysics because it is of great importance to reservoir detection and characterization, geothermal exploration and extraction, underground engineering appraisal, exploration seismology, earthquake engineering, etc. (Reiser et al., 2020; Viswanathan et al., 2022).
Considerable efforts have been devoted to low-intensity wave behaviors across individual fluid-filled rock fractures via laboratory experiments. Place et al. (2016) performed ultrasonic measurements on single fractures fully filled with air, water, or grouts at different cement contents, where the filling fluid was almost at rest or moving at a controlled flow rate through the fracture.
They found that the fluid type highly affects reflected wave spectra and energy, while the internal fluid flow has negligible influences on wave reflection. Kamali-Asl et al. (2019) conducted the flow-through-fracture tests with concurrent measurements of the ultrasonic P- and cross-polarized S-waves propagation along the fracture. Their test results showed that the decreasing fracture aperture caused less P-wave attenuation and higher P-wave velocity while increasing the amplitude of cross-polarized S-waves. Yang et al. (2020, 2021) performed massive ultrasonic pulse-transmission tests on individual fluid-filled rock fractures, clarifying the role of the fluid composition and spatial distribution, fracture orientation, and temperature in P-wave signatures across single fluid-filled rock fractures.