Laboratory Investigation on Seismic Response of Rock Fractures Filled with Granular Materials

Filling materials may exist in all scales of rock fractures, not only influencing seismic wave attenuation, but controlling rock mass instability. The objective of this study is to experimentally investigate the seismic response of rock fractures filled with granular materials. This experimental study simplifies the complex interaction between seismic waves and filled rock fractures and considers the seismic-induced compressive responses of a filled single fracture and a set of filled parallel fractures and the seismic-induced frictional slip on a filled fracture. The seismic-induced compressive response of a filled single fracture is investigated by a split Hopkinson rock bar (SHRB) test. The SHRB test simulates the P-wave propagation across the filled fracture and verifies the analytical solutions predicted by the displacement and stress discontinuity model. The seismic wave attenuation across the filled fracture is due to wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The specific fracture stiffness and the specific initial mass of the filling materials are two key parameters in interrelating the physical, mechanical and seismic properties of the single fracture filled with dry granular materials. The SHRB test is modified to study the seismic-induced compressive responses of a set of filled parallel fractures and to verify the analytical solutions predicted by the modified recursive method based on the displacement and stress discontinuity model. The seismic wave attenuation across the filled parallel fractures is due to multiple wave reflections between the filled fractures, as well as wave reflection at the fracture interfaces and dynamic compaction of the filling materials. The dynamic compaction of the filling materials induces the decreases of loading rate and dominant frequency when the P-wave propagates across each filled fracture in a fracture set. The seismic-induced frictional slip on a filled fracture is performed by a dynamic-induced direct-shear (DIDS) test. A generated plane P-wave propagates as a dynamic shear load and induces the frictional slip along a layer of filling materials. The dynamic shear stress is non-uniformly accumulated in the filling materials. The shear stress at the trailing edge controls the dynamic triggering of frictional slip owing to the highly compacted filling materials. The dynamically triggered frictional slip is quickly arrested by the re-connection of sand contacts after the instantaneous disturbance, while the statically triggered frictional slip is unrecoverable during a short duration and contains a main slip and a few short slips before and after the main slip. The seismic response of rock fractures filled with granular materials is decomposed into the seismic-induced compressive responses and the seismic-induced frictional slip in this study. Both seismic-induced responses are strongly related to the dynamic response of the filling materials. The analytical models and the experimental experiences are possible to be numerically combined to study random wave propagation across rock fractures filled with granular materials and to be applied to estimate seismic wave attenuation and rock mass instability.