## 4 Discussion  
### Particle Crushing Behavior  
Particles experienced compaction and crushing with high-amplitude stress waves propagation across a filled fracture. The process of particle crushing was not only related to the dynamic compression process of the whole specimen, but also determined the amount of energy dissipation when the seismic wave transmission across the filling material. By analyzing the degree of the particle crushing, the deformation mechanism in the whole process could be obtained indirectly [24]. Previous works have found that particle crushing was affected by many factors, such as initial void, particle size, particle grading composition, strain rate, loading condition, constraint condition, etc. [25, 26]. However, when the specimen was saturated, the crushing of particles could not be fully reflected from the gradation curve because the water was incompressible, so it needs further discussion. The test work results were quantified by the relative breakage potential \(B\) which was given by [PERSON] [27]:  
\[B=\frac{B_{t}}{B_{p}} \tag{7}\]  
where \(B_{p}\) is the breakage potential, which was defined as the area between the lower bound of the 0.074 mm sieve opening and the original size distribution curve, and \(B_{t}\) is the total breakage, which was defined as the area between the original and the final size distribution curves. As shown in Figure 7a, as the weight fraction of Kaolin decreased from 70% to 0%, a considerable number of particles were crushed into smaller particles. The dependence of filler deformation on the amplitude of the stress wave could be divided into three stages: initial compaction stage, crushing stage, and compaction and crushing stage. When the gas pressure was set to 0.20 MPa, the particle skeleton of the pure quartz sand specimen mainly in the compaction and crushing stage, the particles were seriously crushing, and the relative breakage potential \(B\) was the largest, which was 0.29. With the increase of clay content, the volume of clay was compressed under high amplitude impact. Because the clay could not be further crushed, the clay filled the space between the skeletons and reduced the particle deformation space. The stiffness of the artificial joint was increased, and the wave attenuation was reduced. When the Kaolin weight fraction was 30%, after the clay was compressed, the quartz sand skeleton still had deformation space, the relative breakage potential \(B\) decreased less, and the wave attenuation was larger. When the weight fraction  
Figure 6: The artificial filled joints with different saturation \(S_{r}\) correspond to: (**a**) incident wave and transmission wave; (**b**) Attenuation coefficient curve. of Kaolin was 50%, the void of the specimen was completely filled by the compressed clay during impact, and the rigidity of the specimen reached the maximum, and the wave attenuation was small. When the weight fraction of Kaolin further increased, the particle skeleton was no longer the force subject, and waves were absorbed and attenuated during the compression process of clay. With the increase of clay content, the wave attenuation performance decreased first and then increased, the relative breakage potential \(B\) decreased continuously, and the same trend did not appear. According to the test data of T-3, T-4, T-11 to T-20 groups in Figure 8, it could be found that when the Kaolin weight fraction was 30%, with the increase of saturation, the relative breakage potential \(B\) decreased first, then increased and then decreased. As the saturation increased from 0% to 25%, the relative breakage potential \(B\) decreased from 0.24 to 0.16. Compared with the saturation of 50%, the relative breakage potential \(B\) was 0.19. Analyzing the reason was that when the saturation was 25%, under the high amplitude impact, the existence of water between particles led to the decrease of friction between particles. First, the particles slid relatively to the space compaction and then the particles crushed.