## 2 Test Method and Test Materials
### Specimen Preparation and Test Process  
The test material was quartz sand and Kaolin clay. Quartz sand with a single mineral composition was a kind of granular material often used in the study of stress wave propagation in filled rock joints. Kaolin clay was commonly found in natural rock discontinuities. At the same time, it had good water absorption, and did not expand after water absorption. In the preparation of partially saturated specimens, it could be used as a water retaining agent to make the water distribution in partially saturated specimens more uniform.  
The quartz sand was Fujian standard quartz sand and the major particle size was between 0.5 mm and 1 mm. The granule sizes of 0.5-0.8 mm accounted for about 70%, and 0.8-1.0 mm accounted for about 30%. The standard quartz sand oven dried at 105 \({}^{\circ}\)C. The initial dry density and voidage of quartz sand was 1.567 g/cm\({}^{3}\) and 41%. The clay was selected from Lingshou County washed Kaolin, which is a kind of clay widely existing in nature. The initial dry density and particle size of clay was 1.59 g/cm\({}^{3}\) and 100-200 mesh. The particle size distribution curves of sand and kaolin has been shown in Figure 4.  
Each test confined the specimen in a 6 mm thick chamber. The gas chamber pressure constant was set to 0.2 MPa. There were 10 working conditions in the test, two parallel tests were carried out in each group of working conditions, and a total of 20 tests were conducted, with test numbers from T-1 to T-20 in sequence. T-1 to T-10 tests were conducted to investigate the influence of clay content on the propagation characteristics of the stress wave in artificial filled joints. The weight fractions of clay were 0%, 30%, 50%, 70% and 100%, respectively, and the corresponding dry densities were 1.569 g/cm\({}^{3}\), 1.575 g/cm\({}^{3}\), 1.578 g/cm\({}^{3}\), 1.582 g/cm\({}^{3}\) and 1.590 g/cm\({}^{3}\), respectively. The quartz sand and clay were uniformly mixed according to the mass ratio, and filled into the sleeve according to the fixed mass, which were filled with 18.45 g, 18.50 g, 18.54 g, 18.60 g and 18.68 g, respectively. T-3, T-4, T-11 to T-18 tests were conducted to investigate the influence of moisture content on the propagation characteristics of stress wave in artificial filled joints. Since the water in quartz sand mostly existed in the form of free water, it would sink into the bottom of the specimen because of the action of gravity during the test. Referring to the test method  
Figure 3: Typical response signals of incident, reflected and transmitted waves.  
of [PERSON] et al. [20], clay was used as the water retention agent. Specimens with different saturation were prepared by adding a certain amount of water into sand-clay mixtures with 30% clay weight fraction. It was found that the specimen could basically approach a complete saturation state when 18.50 g sand-clay mixtures mixed with 4 g water. Therefore, 0 g, 1 g, 2 g and 3 g water were added to 18.50 g sand-clay mixtures, respectively, and the corresponding saturation \(S_{r}\) was 0%, 25%, 50% and 75%, respectively. When preparing saturated specimens, putting 18.50 g dry sand-clay mixtures into the sleeve, then putting sleeve into the vacuum saturation device, adding water until completely submerged. Opening the vacuum pump to pump the saturation device to vacuum, soaking for 10 min, then taking it out, so that the mixtures in the sleeve was saturated, then completing the impact test within 2 min. In the T-19 and T-20 group tests, the saturated quartz sand specimens were made by the above method as a comparative test.