## 3 Results  
The transmission coefficient based on peak stress was an important index to judge stress wave attenuation in rock joints. The wave transmission coefficient was defined as the ratio of the amplitude of the transmitted wave to that of the incident wave [21]; that is:  
\[T=\frac{\sigma_{\mathrm{max}}}{\sigma_{\mathrm{max}}}=\frac{\varepsilon_{ \mathrm{max}}}{\varepsilon_{\mathrm{max}}} \tag{5}\]  
where \(T\) is the wave transmission coefficient, \(\sigma_{\mathrm{max}}\) and \(\sigma_{\mathrm{max}}\) are the peak stresses of the incident wave and the transmitted wave, \(\sigma_{\mathrm{max}}\) and \(\sigma_{\mathrm{max}}\) are the strains corresponding to the peak stresses of the incident wave and the transmitted wave. The wave transmission coefficient \(T\) defined based on the peak stress of the incident wave and the transmitted wave represents the wave attenuation capacity of a specimen but not the infill materials, which are greatly affected by the thickness of the specimen. To describe the stress attenuation ability of granular material quantitatively, a dimensionless  
Figure 4: Particle size distribution curves of quartz sand and Kaolin before impact. modified attenuation coefficient \(k\) was proposed by removing the relative impedance of the bar to the specimen and considering the effects of the size of the specimen [22], as follows:  
\[k=-\frac{D}{L_{s}}\ln(T) \tag{6}\]  
where \(L_{s}\) is the length of the specimens, \(D\) is the diameter of the specimen, and \(T\) is the wave transmission coefficient. The main results of the test are given in Table 1.  
According to Table 1, when the gas pressure was 0.20 MPa, the amplitudes of the incident waves ranged from 149.3 MPa to 169.2 MPa. For the same gas pressure, the incident wave amplitude was similar and the test conditions were basically the same. Therefore, the tests had good repeatability and high reliability. The incident wave and transmission wave of artificial filled joints with different Kaolin weight fractions are plotted in Figure 5a. Under the condition of similar incident wave amplitude, the amplitude of the transmitted waves of 50% Kaolin weight fractions is much higher than that of transmitted waves corresponding to other working conditions. When the gas pressure is 0.20 MPa, the amplitudes of the transmitted waves of the 50% Kaolin weight fractions ranged from 48.8 MPa to 55.1 MPa. With the increase of Kaolin weight fractions, the amplitude of transmitted wave increases first and then decreases. When the weight fraction of Kaolin was 100%, the amplitude of transmitted wave is the smallest, only ranging from 10.4 MPa to 13.1 MPa, which is decreased by about 77% compared with the highest amplitude of transmitted wave. According to Table 1, as shown in Figure 5b, with the increase in the weight fractions of Kaolin, the average attenuation coefficient \(k\) decreases first and then increases. When the weight fraction of Kaolin is 30%, the average attenuation coefficient is close to that of quartz sand. When the weight fraction of Kaolin was 50%, the minimum attenuation coefficient was 9.70. When the weight fraction of Kaolin was 100%, the maximum attenuation coefficient was 21.92. It could be considered that under the impact of high amplitude  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Test** \\ **Number** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Clay** \\ **Weight** \\ **Fraction** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Saturation** \\ **\(S_{s}\)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Incident Wave** \\ **Amplitude/MPa** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Transmitted** \\ **Wave** \\ **Amplitude/MPa** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Attenuation** \\ **Coefficient \(k\)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Incident** \\ **Energy** \\ **\(W_{i}\)/J** \\ \end{tabular}  } & \multirow{2}{*}{