## 4 Results
### Compaction Characteristics  
In the compaction seepage experiment of lithological combination samples, the axial stress was 0-5 MPa, and 1 MPa was used as the graded loading gradient to test the seepage characteristics under 0 MPa, 1 MPa, 2 MPa, 3 MPa, 4 MPa, and 5 MPa axial stress. The'stress-displacement' and'stress-strain' recorded during the whole compaction seepage experiment are shown in Figure 7. The results show that the axial compression of 2 MPa is the dividing line. When the axial compression is less than 2 MPa, the order of strain from large to small under the same axial compression is C-2 > C-4 > C-3 > C-1. When the axial compression is greater than 2 MPa, the order of strain from large to small under the same axial compression is C-2 > C-4 \(\approx\) C-3 > C-1. In the C-1 case, the stress decreased and then increased. The compaction curves of C-4 and C-3 are close to the same compaction characteristics after 2 MPa axial compression. Creep phenomenon is common under five-stage axial compression in the range of 1-5 MPa. The seepage water pressure of the C-3 has the least influence on the pulse of the load under constant load. According to the results, the seepage and compaction characteristics of the combined crushed stone have the following characteristics: the strain of the combined crushed stone within 0-2 MPa axial compression is affected by many factors such as lithological strength, softening characteristics, layered thickness, number of interfaces, accumulation structure, and initial PV. Therefore, the'mudstone-coal' combination condition has the largest deformation during the initial compaction process. From a comprehensive perspective, the compaction characteristics are still dominated by the initial PV, and the order of strain and \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Scheme Number** & **Combined Lithology** & **Thickness/mm** & \(\mathbf{K_{\mathrm{p}}}\) & & \(\mathbf{\varphi}\)**/\%** & \\ \hline C-1 & sandstone & 77 & 1.48 & & 32 & \\  & mudstone & 73 & 1.47 & & 32 & \\ C-2 & mudstone & 73 & 1.47 & & 32 & \\  & coal & 86 & 1.78 & & 44 & 38 \\  & sandstone & 79 & 1.51 & & 34 & \\  & coal & 87 & 1.80 & & 44 & 39 \\  & sandstone & 47 & 1.50 & & 33 & \\ C-4 & mudstone & 46 & 1.55 & 1.57 & 35 & 36 \\  & coal & 33 & 1.71 & & 42 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The bulking characteristics of combined lithologic samples. Figure 7: Compaction characteristics of lithology combination broken rock: (**a**) stress–strain; (**b**) stress–displacement. initial PV in the compacted state is finally consistent. In a non-argillaceous rock combination sample of 'coal-sandstone', the contribution rate of seepage water pressure to the external load of broken rock is small. The void and bulking characteristics of each sample under different axial stresses are shown in Figure 8. The compaction characteristic parameters of the four composite samples fitted by Formula (2) are shown in Table 5.  
\[\varphi=m+ue^{-\frac{\varphi_{0}}{\varphi}} \tag{2}\]  
The creep amount of the load-holding stage during the compaction process is shown in Figure 9. From Figure 9a, it can be seen that the overall regularity of the change in creep amount in the same period under fixed axial stress is poor. For C-3 and C-4, the change trend of creep under each axial stress is similar. Combined with the above-mentioned stress-strain law, the compaction characteristics of C-3 and C-4 are similar as a whole. The creep amount of C-2 is greater than that of C-1 in the range of no more than 3 MPa axial compression. After the axial compression is greater than 3 MPa, the creep amount of C-1 and C-2 has the same trend, and the creep amount of C-1 is greater than that of C-2. However, the overall regularity of creep accumulation under each axial stress is good. As shown in Figure 9b, the creep accumulation is sorted into two sections from large to small. No more than 2 MPa: C-2 > C-1 > C-3 > C-4; more than 2 MPa: C-2 > C-3 > C-1 > C-4.