## 4 Results
### Seepage Characteristics  
Compaction seepage experiments were carried out on four samples with different lithological combinations. The seepage characteristics under different axial stress are shown in Figure 10.  
Figure 10: Compaction seepage characteristics of lithology combination samples: (**a**) C-1; (**b**) C-2; (**c**) C-3; (**d**) C-4.  
Figure 9: Creep characteristics of equal duration under different axial stress: (**a**) creep increment; (**b**) creep cumulative amount.  
The results show that the seepage characteristics of C-1 change from unstable to stable in the range of 2-3 MPa. Subsequently, for the convenience of narration, the axial compression interval in which the seepage characteristics change from unstable to stable is called seepage stabilized axial compression (SSAC). In the axial compression range of 3-5 MPa, the permeability decreases by one order of magnitude for each 1 MPa increase, the flow rate decreases sharply, and the nonlinear seepage characteristics are stable. The SSAC of C-2 is 1-2 MPa, and the flow velocity decreases sharply under the axial stress of 1 MPa, and then the decrease of flow velocity becomes smaller. The SSAC of C-3 is 0-1 MPa. In the axial compression range of 3-5 MPa, the flow velocity decreases obviously with the increase of 1 MPa, and the flow velocity tends to be stable with the increase in water pressure gradient. The flow velocity of C-4 decreased sharply after axial compression loading. With the gradual compaction, the flow velocity changed little and fluctuated in a small range. In the case of various lithological combinations, the flow velocity of sandstone and coal composite specimens under the same axial stress is the largest. The remaining samples have mudstone combinations, and the variation range of flow velocity is close.  
The permeability and non-Darcy factor are obtained by fitting, and then the changes in \(k\) and \(\beta\) under different \(\varphi_{\rm d}\) are analyzed as shown in Table 6. The results of compaction-seepage process show that: For C-1, \(\varphi_{\rm d}\) is decreased by 13.65%, \(k\) increases first and then decreases, and the absolute value of \(\beta\) increases monotonously. For C-2, \(\varphi_{\rm d}\) decreased by 17.78% during the compaction and seepage process, \(k\) experienced two 'decrease-increase' fluctuations, and the absolute value of \(\beta\) increases monotonously. For C-3, \(\varphi_{\rm d}\) decreases by 16.21%, \(k\) has two 'increase-decrease' fluctuation changes, and the absolute value of \(\beta\) increases monotonously. For C-4, \(\varphi_{\rm d}\) decreased by 16.70%, \(k\) had three 'decrease-increase' fluctuations, and \(\beta\) had two 'increase-decrease' fluctuations.  
The experimental results show that the seepage characteristics of the combined rock samples under low axial stress are unstable, and the curves of water pressure gradient and flow velocity show tortuous characteristics. The flow velocity of the combined samples with  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Scheme Number** & **Axial Stress/MPa** & \(\varphi_{\rm d}\)/\%** & \(k\)/D & \(\beta\)/m\({}^{-1}\) & \(R^{2}\) \\ \hline \hline \multirow{8}{*}{C-1} & 0 & 32.02 & \(-1.76\times 10^{-1}\) & \(1.65\times 10^{6}\) & 0.946 \\  & 1 & 25.43 &  
mudstone is small, and the flow velocity does not exceed 1.6 mm/s. The flow velocity of the combined samples without mudstone is larger, and the flow velocity exceeds 2.5 mm/s. Under the same axial stress and water pressure gradient, the flow velocity of mudstone can be reduced by about 36%. The change trend and change times of CSC of lithology combination are related to the order and number of interfaces of lithology combination.