## 3 Results and Discussion
### Experimental Results for the Four Typical Crack Models
#### 3.3.3 Fixed Confining Pressure  
The flow rate change curves of different crack modes and axial pressures with confining pressures of 20 MPa and 10 MPa are shown in Figure 30. The flow rate change rule was similar with confining pressures of 20 MPa and 10 MPa, but the flow rate value was significantly higher under 10 MPa compared to 20 MPa. This indicates that the flow rate increases as the confining pressure decreases. The seepage flow rate was in the order of type III, type II, type Y, 70-degree, and then type I. As the axial pressure decreased, the seepage flow rate gradually increased and the increment of the seepage flow rate also increased. Figure 28: Flow rate change curves of the Y-type crack model: (**a**) variation in flow rate with the axial pressure and (**b**) variation in flow rate with the confining pressure. Figure 29: Flow rate change curves of different crack modes: (**a**) axial pressure of 40 MPa and (**b**) axial pressure of 10 MPa. When the confining pressure was 10 MPa, the axial pressure decreased from 30 MPa to 5 MPa. The seepage increment of the type I crack model was the largest (6.5 \(\times\) 10\({}^{-6}\)), while that of the other four cracks models was about 4 \(\times\) 10\({}^{-6}\). When the axial pressure was 10 MPa, the confining pressure decreased from 30 MPa to 5 MPa. The seepage increment of the type III crack model was the largest (3.058 \(\times\) 10\({}^{-5}\)), while the seepage increment of the 70-degree model was the smallest (1.329 \(\times\) 10\({}^{-5}\)), as shown in Figure 31a. When the confining pressure was 20 MPa, the axial pressure decreased from 30 MPa to 5 MPa. The seepage increment of the type I crack model was the largest (3.99 \(\times\) 10\({}^{-6}\)), while that of the other four cracks models was about 2 \(\times\) 10\({}^{-6}\). When the axial pressure was 20 MPa, the confining pressure decreased from 30 MPa to 5 MPa. The seepage increment of the type III crack model was the largest (2.886 \(\times\) 10\({}^{-5}\)), while the seepage increment of the type I model was the smallest (1.114 \(\times\) 10\({}^{-5}\)). When the confining pressure was at a constant value (10 MPa in Figure 31a and 20 MPa in Figure 31b) and the axial pressure decreased from 30 MPa to 5 MPa, the seepage increment of the type I crack model was the largest, and the seepage increments of the type III, type II, type Y, and 70-degree crack models were almost stable at the same value, indicating that these crack models have similar levels of sensitivity to axial pressure. The seepage increment (10\({}^{-5}\)) when the confining pressure reduced from 30 MPa to 5 MPa was an order of magnitude greater than the seepage increment (10\({}^{-6}\)) when the axial pressure reduced from 30 MPa to 5 MPa (10\({}^{-6}\)), indicating that the permeability of a crack is more sensitive to confining pressure than to axial pressure. The order of seepage increment from large to small cracks was type III, type II, type Y, type I, and then the 70-degree model, which indicates that the order of sensitivity of the permeability of the crack to the confining pressure was type III, type II, type Y, type I, and then the 70-degree model. Studying stress-sensitive characteristics is one of the most important issue in mining technology. There are essential differences in seepage characteristics of rocks under unloading and loading conditions. In order to fit the real stress environment of rock mass engineering, the engineering properties of rock under unloading stress path must be studied deeply. In recent years, with the rapid development of computer computing performance, numerical simulation calculation method has been more tried in the field of fractured rock seepage. In this paper, the numerical simulation calculation method was conducted on the basis of a new method for hydraulic parameter fitting and crack reconstruction technology involving the typical experimental failure form. DEM is used to investigated the seepage characteristics of different fracture modes under unloading action.