## 2 Theory of Slug Test on Determining Permeability Coefficient of Anisotropic Fractured Rock Mass
### The Analysis of Conventional Water Pressure Test Results  
According to the Chinese \"Specification for Water Pressure Test in Borehole of Hydropower Projects\" (NB/T 35113-2018) [26], generally, a conventional water pressure test [27; 28] is always carried out with five phases (\(P_{1}-P_{2}-P_{3}-P_{4}(=P_{2})-P_{5}(=P_{1})\), \(P_{1}<P_{2}<P_{3}\)) and three pressure stages, \(P_{1}\), \(P_{2}\) and \(P_{3}\), which are 0.3 MPa, 0.6 MPa and 1 MPa, respectively. The water permeability of the test section is calculated by the pressure value (\(P_{3}\)) and flow rate (\(Q_{3}\)) of the third pressure stage in the Equation (17).  
\[q=\frac{Q_{3}}{LP_{3}} \tag{17}\]  
Among them, \(q\) is the water permeability of the test section (Lu _or_ L/min/m); \(L\) is the length of the test section (m); \(Q_{3}\) is the flow rate of the third pressure stage (L/min); \(P_{3}\) is the test section pressure of the third pressure stage (MPa).  
Three conventional water pressure tests were conducted on site. The test section ranges of the first group test are 1.06-4.76 m, the second 4.37-10.12 m and the third 1.37-10.12 m.  
The \(P-Q\) curves of the three water pressure tests are shown in Figure 5. It can be observed from Figure 5 that the \(P-Q\) curve of the first group water pressure tests belong to the expansion type (\(C\) type), and the pressure rise curve in the \(P-Q\) curve is basically the curve of the convex to \(P\) axis. The pressure drop curve and the pressure rise curve basically coincide. The flow increases significantly when the pressure reaches a certain pressure, indicating that the fracture status changes and the fracture permeability increases under the action of the test pressure.  
However, the flow rate of the fourth phase and the second phase, and the fifth phase and the first phase basically coincide, indicating that this fracture status change is temporary and reversible. With the decrease in test pressure, the fracture returns to its original status, which showing an elastic expansion property. The \(P-Q\) curves of the second and third groups water pressure tests belong to the turbulent flow type (\(B\) type), and the pressure rise curve in the \(P-Q\) curve is basically the curve of the convex to the \(Q\) axis. Similarly, the pressure drop curve and the pressure rise curve basically coincide. The fracture status has not changed during the whole test period.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Footage No.**} & \multirow{2}{*}{**Fracture No.**} & \multicolumn{2}{c}{**Occurrence Measured**} & \multirow{2}{*}{**Dip Angle (\({}^{\circ}\))**} \\  & & **by Slug Test System** & & **Measured by** \\ \cline{3-4}  & & **Dip Direction (\({}^{\circ}\))** & **Dip Angle (\({}^{\circ}\))** & **Compass after Coring** \\ \hline \hline \multirow{2}{*}{Footage No. 2} & Fracture No. 1 & 159 & 29 & 27 \\ \cline{2-4}  & Fracture No. 2 & 48 & 59 & 60 \\ \hline \hline Footage No. 3 & Fracture No. 3 & 50 & 61 & 59 \\ \hline \hline Footage No. 4 & Fracture No. 4 & 72 & 53 & 52 \\ \hline \hline Footage No. 5 & Fracture No. 5 & 120 & 73 & 71 \\ \hline \hline Footage No. 6 & Fracture No. 6 & 110 & 55 & 51 \\ \hline \hline Footage No. 7 & Fracture No. 7 & 166 & 47 & 45 \\ \hline \hline Footage No. 8 & Fracture No. 8 & 162 & 48 & 48 \\ \hline \hline Footage No. 8 & Fracture No. 9 & 129 & 64 & 65 \\ \hline \hline Footage No. 9 & Fracture No. 10 & 178 & 63 & 60 \\ \hline \hline Footage No. 10 & None & None & None & None \\ \hline \hline Footage No. 11 & None & None & None & None \\ \hline \hline \end{tabular}
\end{table}
Table 2: The contrast of geological information obtained by the slug test system and manual measurement after coring.  
The water permeability of the corresponding test section in three group tests are calculated as well [28], and the calculation results are shown in Table 3. It can be observed from Table 3 that with the increase in the buried depth of the test section, the water permeability decreases gradually.