# Research on the Characteristics of Seepage Failure in the Surrounding Rock (Coal) of the Goafs
## 2 Materials and Methods
### Experimental Principles and Procedures  
Permeability (k) is one of the main parameters for studying the permeability of rock formations, representing the permeability of coal rock. From the perspective of coal rock damage, permeability can be considered the dislocation and fracture of bonding bonds at the micro-level, the expansion and penetration of microcracks at the meso-level, and changes in mechanical behavior at the macro-level. The stress changes experienced by coal rock are usually accompanied by a damage process [27; 28; 29; 30; 31]. In this study, a seepage system was used to test 10 selected sets of coal samples. To express the permeability performance in the experiment, the coal samples were assumed to \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Coal Sample Number** & **Diameter (mm)** & **Length (mm)** & **Mass (g)** & \multicolumn{2}{c}{**Volumetric Weight (N/m3)**} & **Fissure Development** \\ \hline M-1 & 24.76 & 30.10 & 17.20 & 12.32 & 3 microfractures \\ M-2 & 24.82 & 30.22 & 19.23 & 12.96 & 1 microfractures \\ M-3 & 24.82 & 30.24 & 19.19 & 12.80 & 2 microfractures \\ M-4 & 24.74 & 30.00 & 16.81 & 12.32 & 1 microfractures \\ M-5 & 24.82 & 30.20 & 17.82 & 13.01 & One X-shaped microfissure \\ M-6 & 24.74 & 30.12 & 20.21 & 15.02 & Multiple microfissures \\ M-7 & 24.86 & 30.10 & 19.97 & 13.18 & 1 microfractures \\ M-8 & 24.78 & 30.20 & 17.23 & 12.60 & 1 microfractures \\ M-9 & 24.72 & 30.12 & 19.38 & 12.83 & Multiple microfissures \\ M-10 & 24.88 & 30.20 & 19.26 & 13.01 & Multiple microfissures \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristic parameter table of coal sample to be measured. Figure 1: Locations for coal rock samples. be equivalent to a porous medium with the same cross-sectional area, uniformly distributed, in accordance with the experimental assumptions of [PERSON]'s law [32; 33]:  
\[Q=\frac{k\cdot\Delta p\cdot S}{\eta\cdot L} \tag{1}\]  
where \(Q\) is the flow rate; \(S\) is the cross-sectional area of the coal sample; \(L\) is the length of the coal sample; \(\eta\) is the viscosity coefficient of the fluid; \(k\) is the permeability of the coal sample; and \(\Delta P\) is the pressure difference between the upstream and downstream of the test sample. Permeability can be measured in two ways: one is by applying a stable pressure difference \(\Delta P\) at the upstream and downstream ends of the coal sample and measuring the flow rate \(Q\) through the sample to obtain the permeability; the other is by giving a certain flow rate \(Q\) and then calculating the permeability based on the pressure difference \(\Delta P\) between the upstream and downstream. In this experiment, a stable flow rate injection method was used to obtain the parameters (Table 2). The steps of the experiment are as follows:  
1. Check the Instrument: Before inserting the core, check the instrument to ensure that the confining pressure has been released, remove the end caps, and ensure that there are no cores or spacers in the holder. Prepare for the experiment. 2. Insert the Coal Sample: Wrap the prepared coal sample with waterstop tape and insert it into the holder. Fix the end caps with two protruding clips, aligning them with the grooves on the end cover, and rotate clockwise about 1/4 turn until it cannot be turned further, ensuring the position is fixed while tightening the adjustment cap. 3. Insert Spacers: Insert appropriate length spacers based on the coal sample length (30 mm), ensuring the total length of the core and spacers is within 80-100 mm. 4. Check Sealing: After inserting the test sample, turn the adjustment sleeve counter-clockwise by hand. If it moves easily, tighten it further. Ensure the sleeve is tight and not easy to move, completing the inspection. 5. Apply Confining Pressure: Manually pressurize the confining pressure pump. After loading the confining pressure, check the stability and reliability of the seepage system. If there is any pressure relief, check for leaks and repair them promptly to ensure the instrument works properly. 6.