## 6 Conclusions  
This study investigated the effects of the nozzle diameter, jet pressure, rotation speed, and slotting time on the cutting depth. A model for calculating a slotting depth suitable for field application was established, and optimized slotting parameters were obtained. Based on water jet slotting field tests, the proposed calculation model was verified. The following conclusions were drawn from this study:  
1. The attenuation coefficient of the jet axial dynamic pressure first decreased and then increased with the increase of the nozzle diameter. For a much higher jet impact velocity, there existed an optimal nozzle diameter. Additionally, the cutting depth linearly increased with the jet pressure and decreased as a power function with the increase of the jet translation speed. Moreover, the number of cuttings had a significant impact on the cutting depth, and the several previous cuttings played a major role in the jet cutting. With the further increase of cutting times, the cutting depth slowly increased, but the increment was small.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Number**} & \multirow{2}{*}{**Test Condition**} & \multicolumn{2}{c}{**Whether There is Water**} & \multirow{2}{*}{**Slotting Depth (m)**} & \multirow{2}{*}{**Prediction Depth (m)**} \\  & & **Flowing Out from Inspection Hole or Not** & & & \\ \hline
1 & n = 60 r/min, t = 5 min & 11\#-2\#: Yes, 34\#-6\#: No & 1.0\(\sim\)1.5 m & 1.37 m \\
2 & n = 50 r/min, t = 10 min & 11\#-2\#: Yes, 4\#-6\#: No & 1.0\(\sim\)1.5 m, close to 1.5 m & 1.48 m \\
3 & n = 40 r/min, t = 15 min & 11\#-3\#: Yes, 4\#-6\#: No & 1.5\(\sim\)2.0 m & 1.69 m \\
4 & n = 30 r/min, t = 20 min & 11\#-3\#: Yes, 5\#-6\#: No & 1.5\(\sim\)2.0 m, close to 2.0 m & 1.91 m \\ \hline \hline \end{tabular}
\end{table}
Table 1: Results of water jet slotting in coal seam under different test conditions (rotation speed and slotting time).  
Figure 14: Layout of slotting borehole and inspection holes.  
2. Water jet slotting experiments were conducted with different rotation speeds and slotting times. The results revealed that the slotting depth increased with the slotting time, but the growth rate gradually decreased and tended to be stable. As the rotation speed increased, the slotting depth became greater at the initial period, and the limit depth was reached faster. At the early slotting stages, more slotting repetitions were helpful in increasing the slotting depth. At the later slotting stages, a longer single impact improved the slotting efficiency.
3. A model for calculating the water jet slotting depth was established according to the effects of key parameters on the cutting depth. This model was subsequently verified using the rotatory slotting experiment data, and the results revealed that the fitting was good. Based on the proposed model, the slotting depths under different jet pressures and the threshold rock-breaking pressures were calculated.
4. Water jet slotting field tests were carried out. The slotting depths at different rotation speeds with different slotting times were analyzed. Comparisons with the depths predicted by the calculation model were made, which revealed that the differences are acceptable.  
[PERSON], [PERSON], [PERSON], and [PERSON] contributed to conceiving and designing the experiments, analyzing the data, and writing the paper. [PERSON] and [PERSON] performed the experiments.  
This research was funded by the National Natural Science Foundation of China, grant numbers 51504046, 51774055, and the National Science and Technology Major Projects of China, grant number 2016 ZX05045.  
We thank [PERSON], Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.  
The authors declare no conflict of interest.