(**c**) Borehole under a surrounding rock

**Figure 14.** The variation curve of surrounding rock displacement of the borehole with time.

2.3.2. Analysis of the Plastic Failure

The distribution of plastic failure zones of the boreholes before and after protection is shown in Figure 15. From the plastic distribution diagram, it can be seen that the area of plastic damage of rock mass in 160 mm and 200 mm boreholes are obviously reduced after the application of borehole protection tubes; especially, the area of plastic damage of rock mass in the upper and lower sides of boreholes is the largest. The reduction of the plastic failure zone indicates that the failure instability of the rock mass around the borehole is alleviated, the degree of displacement and deformation is reduced, and the stability of the rock mass around the borehole is strengthened.

**Figure 15.** Distribution of plastic failure of boreholes with different diameters. (**a**) The distribution plastic failure of the borehole of diameter of 100 mm without protection pipe; (**b**) the distribution plastic failure of the borehole of diameter of 160 mm with protection pipe; (**c**) the distribution plastic failure of the borehole of diameter of 200 mm without protection pipe; (**d**) the distribution plastic failure of the borehole of diameter of 200 mm with protection pipe.

Therefore, it is found that the stress concentration around the borehole is relieved according to the FLAC 3D simulation, the displacement of rock mass around the borehole is reduced, and the plastic damage is reduced, which indicates that the borehole protection tube can provide some protection support. However, for the large boreholes of 160 mm and 200 mm, the strong stress concentration and plastic damage obviously still exist after the protection, and the stability of the borehole needs to be further strengthened.

### **3. Experimental Study of the Internal Support Borehole Protection Pipe**

According to the above analysis of borehole stability, the stability of the directional long borehole on the roof needs to be further improved. The borehole protection pipe is the main measure to change the stability of the borehole, but after using the conventional borehole protection pipe, there is still serious plastic damage and stress concentration phenomenon in directional long boreholes of large diameters. Therefore, the conventional pipe can only improve the stability of the directional long hole in the roof to a certain extent. Therefore, various internal supporting structures are designed to enhance the effect of hole protection, and their effects are studied through compression experiments.
