## 4 Floor Heave Control with Anti-Shear Pile Technology
### Effectiveness of Anti-Shear Pile Method
The elaboration of schemes for the second simulation stage was based on the concept that the summary length of the piles should not be more than 4.0 m (more than at the first simulation stage), since the borehole drilling and pile consumption depend on it. At this stage, it was assumed that two piles would be installed on the immediate floor. The position of one of these piles was determined in the results of the analysis of the investigation at the first simulation stage. This was position #5. Pile installation in this position leads to the highest efficiency among the compared ones. Thus, the most effective position for the second pile was searched for in the second simulation stage. The distributions of vertical strain and shear strain around the entry before and after pile installation are shown in Figures 12 and 13. Figure 11: The vertical strain distributions around the gob-side entry retaining after pile installation with schemes (**a**) 1 pile 5 (2); (**b**) 1 pile 4 (2); (**c**) 1 pile 3 (2). Figure 10: The entry floor heave (**a**) and horizontal strain distributions in surrounding rock (**b**) before and after pile installation at the 1 st simulation stage. It can be seen that installing a second pile in any of the compared positions reduces the size of the post-peak strain zone on the immediate floor. The best effect is obtained in the case of the 2 pile 5+2 scheme. The size of the vertical delamination zone (Figure 11(b)) and the post-peak shear strain zone (Figure 11(b)) is the smallest in this case. It is clear that this is reflected in the size of the floor heave, which is minimal for this scheme. The scheme 2 pile 5+1 is less effective, as can be seen from the larger size of the post-peak shear strain (Figure 11(a)) and post-peak vertical strain (Figure 11(a)) on the entry floor. However, the 2 pile 5+1 scheme is better than the scheme 2 pile 5+3. The post-peak strain zones in the 2 pile 5+3 scheme (Figures 11(c) and 11(c)) are the largest among the compared schemes. It can be concluded that the anti-shear effect of pile installation is more efficient than the span-lowering effect for floor heave control in gob-side entry retaining. The entry floor heave before and after pile installation at the second simulation stage is shown in Figure 11(a). The scheme 2 pile 5+2 is the most effective one. With this scheme, the maximum floor heave is 361 mm (Figure 11(a)), and the size of the horizontal post-peak strains is the smallest (Figure 11(b)). Thus, the most efficient scheme has been determined. Figure 12: The vertical strain distributions around the gob-side entry retaining after pile installation with schemes (a) 2 pile 5+1; (b) 2 pile 5+2; (c) 2 pile 5+3. Figure 13: The shear strain distributions around the gob-side entry retaining after pile installation with schemes (a) 2 pile 5+1; (b) 2 pile 5+2; (c) 2 pile 5+3. Third simulation stage. For the practical implementation of the proposed method, it must satisfy two conditions: be technologically simple and inexpensive. There is no doubt that the piles should be installed in the gateroad outside the longwall influence zone. This will allow us to start floor heave control in a timely manner. Hole drilling for piles is a necessary process for floor support technology, and there is only one possible way to implement it. Therefore, the attention at the third simulation stage was focused on finding the optimal type of pile. In order to find more economical and simple solutions, a technical and economic comparison of different types of piles was carried out. A metal tube with a concrete filling was the initial pile type, for which all previous simulation stages were carried out. This type of pile was called the \"steel pile\" (Figure 15a). As an alternative, pile variants were proposed, such as tubes without filling (scheme 2 pile 5+2(tube)), called \"steel tubes\" (Figure 15b). Concrete filling piles with different concrete classes (2 pile 5+2(B-20...B-60)) were also studied.