# A Dynamic Numerical Simulation on the Grouting Timing in Retained Rib of Pillarless Mining
## 5 Discussion  
The timing of grouting is one of the most important parameters for grouting in mining roadways. Timely grouting in the surrounding rock can repair the damaged bolt structure in time. The roadway support structure and the surrounding rock can be effectively coupled and become an effective bearing structure again [45,46]. Grouting in retained coal rib was carried out at different distances from the working face in this simulation. If grouting is performed at different sites before and after the working face, Figure 8 shows the deformation of the retained coal rib when the grouting position is 120 m behind the working face. On the whole, whenever grouting was used to consolidate the fragmented surrounding rocks, the deformation of the retained coal rib was effectively controlled to varying degrees, as shown in Figure 8. The curve in Figure 8 presents a \"U\" shape, indicating an optimal timing of grouting in the retained coal rib of pillarless mining. The principle of optimal grouting timing in the retained coal rib of pillarless mining is shown in Figure 9. The solid line 1 in the figure represents the deformation of the retained coal rib without grouting reinforcement, and the dashed lines 2-5 represent the deformation of the retained coal rib after the grouting is carried out at different sites before and after the working face. As illustrated by solid line 1, the roadway experiences some large deformations and even instability without the grouting. In this case, the surrounding rock of the roadway is in a loose and fragmented state, accompanied with a complete loss of the anchoring force. The roadway is in a plastic flow state under the low support force. Figure 8: Retained coal rib deformation grouted at different distances from working face. Figure 7: Relationship between diffusing range, rib deformation and grouting site. The dashed line 2 represents the deformation of the retained coal rib when grouting was added at a later than optimal timing. In the mining-affected stable section, the main roof above the roadway forms a stable voussoir beam structure, with a low mining-induced stress in the surrounding rock. In addition, the deformation speed was also low. However, the fragmented range expanded beyond the anchoring range, and the roadway surrounding rock enters a state of slow and continuous plastic flow. The dashed line 3 represents the deformation of the retained coal rib when the grouting was added at an earlier than optimal timing. When not affected by mining-induced stress, the fissures in the retained rib are original joints and small cracks caused by roadway excavation. The crack apertures were small and the connectivity degree was low[PERSON]  
The dashed lines 4 and 5 are representative points of the optimal grouting timing[PERSON] The best support effect can be obtained by grouting at the sections affected by high mining-induced stress before and behind the working face. According to the study by [PERSON] [33], the observation of the bore scope could be used to study the development of cracks in the retained coal rib and optimize the grouting time and reasonable reinforcement methods. Under the influence of mining-induced stress, many cracks appear in the coal rib, leading to the continuous expansion of cracks and the original joints in the coal seam. It should be noted that the fractured zone of the coal rib is still within the anchoring range. In this case, the interaction system between the bolt structure and surrounding rock is in a critically stable state, and the bolt system exerts its maximum supporting effect, as shown in Figure 10[40]. In the minding sites, the fast-hardening sulfoaluminate cement is a commonly used grouting cement, whose uniaxial compressive strength can reach 30 MPa in 1 day and can be increased to 42.5 MPa in 3 days. The recommended method of support is to use a grout with a water-cement ratio of [PERSON], grouting 20-30 m before the working face [40].