## 4 Engineering Application  
### Design for the Construction of the Synergistic Support System of \"LDAGF\"  
According to the synergistic support system, its construction process in the ESFMCFR is as follows: \"L\"--floor breaking and side lowering\(\rightarrow\)\"D\"--bolt advance support\(\rightarrow\)\"G\"--advance water injection and grouting\(\rightarrow\)\"F\"--flat-roof U-shaped steel shed support\(\rightarrow\)\"A\"--anchor cable support. First, according to the dip angle of the coal seam in the extremely soft and fragile fully mechanized caving face, the floor breaking and side lowering begins. After the roadway positions are determined, self-drilling screw steel bolts for grouting are arranged along the roof of the shed beam for advance roof protection. Afterwards, the bolts undergo advance grouting to solidify the roof. Meanwhile, advance water injection is performed on the coal at the head to enhance the coal's plasticity. After the advance support is completed, during slurry solidification, the flat-roof U-shaped steel sheds are used to support the ESFMCFR in thick seams. Then, the roadway roof, sides and flat-roof U-shaped steel shed legs are reinforced by anchor cables to enhance stability. The synergistic support system of \"LDAGF\" for the ESFMCFR in thick seams is shown in Figure 7. Its specific design is as follows:  
(1) \"L\"--floor breaking and side lowering  
Figure 6: Schematic diagram of the synergistic support system of “LDAGF”. Figure 7: Synergistic support system of “LDAGF”. According to the dip angle of the coal seam, the floor breaking and side lowering construction is performed. The lower side is lowered by 200 mm so that most of the higher side is located in the floor strata of the coal seam, hence improving the SR environment of the roadway sides. Meanwhile, floor breaking and side lowering into the floor strata can ensure that the flat-roof U-shaped steel shed legs at the two sides take root firmly and improve the lithologic strength of the roadway sides (Figure 8). 2. \"D\"--bolt advance support  
The advance support of the EESFMCFR is achieved by self-drilling screw steel bolts for grouting, which are arranged according to the section contours. The bolt spacing is about 200 mm (the spacing can be adjusted according to the distance between the coal seam and gangue). The bolts are arranged at a 3-5 deg angle with the roadway construction slope to ensure that the bolts are on the same plane. The anchor bolts are 4800 mm long, but the overlapping length of each group of bolts is \(\lesssim\)1.0 m. Over-excavation is prohibited (the circulating footage distance is controlled according to the length of the anchor bolts and overlapping distance). The layout of the advance self-drilling screw steel anchor bolts for grouting is shown in Figure 9.  
3. \"G\"--advance water injection and grouting in the roadway  
An electric grouting pump is adopted. Then, 3-5 advance self-drilling screw steel anchor bolts can be grouted at the same time by installing a diverter at the outlet hole of the electric grouting pump. When the grouting pressure meets the requirement, the grouting pipeline is closed; the holes are changed to continue grouting. The layout of the grouting pipeline is shown in Figure 10a. Figure 8: Schematic diagram of floor breaking and side lowering of the roadway. To enhance the coal's plasticity, the coal body at the head is injected with water in advance. According to previous experience and the results of numerical simulation optimization, two holes are injected with water in each cycle. The other parameters are as follows: the hole spacing is 3 m (arranged separately according to the axis of roadway cross section), the holes are about 1.7 m away from the roof and the drilling depth is 10 m with an elevation angle of 3-5\({}^{\circ}\) with the roadway direction. The holes are sealed with hole-sealers, and the water injection pressure is 0.5 MPa. The arrangement of the water injection holes is based on the infiltration range of water in the coal body.