## 2 Experiments and Methods  
### The Experimental System  
The experimental system (Figure 2) is modeled on the actual grouting injection and mining conditions at a ratio of 1:400. The outer part of this system is composed of a box spliced with 20 mm thick acrylic sheets, and the inner dimensions of the box are 1200 mm \(\times\) 500 mm \(\times\) 250 mm (length \(\times\) width \(\times\) height). This system is composed of a mining module, grouting module, pressure displacement monitoring module, similar simulation material module, saturation monitoring module, and water leakage of working face monitoring module, which can be used to implement the IOGI system during the coal seam excavation and monitor the seepage of the slurry water in the overburden.  
The mining module uses acrylic sheets with a thickness of 1 cm and a width of 10 cm as a mining unit. Each piece of acrylic sheet is pulled out to simulate the mining of 40 m  
Figure 2: Three-dimensional visualization of experimental system for the seepage of IOGI slurry-bled water: (**a**) Design diagram and (**b**) Physical diagram [30].  
of the coal seam along the strike direction. The excavation of the coal seam is simulated by pumping acrylic sheets. The grouting module (Figure 2) is composed of a slurry pump (BT300M yz1515x, Jingkecheng (Hangzhou) Scientific Instrument Manufacturing Co., Ltd., Hangzhou, China), a slurry mixer, a slurry storage tank, a grouting pipe, a grouting hole, and a grouting pipe-hole connector. It should be noted that in order to visualize the slurry-bled water process and the layout of the pressure sensors, the upper overburden of the injection layer was temporarily disregarded, and the upper part of the injection layer was assumed to be the water-proof layer, which should not affect the final research results. The injection layer is located at the interface between the roof of the system and the top of similar materials. Before the pumping of acrylic sheets, the opening of the interface is almost zero. When the acrylic sheets are continuously pulled out, the opening of the interface gradually increases according to the subsidence law of similar materials and gradually expands in the strike direction.  
The slurry pressure in the injection layer was determined by drilling holes in the top cover of the experimental system and installing pressure sensors (CYT-102-A-P6-S1-V1-C1, Beijing Tianyu Hengchuang Sensor Technology Co., Ltd., Beijing, China) at the grouting holes (Figure 2). Second, the module for monitoring water leakage from the working face is located at the interface of the water-bearing fracture zone above the working face. It consists of a strainer, a silicone pad, 18 aqueducts, 18 polyvinyl chloride (PVC) casing pipes, and glass-measuring cylinders (Figure 3). The boundary around the silicone pad and the frame around the model were glued by quick-setting adhesive, and the interface was sealed with petroleum jelly at the connection point to prevent the bled water from leaking into the lower overburden from the measured boundary between the silicone pad and the experimental system frame. Aqueducts attached to the prefabricated holes on the silicone pad were inserted into the measuring cylinders, and the visual monitoring of slurry-bled water leakage in the working face was realized by monitoring the volume of water in the measuring cylinders. Since the aqueducts are soft silicone pipes, PVC casing pipes were inserted on the outside of the aqueducts as protective pipes to prevent similar materials moving during mining from compressing the aqueducts so that the water did not flow through the aqueducts. The PVC casing pipes and the aqueducts passed through the lower traditional similar materials, the acrylic sheets, and the bottom plate of the system box to enter the measuring cylinders at the bottom of the system box. To prevent similar materials from blocking the aqueducts, a layer of strainer was placed on the silicone pad.